improvement of treatment plans developed with intensity

8/4/2019 Improvement of Treatment Plans Developed With Intensity

1/17

Improvement of Treatment Plans

Developed with Intensity-modulated

Radiation Therapy for Concave-shapedHead and Neck Tumors1

1. Nesrin Dogan, PhD,

2. Leonid B. Leybovich, PhD,3. Stephanie King, CMD,

4. Anil Sethi, PhD and

5. Bahman Emami, MD

+ Author Affiliations

1. 1From the Department of Radiation Oncology, Loyola University Chicago

Medical Center, 2160 S First Ave, Maywood, IL 60153. Received May 29, 2001;revision requested June 25; revision received August 23; accepted September 20.

Address correspondence to N.D. (e-mail: [email protected]).

Next Section

Abstract

PURPOSE: To improve dose conformity and normal tissue sparing in patients with

concave-shaped head and neck cancers by using tomotherapy and static step-and-shootintensity-modulated radiation therapy (IMRT) and by comparing results with those of

three-dimensional (3D) conformal radiation therapy (CRT) and two-dimensional (2D)radiation therapy.

MATERIALS AND METHODS: Treatment planning in 10 patients with concave-

shaped head and neck tumors was performed by using tomotherapy and step-and-shoot

IMRT, 3D CRT, and 2D techniques. IMRT plans were modified by placing virtualcritical structures in regions outside the target where hot spots occurred. These modified

plans were used for comparison because they provided better dose conformity. Critical

structures were the spinal cord, the parotid glands, and the mandible. Comparisons were

performed by means of dose-volume histograms, clinical target volume (CTV), targetcovered by 95% isodose (D95%), dose received by 5% of the critical structure volume

(D5%), maximum dose, mean dose, and normal tissue complication probability for criticalstructures.

RESULTS: Original IMRT plans showed more conformal dose distributions than those

in 3D CRT and 2D plans. However, hot spots developed in the posterior and anterior

neck. Introduction of virtual critical structures in IMRT plans resulted in removal of these
http://radiology.rsna.org/content/223/1/57.full#aff-1http://radiology.rsna.org/search?author1=Nesrin+Dogan&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Leonid+B.+Leybovich&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Leonid+B.+Leybovich&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Stephanie+King&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Stephanie+King&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Anil+Sethi&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Bahman+Emami&sortspec=date&submit=Submithttp://radiology.rsna.org/content/223/1/57.fullhttp://radiology.rsna.org/content/223/1/57.full#notes-1http://radiology.rsna.org/content/223/1/57.full#aff-1http://radiology.rsna.org/search?author1=Nesrin+Dogan&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Leonid+B.+Leybovich&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Stephanie+King&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Anil+Sethi&sortspec=date&submit=Submithttp://radiology.rsna.org/search?author1=Bahman+Emami&sortspec=date&submit=Submithttp://radiology.rsna.org/content/223/1/57.fullhttp://radiology.rsna.org/content/223/1/57.full#notes-1


2/17

hot spots without affecting target coverage. Modified IMRT plans also demonstrated

better CTV coverage than that in 3D CRT and 2D plans. The average D95% was 97.3%

with tomotherapy, 97.1% with step-and-shoot IMRT, 84.7% with 3D CRT, and 69.4%with 2D techniques. D5% for the spinal cord changed from approximately 45 Gy with 3D

plans and 46 Gy with 2D plans to approximately 28 Gy with IMRT.

CONCLUSION: IMRT demonstrated better target coverage and sparing of critical

structures than that of 3D CRT and 2D techniques. Use of virtual critical structuresresulted in removal of hot spots around the spinal cord.

RSNA, 2002

Previous SectionNext Section

RSNA, 2002

Because of the complex anatomy and large number of sensitive normal structures in thevicinity of a target, treatment planning for head and neck cancers is a challenging task.

The clinical target volume (CTV) includes the gross tumor volume and regions of

suspected microscopic disease. These regions usually surround the spinal cord. The need

to spare the spinal cord from excessive radiation dose results in concave-shaped clinicaland planning target volumes. Moreover, the parotid glands also have to be spared because

xerostomia considerably affects a patients quality of life (1). In conventional forward

planning techniques, sparing of the spinal cord with two-dimensional (2D) techniquesand three-dimensional (3D) conformal radiation therapy (CRT) is usually achieved by

combining photon and electron beams. However, matching of photon and electron beams

produces areas of highly nonuniform dose distribution (24). Although the dose to the

spinal cord may not exceed the tolerance level with these techniques, very high doses areusually delivered to the parotid glands (5).

When intensity-modulated beams are used, more conformal dose distributions may be

achieved (68). Development of optimal intensity-modulated radiation therapy (IMRT)plans, however, is practically impossible with conventional forward 3D treatment

techniques.

Inverse treatment planning systems that automatically create IMRT plans are now

commercially available (9,10). In the optimization process, the purpose of these systemsis to satisfy treatment goals, such as assigning prescription dose to the target(s) and dose-

volume constraints to the critical structures. Even though treatment goals may be met,however, dose distributions developed by IMRT planning systems are usually far fromdesired. For example, for aforementioned concave CTV of the head and neck, treatment

plans demonstrate acceptable doses to the spinal cord but high doses to regions around

the cord. Posner et al (11) also reported a similar problem.

Several planning techniques that might alleviate this problem have been reported in theliterature (12,13). De Neve et al (12) developed a method involving beam intensity
http://radiology.rsna.org/content/223/1/57.full#abstract-1http://radiology.rsna.org/content/223/1/57.full#sec-1http://radiology.rsna.org/content/223/1/57.full#ref-1http://radiology.rsna.org/content/223/1/57.full#ref-2http://radiology.rsna.org/content/223/1/57.full#ref-4http://radiology.rsna.org/content/223/1/57.full#ref-5http://radiology.rsna.org/content/223/1/57.full#ref-6http://radiology.rsna.org/content/223/1/57.full#ref-8http://radiology.rsna.org/content/223/1/57.full#ref-9http://radiology.rsna.org/content/223/1/57.full#ref-9http://radiology.rsna.org/content/223/1/57.full#ref-10http://radiology.rsna.org/content/223/1/57.full#ref-11http://radiology.rsna.org/content/223/1/57.full#ref-12http://radiology.rsna.org/content/223/1/57.full#ref-12http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-12http://radiology.rsna.org/content/223/1/57.full#abstract-1http://radiology.rsna.org/content/223/1/57.full#sec-1http://radiology.rsna.org/content/223/1/57.full#ref-1http://radiology.rsna.org/content/223/1/57.full#ref-2http://radiology.rsna.org/content/223/1/57.full#ref-4http://radiology.rsna.org/content/223/1/57.full#ref-5http://radiology.rsna.org/content/223/1/57.full#ref-6http://radiology.rsna.org/content/223/1/57.full#ref-8http://radiology.rsna.org/content/223/1/57.full#ref-9http://radiology.rsna.org/content/223/1/57.full#ref-10http://radiology.rsna.org/content/223/1/57.full#ref-11http://radiology.rsna.org/content/223/1/57.full#ref-12http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-12


3/17

modulation that uses a static technique of beam segmentation in the lower neck and upper

mediastinal regions. Esik et al (13) used a triangle-shaped dummy organ that was

positioned in the posterior neck region. They were able to avoid assigning high doses toareas surrounding the spinal cord. However, the treatment plans were developed for a

simplified phantom-based case that used only the step-and-shoot IMRT method of

delivery. Wu et al (14) used distention of the original spinal cord contours by 0.5 cm.This technique pushed the high-dose region somewhat away from the spinal cord.

However, the high-dose regions still remained in the posterior neck and around the spinal

cord.

The purpose of our study was to improve dose conformity and normal tissue sparing inpatients with concave-shaped head and neck cancers by using tomotherapy and step-and-

shoot IMRT and by comparing results with those of 3D CRT and 2D radiation therapy.


MATERIALS AND METHODS

Ten patients with head and neck cancer and cervical node involvement (node levels 15)

were selected for this study. Patients underwent computed tomography (CT) with a 35-

mm section thickness from the top of the head to the lower portion of the neck. The scans

were transferred to the AcQSIM system (Marconi Medical Systems, Cleveland, Ohio) forcontouring. CTV volumes, prescription doses, and dose constraints to the critical

structures were specified according to ICRU50 recommendations (15). Eight patients

received radiation after resection of the primary lesion. In all cases, CTV1 included lymphnode station levels 15, and prescription dose was 44 Gy. The shape of CTV1 was

concave. CTV2 included all subclinical and microscopic disease around the primary

tumor and within the lymph node drainage area at risk, and prescription dose was 60 Gy.CTV3 included the tumor bed in eight patients, and prescription dose varied from 64 to 66

Gy. In two patients who had an intact tumor, CTV3 included the primary tumor, and

prescription dose was 70 Gy. The critical structures considered in this study were the

spinal cord, the parotid glands, and the mandible. The shape of the CTV was used as theonly criterion for inclusion of patients in the study. Figure 1 shows the 3D volumetric and

2D representations of the target and normal structures in one patient included in the

study.
http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-14http://radiology.rsna.org/content/223/1/57.full#notes-1http://radiology.rsna.org/content/223/1/57.full#sec-2http://radiology.rsna.org/content/223/1/57.full#ref-15http://radiology.rsna.org/content/223/1/57.full#F1http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-14http://radiology.rsna.org/content/223/1/57.full#notes-1http://radiology.rsna.org/content/223/1/57.full#sec-2http://radiology.rsna.org/content/223/1/57.full#ref-15http://radiology.rsna.org/content/223/1/57.full#F1


4/17

View larger version:

In this page

In a new window

Download as PowerPoint Slide

Figure 1. Target and critical structure representations are shown in 3D volumetric (top)

and transverse (bottom) CT images. Yellow = parotid glands, light blue = mandible, darkblue = target, green = spinal cord.

The treatment of all patients was planned and performed with 3D CRT because it is used

routinely in our clinic for patients with head and neck tumors. For comparison, however,

treatment for these patients was also planned with 2D and IMRT techniques. 3D CRTand 2D planning was performed on a FOCUS treatment planning system (CMS, St Louis,

Mo). IMRT plans were developed with a NOMOS-CORVUS treatment planning system

(NOMOS, Sewickley, Pa). All plans were generated by an experienced dosimetrist (S.K.)in collaboration with two physicists (N.D., L.B.L.) who coauthored this article. The same

two physicists analyzed the plans for the presence of hot spots, selected and compared

IMRT plans with 3D CRT and 2D plans, and performed all relevant calculations. Ourinstitutional review board did not require its approval or patient informed consent, since

no patient-specific information was used.

A combination of 6-MV photon beams and 612-MeV electron beams were used in 3D

CRT and 2D treatment plans when necessary. Coned-down fields were used to boost thegross tumor volume. 2D plans were developed by performing CT through the center of

the target. 2D plans used mostly parallel-opposed lateral beams with Ellis compensating

filters. An anterior field was used to treat the lower portion of the neck. To evaluate thetrue target coverage and critical structure sparing, volumetric dose distributions

corresponding to the beam arrangements developed for 2D treatment plans were also

calculated.

In 3D CRT plans, the wedge angles, number of beams, and their orientations andweightings were varied until acceptable volumetric dose distributions were obtained.

Most of the 3D CRT plans used a four-field technique with right and left lateral, anterior,

and lower-anterior neck fields. Non-coplanar beams were also used as needed.
http://radiology.rsna.org/content/223/1/57/F1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F1.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F1http://radiology.rsna.org/content/223/1/57/F1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F1.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F1


5/17

IMRT plans were developed for both tomotherapy and static step-and-shoot techniques.

Tomotherapy plans were developed by using 1-cm MIMiC mode (NOMOS) (16) with no

couch angulation. The step-and-shoot IMRT plans included eight coplanar nonopposing6-MV photon beams. This beam arrangement was selected because our preliminary

studies demonstrated that further increase in the number of beams did not noticeably

improve treatment plans. Multileaf collimator field segments and number of segmentswere determined with the NOMOS-CORVUS planning software. For each case, the

number of multileaf collimator segments for each field varied between 30 and 50. After

IMRT plans were developed, they were analyzed for the presence of hot spots outside thetarget.

Two methods were used to reduce the magnitude of hot spots. The first method made use

of an external avoidance feature, available with the NOMOS-CORVUS system, in

which the system tried to eliminate beams that passed through locations of externalavoidance structures. The second method consisted of the creation of virtual critical

structures (17) in areas that needed dose reduction. Dose constraints to virtual critical

structures were determined interactively during the optimization process. Doseconstraints that resulted in the desired dose reduction without affecting target coveragewere used to produce the final IMRT plan. For each patient, an IMRT plan that

demonstrated superior dose distribution was selected for comparison with 3D CRT and

2D plans.

The planning goals were to cover 95% of the target volume with 95% of the prescribed

dose and to keep the critical structure doses at or below the known tolerance limits. For

the spinal cord, the tolerance dose was 45 Gy. For the parotid glands, the goal was to

limit the volume that received a dose greater than 30 Gy to 33%. For the mandible, thevolume that received a dose greater than 60 Gy was limited to 15%.

Treatment plans were compared by means of dose-volume histograms, maximum dose

(Dmax), dose received by 5% of the critical structure volume (D5%), mean dose (Dmean), and

target volume covered by 95% of the prescription dose (D95%). In addition, doses receivedby 33% and 50% of the parotid glands (D33% and D50%, respectively), the mandible

volume that received 60 Gy or more (V60Gy), and the dose received by 50% of the

mandible (D50%) were evaluated. Standard deviations for the mean values of allparameters were calculated.

Normal tissue complication probability (NTCP) was calculated for all critical structures

by using the Lyman model (18) with the Kutcher-Burman histogram reduction method

(19,20). The parameters for NTCP calculations (volume effect [n], slope [m], and dosethat will cause 50% complication probability [TD50]) were selected according to Emami

et al (21). The following NTCP parameters were used: TD50 = 66.5 Gy, n = 0.05, and m =

0.17 for the spinal cord; TD50 = 46.0 Gy, n = 0.70, and m = 0.18 for the parotid glands;and TD50 = 72.0 Gy, n = 0.07, and m = 0.10 for the mandible. NTCP values were used

only to assist in comparison between rival plans. If two plans had similar dose

distributions but different NTCP values, then the plan with the lower NTCP was
http://radiology.rsna.org/content/223/1/57.full#ref-16http://radiology.rsna.org/content/223/1/57.full#ref-17http://radiology.rsna.org/content/223/1/57.full#ref-18http://radiology.rsna.org/content/223/1/57.full#ref-19http://radiology.rsna.org/content/223/1/57.full#ref-20http://radiology.rsna.org/content/223/1/57.full#ref-21http://radiology.rsna.org/content/223/1/57.full#ref-16http://radiology.rsna.org/content/223/1/57.full#ref-17http://radiology.rsna.org/content/223/1/57.full#ref-18http://radiology.rsna.org/content/223/1/57.full#ref-19http://radiology.rsna.org/content/223/1/57.full#ref-20http://radiology.rsna.org/content/223/1/57.full#ref-21


6/17

considered more beneficial. However, clinical use of the NTCP concept is limited until

improved NTCP models are developed and enough clinical data are obtained.


RESULTS

For both tomotherapy and step-and-shoot IMRT plans, high-dose regions (receiving

80%110% of the prescribed target dose) surrounded the spinal cord and extended to the

posterior and superior portion of the neck (Fig 2). The addition of the external avoidancestructure had a limited effect in removing these hot spots. Moreover, use of the external

avoidance feature increased dose inhomogeneity in the target (Fig 3). On the other hand,

use of virtual critical structures (Fig 4) removed the aforementioned hot spots in all cases.

On average, the maximum and mean doses to the posterior neck were reduced by 22% 8 and 49% 10, respectively.


In this page

In a new window


Figure 2. Original tomotherapy dose distributions shown in the sagittal (top left), coronal(top right), superior transverse (bottom left), and inferior transverse planes (bottom right).
http://radiology.rsna.org/content/223/1/57.full#sec-1http://radiology.rsna.org/content/223/1/57.full#sec-3http://radiology.rsna.org/content/223/1/57.full#F2http://radiology.rsna.org/content/223/1/57.full#F3http://radiology.rsna.org/content/223/1/57.full#F4http://radiology.rsna.org/content/223/1/57/F2.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F2.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F2http://radiology.rsna.org/content/223/1/57/F2.expansion.htmlhttp://radiology.rsna.org/content/223/1/57.full#sec-1http://radiology.rsna.org/content/223/1/57.full#sec-3http://radiology.rsna.org/content/223/1/57.full#F2http://radiology.rsna.org/content/223/1/57.full#F3http://radiology.rsna.org/content/223/1/57.full#F4http://radiology.rsna.org/content/223/1/57/F2.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F2.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F2


7/17


In this page

In a new window


Figure 3. Tomotherapy dose distributions shown in the transverse plane with no external

avoidance (top) and with the addition of the external avoidance structure (bottom).Orange = external avoidance.


In this page

In a new window


Figure 4. Modified tomotherapy treatment plan shows how anterior (orange) and

posterior (purple) virtual critical structures are introduced in hot spot locations. Dosedistributions are shown in the sagittal (top), superior transverse (middle), and inferior

transverse planes (bottom).

Introduction of virtual critical structures resulted in a slight increase (approximately 2%

on average) of maximum dose to nontarget tissue. Figure 5demonstrates dose-volume
http://radiology.rsna.org/content/223/1/57/F3.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F3.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F3http://radiology.rsna.org/content/223/1/57/F4.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F4.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F4http://radiology.rsna.org/content/223/1/57.full#F5http://radiology.rsna.org/content/223/1/57.full#F5http://radiology.rsna.org/content/223/1/57/F4.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F3.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F3.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F3.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F3http://radiology.rsna.org/content/223/1/57/F4.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F4.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F4http://radiology.rsna.org/content/223/1/57.full#F5


8/17

histograms for anterior and posterior neck regions in one patient. A marked dose

reduction was observed after introduction of virtual critical structures. To achieve this

dose reduction, the dose constraint to the virtual critical structure should be 30% of thetarget prescription dose. Because the IMRT plans developed with the use of virtual

critical structures demonstrated better sparing of normal structures (eg, introduction of

virtual critical structures reduced spinal cord D5% from approximately 45 Gy toapproximately 28 Gy) without compromising target coverage (variation of D95% was 2%),

they were selected for comparison with 3D CRT and 2D plans.


In this page

In a new window


Figure 5. Dose-volume histograms developed for the anterior and posterior neck regionsallow comparison of doses received with original and modified IMRT plans.

It was found that the average D95% varied from 69.4% 10.3 with 2D treatment plans to

84.7% 9.0 with 3D CRT to 97.1% 1.3 with tomotherapy to 97.3% 2.7 with step-

and-shoot IMRT. The Tableshows the average dose parameters received by criticalstructures with different treatment planning techniques. As seen from theTable, the

maximum spinal cord dose was reduced from 47.01 Gy 1.33 to approximately 39 Gy as

the complexity of the treatment technique increased (from 2D to IMRT). The maximumdose to the spinal cord was similar with both IMRT techniques (39.77 Gy 5.20 for

tomotherapy and 38.99 Gy 6.07 for step-and-shoot IMRT). A noticeable reduction in

D5% was observed in the spinal cord with IMRT. The average D5% to the spinal cordchanged from 46.17 Gy 1.06 with 2D treatment plans to 45.18 Gy 4.86 with 3D CRT

to 28.61 Gy 3.67 with tomotherapy to 27.51 Gy 3.51 with step-and-shoot IMRT.

Even greater reduction in mean dose to the spinal cord (approximately 50%) was

observed when IMRT was used. IMRT also resulted in a reduction of spinal cord NTCPfrom 2.2% (with 3D CRT and 2D techniques) to 0.049%.

View this table:
http://radiology.rsna.org/content/223/1/57/F5.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F5.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F5http://radiology.rsna.org/content/223/1/57.full#T1http://radiology.rsna.org/content/223/1/57.full#T1http://radiology.rsna.org/content/223/1/57.full#T1http://radiology.rsna.org/content/223/1/57.full#T1http://radiology.rsna.org/content/223/1/57/F5.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F5.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F5.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F5http://radiology.rsna.org/content/223/1/57.full#T1http://radiology.rsna.org/content/223/1/57.full#T1


9/17

In this window

In a new window

Comparison of Average Doses to Critical Structures with Different TreatmentTechniques

2D and 3D treatment plans demonstrated comparable maximum doses to the parotid

glands (approximately 6769 Gy). IMRT reduced the maximum dose to the parotid

glands by approximately 10 Gy. The average D5% showed more variation in doses to theparotid glands. D5% for the parotid glands was reduced from approximately 68 Gy with

2D plans to 66 Gy with 3D CRT to 52 Gy with tomotherapy to 50 Gy with step-and-

shoot IMRT. Much greater variation was observed in mean dose to the parotid glands:

approximately 62 Gy with 2D techniques, approximately 53 Gy with 3D CRT, andapproximately 30 Gy with IMRT. A great reduction was observed in D 33% to the parotid

glands: approximately 64 Gy with 2D plans, 58 Gy with 3D CRT, 35 Gy with

tomotherapy, and 34 Gy with step-and-shoot IMRT. D50% for parotid glands showed a

similar behavior. NTCPs for parotid glands were reduced from approximately 89% with2D plans to approximately 67% with 3D CRT and 7.5% with tomotherapy and 6% with

step-and-shoot IMRT.

The average maximum dose to the mandible was comparable between 2D and 3D plans(67.53 Gy 5.25 vs 68.68 Gy 5.34, respectively). Dmax in the mandible practically did

not change when IMRT was used (63.86 Gy 3.19 with tomotherapy vs 65.33 Gy 3.91

with step-and-shoot IMRT). Again, D5% for the mandible was similar between 2D plansand 3D CRT (66.15 Gy 5.18 vs 67.11 Gy 5.83, respectively). However, some

reduction in D5% was observed with IMRT (57.31 Gy 4.91 for tomotherapy and 56.43

Gy 1.80 for step-and-shoot IMRT). The average mean dose varied as follows: 42.65 Gy

9.79 with 2D plans, 46.58 Gy 9.60 with 3D CRT, 39.83 Gy 5.43 with tomotherapy,and 40.93 Gy 5.18 with step-and-shoot IMRT. The reduction in D50% in the mandible

was not conclusive in 2D versus 3D CRT plans (from 58.34 Gy 7.86 to 52.66 Gy

14.2, respectively) and was somewhat noticeable with IMRT (approximately 43 Gy).V60Gy for the mandible decreased from 48.0% with 2D plans to 30.0% with 3D CRT to

3.8%5.5% with IMRT. Although NTCP values for the mandible increased from 7.6%

with 2D to 12.0% with 3D CRT, IMRT plans reduced the NTCP value to a fraction of apercent.

As expected, the mean dose to nontarget tissue increased with the complexity of the

treatment plans: The lowest value (approximately 6.9 Gy) was observed with 2D plans.

In 3D CRT plans, the mean dose became approximately 9.3 Gy, and in IMRT plans, thedose increased to 11.4 Gy for tomotherapy and 12.8 Gy for step-and-shoot techniques.

Figures 6 and7 show comparisons of dose distributions and dose-volume histograms for

the target, spinal cord, parotid glands, and mandible in one patient by using all four

radiation delivery techniques. These figures clearly demonstrate that IMRT plans with theuse of virtual critical structures produced more conformal target coverage and better

sparing of critical structures than that of 3D CRT and 2D techniques.
http://radiology.rsna.org/content/223/1/57/T1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/T1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57.full#F6http://radiology.rsna.org/content/223/1/57.full#F7http://radiology.rsna.org/content/223/1/57.full#F7http://radiology.rsna.org/content/223/1/57/T1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/T1.expansion.htmlhttp://radiology.rsna.org/content/223/1/57.full#F6http://radiology.rsna.org/content/223/1/57.full#F7


10/17


In this page

In a new window


Figure 6. Comparison of isodose distributions with (top left) tomotherapy, step-and-

shoot IMRT (top right), 3D CRT (bottom left), and 2D (bottom right) techniques.


In this page

In a new window


Figure 7a. Comparison of dose-volume histograms for(a) target, (b) left parotid gland,

(c) mandible, and (d) spinal cord with tomotherapy (dashed and dotted line), step-and-

shoot IMRT (dashed line), 3D CRT (solid line), and 2D (dotted line) techniques. IMRT

plans were developed with the use of virtual critical structures. Target coverage was moreconformal and critical structure sparing was improved in both IMRT plans when

compared with 3D CRT and 2D plans. The parotid glands received noticeably lower dose

with step-and-shoot IMRT than with tomotherapy.
http://radiology.rsna.org/content/223/1/57/F6.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F6.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F6http://radiology.rsna.org/content/223/1/57/F7.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F7.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F7http://radiology.rsna.org/content/223/1/57/F7.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F6.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F6.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F6.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F6http://radiology.rsna.org/content/223/1/57/F7.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F7.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F7


11/17


In this page

In a new window


Figure 7b. Comparison of dose-volume histograms for(a) target, (b) left parotid gland,


shoot IMRT (dashed line), 3D CRT (solid line), and 2D (dotted line) techniques. IMRTplans were developed with the use of virtual critical structures. Target coverage was more

conformal and critical structure sparing was improved in both IMRT plans whencompared with 3D CRT and 2D plans. The parotid glands received noticeably lower dose



In this page

In a new window


Figure 7c. Comparison of dose-volume histograms for(a) target, (b) left parotid gland,





http://radiology.rsna.org/content/223/1/57/F8.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F8.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F8http://radiology.rsna.org/content/223/1/57/F9.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F9.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F9http://radiology.rsna.org/content/223/1/57/F9.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F8.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F8.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F8.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F8http://radiology.rsna.org/content/223/1/57/F9.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F9.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F9


12/17


In this page

In a new window


Figure 7d. Comparison of dose-volume histograms for(a) target, (b) left parotid gland,







DISCUSSION

Because head and neck tumors are surrounded by many critical structures, radiation

treatment for these tumors requires the use of complex treatment planning. The targetvolume is usually concave in shape and surrounds the spinal cord. Moreover, parotidglands are usually near the target. With 2D treatment plans, parotid glands receive doses

higher than the tolerance level. This leads to xerostomia that may cause discomfort for a

patient. Sparing of these organs while delivering the curative dose of radiation presentsan intractable problem for 2D treatment planning.

Although 3D CRT provided the means to create more conformal dose distributions

through the use of complex beam arrangements, development of satisfactory treatment

plans was time consuming and was not always possible. Even in plans that wereconsidered satisfactory, doses to critical structures often came close to or exceeded

tolerance levels.

IMRT may provide more conformal dose distributions and better sparing of critical

structures. Although the development of treatment plans is performed automatically bythe computer, several attempts are usually necessary because many modifications in

target prescription doses and dose-volume constraints to critical structures are required

until treatment goals are achieved. While goals may be achieved in optimized IMRTtreatment plans, the dose distributions may not be satisfactory. According to the results of
http://radiology.rsna.org/content/223/1/57/F10.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F10.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F10http://radiology.rsna.org/content/223/1/57.full#sec-2http://radiology.rsna.org/content/223/1/57.full#sec-4http://radiology.rsna.org/content/223/1/57/F10.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F10.expansion.htmlhttp://radiology.rsna.org/content/223/1/57/F10.expansion.htmlhttp://radiology.rsna.org/powerpoint/223/1/57/F10http://radiology.rsna.org/content/223/1/57.full#sec-2http://radiology.rsna.org/content/223/1/57.full#sec-4


13/17

our study, both tomotherapy and step-and-shoot IMRT plans demonstrated high-dose

regions in the posterior portion of the neck that surrounded the spinal cord. Moreover,

high-dose regions occurred in the anterior portion of the neck. This problem has alsobeen reported elsewhere (11,13).

The use of virtual critical structures markedly reduced doses in the posterior and anteriorneck regions. This technique reduced the integral dose received by normal tissues;

moreover, it removed hot spots near (within 510 mm of) the spinal cord. The presenceof hot spots in the vicinity of the spinal cord or other sensitive structures presents a

concern because of uncertainties in critical structure localization, dose calculation

algorithm, and patient immobilization. These uncertainties may cause occurrence of high-dose levels in the critical structures. Besides removing hot spots from posterior and

anterior neck regions, the introduction of virtual critical structures resulted in some

reduction (5%6%) of mean dose to the spinal cord compared with that observed inoriginal IMRT plans (without use of virtual critical structures).

Esik et al (13) used a similar technique to reduce hot spots in the posterior neck region.They positioned a triangle-shaped at-risk dummy organ in the area that needed dose

reduction. However, the method was tested for a simplified phantom-based case with thestep-and-shoot IMRT method of delivery. Our technique used more realistically shaped

virtual critical structures to reduce the hot spots in both anterior and posterior neck

regions in real clinical cases.

The use of the external avoidance structure available with the NOMOS-CORVUS systemdid not reduce the magnitude of hot spots in either anterior or posterior neck regions. It

seems that the external avoidance structure has a lower priority in the optimization of

dose distributions. If avoiding radiation by means of the external avoidance structure

compromises target coverage, then the limitations imposed by this structure are ignoredby the NOMOS-CORVUS system. This may explain the ineffectiveness of the external

avoidance technique in the elimination of hot spots in the anterior neck region.

As expected, target coverage that was characterized by D95% was poor in 2D plans(69.4%), was noticeably improved in 3D CRT plans (84.7%), and was further improved

(by an additional 15%) in IMRT plans. Even though the average D95% for 3D CRT plans

was lower than the set treatment goal, these plans were used for treatments because it wasnot technically possible to increase D95% without exceeding the tolerance of critical

structures. However, 3D CRT plans were approved for treatments because the target

areas that received less than 95% of the prescribed dose were medically considered to be

at low risk for tumor recurrence in these sets of patients. Both IMRT plans reached theprescribed goal of target coverage; however, the most pronounced effect of the IMRT

plans was the sparing of critical structures. Mean dose to the spinal cord was reduced by

50%. Although 3D CRT exhibited improved target coverage when compared with 2Dtechniques, it did not reduce doses to the spinal cord. When compared with 2D

techniques, 3D CRT reduced the mean dose to the parotid glands by only 15%, and

IMRT techniques reduced this dose by more than 50%. This result may be important inpreserving function of the parotid glands. IMRT techniques also noticeably reduced the
http://radiology.rsna.org/content/223/1/57.full#ref-11http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-11http://radiology.rsna.org/content/223/1/57.full#ref-13http://radiology.rsna.org/content/223/1/57.full#ref-13


14/17

dose to the mandible: Volume of the mandible that received 60 Gy or more was reduced

from 48.0% with 2D techniques and 30.0% with 3D CRT to approximately 5% with

IMRT. This resulted in the reduction of NTCP values to a fraction of a percent.

Reduction in the daily dose received by critical structures with IMRT techniques should

provide an extra biologic benefit in the prevention of late complications and potential fordose escalation. NTCP calculations in this study also demonstrated potential reduction in

morbidity when head and neck tumors were treated with the use of IMRT. However,NTCP values cannot be used for accurate prediction of complications until both data and

models required for these calculations become more reliable.

In conclusion, it seems that both tomotherapy and static step-and-shoot IMRT techniques

will provide visible improvement in target coverage and in the sparing of normalstructures, especially when the virtual critical structure method is used. This should result

in the reduction of acute and late complications in patients with head and neck cancers. In

our department, the virtual critical structure method is currently used for the development

of IMRT treatment plans for targets in various sites.



Acknowledgments

The authors thank Andrew Kahn, DO, for editing the manuscript.


Footnotes

Abbreviations: CRT = conformal radiation therapy, CTV = clinical target volume,

IMRT = intensity-modulated radiation therapy, NTCP = normal tissuecomplication probability, 3D = three-dimensional, 2D = two-dimensional

Author contributions: Guarantors of integrity of entire study, all authors; study

concepts and design, all authors; literature research, N.D.; clinical studies, N.D.,B.E.; data acquisition, N.D., L.B.L., B.E., S.K.; data analysis/interpretation, all

authors; statistical analysis, N.D.; manuscript preparation and definition of

intellectual content, all authors; manuscript editing, revision/review, and final

version approval, N.D., L.B.L., B.E.

Index terms: Head and neck neoplasms, therapeutic radiology, 10.1299, 20.1299;

Therapeutic radiology, comparative studies

Previous Section

References
http://radiology.rsna.org/content/223/1/57.full#sec-3http://radiology.rsna.org/content/223/1/57.full#ack-1http://radiology.rsna.org/content/223/1/57.full#sec-4http://radiology.rsna.org/content/223/1/57.full#fn-group-1http://radiology.rsna.org/content/223/1/57.full#ack-1http://radiology.rsna.org/content/223/1/57.full#ref-list-1http://radiology.rsna.org/content/223/1/57.full#fn-group-1http://radiology.rsna.org/content/223/1/57.full#sec-3http://radiology.rsna.org/content/223/1/57.full#ack-1http://radiology.rsna.org/content/223/1/57.full#sec-4http://radiology.rsna.org/content/223/1/57.full#fn-group-1http://radiology.rsna.org/content/223/1/57.full#ack-1http://radiology.rsna.org/content/223/1/57.full#ref-list-1http://radiology.rsna.org/content/223/1/57.full#fn-group-1


15/17

1.

Eisbruch A, Ship JA, Martel MK, et al. Parotid gland sparing in patients

undergoing bilateral head and neck irradiation: techniques and early results. IntJ Radiat Oncol Biol Phys 1996; 36:469-480.

2.

Lance JS, Morgan JE. Dose distribution between adjoining therapy fields.

Radiology 1962; 79:24-29.

3. Williamson TJ. A technique for matching orthogonal megavoltage fields. Int J

Radiat Oncol Biol Phys 1979; 5:111-116.

4.

Armstrong DI, Tait JJ. The matching of adjacent fields in radiotherapy.

Radiology 1973; 108:419-422.

5.

Marks JE, Davis C, Gottsman Y, et al. The effects of radiation on parotid salivary

function. Int J Radiat Oncol Biol Phys 1981; 7:1013-1019.

6.

Ling CC, Burman C, Chui CS, et al. Conformal radiation treatment of prostate

cancer using inversely-planned intensity-modulated photon beams produced with

dynamic multileaf collimation. Int J Radiat Oncol Biol Phys 1996; 35:721-730.

7. Ling CC, Burman C, Chui CS, et al. Implementation of photon IMRT with

dynamic MLC for the treatment of prostate cancer. In: Sternick ES, eds. The

theory and practice of intensity-modulated radiation therapy. Madison, Wis:Advanced Medical Publishing, 1997; 219-228.

8.

Shiao Y, Woo MD, Butler B, et al. Clinical experience: benign tumors of the CNS

and head & neck tumors. In: Sternick ES, eds. The theory and practice of

intensity modulated radiation therapy. Madison, Wis: Advanced Medical

Publishing, 1997; 195-198.

9.

Carol MP. Peacock: a system for planning and rotational delivery of intensity-

modulated fields. Int J Imaging Syst Technol 1995; 6:56-61.

10.
http://radiology.rsna.org/content/223/1/57.full#xref-ref-1-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-2-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-4-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-5-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-6-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-8-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-9-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-10-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-1-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-2-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-4-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-5-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-6-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-8-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-9-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-10-1


16/17

Haas B, Bohsung J, Groll J, et al. First IMRI patient treatments with VariansIMRT solution-planning, delivery and QA In: Proceedings of Varian IMRT

meeting. Palo Alto, Calif: Varian Medical Systems, 2000.

11.

Posner MD, Quivey JM, Akazawa PF, et al. Dose optimization for the treatment

of anaplastic thyroid carcinoma: a comparison of treatment planning techniques.

Int J Radiat Oncol Biol Phys 2000; 48:475-483.

12.

De Neve W, De Wagter C, De Jaeger K, et al. Planning and delivering high dosesto targets surrounding the spinal cord at the lower neck and upper mediastinal

levels: static beam-segmentation technique executed with multileaf collimator.

Radiother Oncol 1996; 40:271-279.

13.

Esik O, Bortfelt T, Bendl R, Nemeth G, Schlegel W. Inverse radiotherapyplanning for a concave-convex PTV in cervical and upper mediastinal regions.

Strahlenther Onkol 1997; 173:193-200.

14.

Wu Q, Manning M, Schmidt-Ullrich R, Mohan R. The potential for sparing of

parotids and escalation of biologically effective dose with intensity-modulated

radiation treatments of head and neck cancers: a treatment design study. Int JRadiat Oncol Biol Phys 2000; 46:195-205.

15.

International Commission on Radiation Units and Measurements Report 50:

Prescribing, recording, and reporting photon beam therapy. Bethesda, Md:

International Commission on Radiation Units and Measurements1993.

16.

Curran B.. Conformal radiation therapy using a multileaf intensity modulatingcollimator. In: Sternick ES, eds. The theory and practice of intensity-modulatedradiation therapy. Madison, Wis: Advanced Medical Publishing, 1997; 75-77.

17.

Leybovich LB, Dogan N, Sethi A, Krasin MJ, Emami B. A method of correcting

dose distributions produced by IMRT planning systems In: Proceedings of the
http://radiology.rsna.org/content/223/1/57.full#xref-ref-11-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-12-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-13-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-14-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-15-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-16-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-17-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-11-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-12-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-13-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-14-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-15-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-16-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-17-1


17/17

World Congress on Medical Physics and Biomedical Engineering. College Park,Md: American Association of Physicists in Medicine, 2000.

18.

Lyman JT. Complication probability as assessed from dose volume histograms.Radiat Res 1985; 104:13-19.

19.

Kutcher GJ, Burman C, Brewster L, et al. Histogram reduction method for

calculating complication probabilities for three dimensional treatment planning

evaluations. Int J Radiat Oncol Biol Phys 1991; 21:137-146.

20.

Burman C, Kutcher GJ, Emami B, Goitein M. Fitting normal tissue tolerancedata to an analytic function. Int J Radiat Oncol Biol Phys 1991; 21:123-135.

21.

Emami B, Lyman J, Brown A, et al. Tolerance of normal tissue to therapeuticradiation. Int J Radiat Oncol Biol Phys 1991; 21:109-122.
http://radiology.rsna.org/content/223/1/57.full#xref-ref-18-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-19-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-20-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-21-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-18-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-19-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-20-1http://radiology.rsna.org/content/223/1/57.full#xref-ref-21-1

improvement of treatment plans developed with intensity

Documents