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Louise Francis

April Case Study

April 24, 2012

Irradiation Using Mixed Photon-Electron Beam Field Arrangements

Abstract:

Introduction: The purpose of this study is to highlight the advantages of using a mixed photon-

electron beam technique to deliver a homogeneous dose distribution to a tumor volume while

sparing dose to organs at risk (OR) in close proximity to the irradiated volume. The separate

contribution of the photon and electron beams to the overall dose distribution will vary

depending on the radiation oncologist’s objective.

Case Description: The treatment technique using a photon-electron beam mix has many clinical

applications including: re-irradiation of a previously treated area, limiting dose to normal tissue

and dose sparing to underlying critical structures. The presented case study highlights the

advantages of utilizing a photon-electron beam combination for treatment to multiple disease

sites. Patient one is a 68 year-old Caucasian male who first noticed a skin lesion on his right neck

and upon further evaluation was discovered to have squamous cell carcinoma of the right parotid

Patient two is a 76 year-old Caucasian male with a six to eight month history of a slightly

discolored quarter-sized skin lesion at the level of the midsternum in an area previously positive

for cancer. The current lesion was pathologically identified as desmoplastic melanoma. Patient

three is a 46 year-old Pakistani woman diagnosed with inflammatory breast cancer requiring

irradiation to the left chest wall following a mastectomy.

Conclusion: All plans were visually evaluated by radiation oncologists to determine effective

coverage of tumor volumes. Since gross tumor volumes (GTV), clinical tumor volumes (CTV)

and planning tumor volumes (PTV) were not contoured by all the radiation oncologists,

irradiated volume coverage could not be solely evaluated using a dose volume histogram (DVH).

The DVH did serve as a valuable tool when comparing multiple plans for each case study. It

allowed for objective evaluation of doses to critical structures for a given plan.

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Key Words: Chest wall irradiation, Mixed photon-electron beams, Parotid irradiation,

Desmoplastic melanoma.

Introduction

The use of radiation to treat superficial lesions has been used successfully for many years. This

has been accomplished by using low energy photon beams, varying energies of electron beams

and appropriately chosen orthovoltage energies. However, if a treatment scenario arises to which

neither of the previously mentioned modalities offered a viable solution; an alternate treatment

option would need to be considered. A possible solution could involve radiation treatments that

utilized a mixed photon-electron beam approach. The selection of the most appropriate energies

would depend upon the depth of the target volume and the depth of the structure that the

radiation oncologist wished to avoid. Often a compromise may need to be reached regarding the

two objectives.

The dose fractionation for the presented case studies does not differ from the traditional radiation

daily dosages of 1.8-2.0 Gy/ fraction for curative treatment. What will vary is the contribution

from the respective photon and electron beams necessary to accomplish the overall treatment

goal. The photon portion of the beam will assist by increasing the depth of the field coverage and

decreasing the superficial dose especially to the skin. The electron contribution of the beam will

decrease the amount of exit dose to underlying structures and increase dose to superficial areas

and the skin surface. Caution should be used when choosing electron energies. The higher the

beam energy, the higher the exit dose to underlying structures and the higher the skin dose. This

may not be a desirable goal since an unnecessarily high dose to the skin may cause skin

breakdown and necessitate a break in treatment.

Ideally, in order to evaluate target coverage, a volume should be clearly delineated by the

radiation oncologist. This would aid in the evaluation of both the visual isodose line distribution

and DVH. Three dimensional (3-D) planning allows for visual observance of isodose line

changes as adjustment of the photon electron contributions are made. This is a valuable tool

because the radiation oncologist can request changes be made that may not easily translate into a

written format. For instance, if the skin dose is above a desired amount, but the area of interest at

depth is not covered by the prescription dose, it may be difficult for the radiation oncologist to

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convey how much to decrease the skin dose and increase the dose at depth. This adjustment

could not only necessitate an adjustment in the contributions of the electrons and photons to the

beam mix, but possibly changing the electron energy entirely.

Methods and Materials

Patient Selection

Patient selections for this case study varied by treatment site, but were unified by the necessity to

avoid or limit dose to an underlying or adjacent structures. Patient one required irradiation to the

right parotid bed. The radiation oncologist requested that treatment be delivered with a unilateral

treatment technique. His chief concerns were dose to the oral cavity and the contralateral parotid.

He also noted that the right parotid gland had been removed and the remaining tumor bed was

very superficial. Patient two presented with a skin lesion at the midsternal level. The patient had

a previous skin lesion removed from this area with a different pathology and was treated with

surgical resection only. Once appropriate margins were drawn, the radiation oncologist

expressed concern regarding the dose to the underlying heart and lung from irradiation of the

skin lesion. Patient three had inflammatory breast carcinoma of the left breast with positive

internal mammary nodes. Her diagnosis required dosage to the skin and internal mammary nodes

while limiting the dose to the heart and left lung. The dose to the heart is always a concern for

patients that have left-sided breast disease because of possible cardiac complications in the

future.1

Patient Setup

Following informed consent, each patient was appropriately immobilized for their respective

disease site and scanned using a Philips Big Bore multi-slice Computed Tomography (CT) unit.

All scans were done using a scan slice thickness of 3mm. Patient one was simulated supine on a

full egg crate and a custom Aquaplast® mask was made for immobilization of his head and neck

area. Patient two was also simulated supine with a full egg crate. His arms were positioned above

his head utilizing a Q-Fix Arm Shuttle™ device and his head rested on a cushion supported by the

device. Patient three was positioned on the AccuFix™ Quest carbon fiber breast board with her

left arm raised and head turned to the right. All patients were given knee cushions for additional

support.

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Target Delineation

The CT data sets for all patients were sent to dosimetry and imported into version 9.0 of ADAC

Pinnacle treatment planning system (TPS). The isocenters for all patients were originally set in

simulation by the radiation therapist per the radiation oncologist’s written simulation instructions

and were imported with the planning data set. Once imported, the medical dosimetrist contoured

cervical spinal cord, mandible, and brainstem for patient one. For patient two lungs, esophagus,

spinal cord, and heart were contoured. For patient three lungs, heart, cervical spinal cord, and

internal mammary nodes were contoured. Wires placed at the time of simulation to outline scars

and other areas of interest were also contoured by the medical dosimetrist and their density was

overridden to that of air for treatment planning. The wires would later assist in field delineation.

For patient one, the planning treatment volume (PTV) drawn by the radiation oncologist

involved an expansion of the wires placed on the skin during simulation and the physician

contoured the parotid bed. The expansion was adjusted by the radiation oncologist to form a final

PTV. For patient two, the radiation oncologist contoured the tumor bed and expanded it 1.5 cm

in the inferior, superior and lateral directions. There was no expansion in the anterior direction

and 0.5 cm expansion in the posterior direction. For patient three, there was not a PTV

contoured. The patient had undergone a mastectomy, therefore the chest wall and the internal

mammary nodes were the target volume.

Treatment Planning

Prior to the start of treatment planning, all case study objectives were discussed by the radiation

oncologist and medical dosimetrist. It was determined that all of the patients would benefit from

treatment plans involving a photon-electron mix. It was further decided that the contribution of

photon and electron energies to each field would depend on the specific treatment objectives.

The medical dosimetrist elected to begin with an equal contribution of photon and electron

energies and adjusted weighting to produce the most desirable treatment plan.

Patient one required treatment to the right parotid tumor bed. The radiation oncologist contoured

PTV was subtracted 5mm from the skin surface to form PTV-5mm. This was done by the

medical dosimetrist because the volume was within the buildup region of available energies and

to honor a verbal request by the physician to avoid a high dose to the skin. The medical

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dosimetrist also contoured a structure called “ear plug” in the adjacent ear canal that would be

later constructed from Aquaplast® and used during daily treatment. The structure was assigned a

tissue density and would be placed in the ear canal to enhance the dose distribution around the

PTV-5mm and aid in decreasing auditory damage such as hearing loss. Initially enface photon

and electron fields were constructed to treat this area, but after review of this field arrangement

compared to an anterior-posterior/ posterior-anterior (AP/PA) fields paired with an enface

electron field, the latter arrangement was chosen. The AP/PA field arrangement, coupled with

the en face electron field, spared more of the oral cavity and contralateral parotid which was the

objective expressed by the radiation oncologist. The AP/PA field block margins were

constructed by the physician to give 1cm margin medially and a 0.5 cm margin laterally to the

PTV not subtracted 5mm from the skin. His rational for using that volume was because it already

included adequate margin and extended beyond the skin surface. The daily tumor dose was 2 Gy

per fraction for a total dose of 60 Gy. The photon fields utilized 6MV beams and were optimized

using a step-and-shoot technique. The enface electron field energy utilized a 12MeV beam

prescribed to d90 that aided in delivering additional dose to the superficial PTV-5mm region.

Initially the photon-electron contribution was more evenly distributed, but observing the isodose

distribution while adjusting the contributions yielded a final distribution of 10% photon and 90%

electron.

Patient two required treatment to a midsternal melanoma skin lesion. The biopsied area had been

previously positive for a spindle cell skin lesion that was surgically removed, but not irradiated.

The radiation oncologist created a PTV by expanding the tumor bed 1.5cm inferiorly, superiorly,

laterally and 0.3 cm posteriorly. There was no expansion in the anterior direction. Upon

examination of the PTV, it became evident that the expanded tumor volume had superficial and

deep components that would not adequately be covered by using photons or electrons alone. The

PTV was also directly anterior the heart and lungs and limiting dose to these volumes was of

concern to the radiation oncologist. To compensate for the expanded volume being at the skin

surface, 3 mm of bolus was added to the plan. Bolus is often used to shift the dmax of the photon

and electron beams closer to the skin surface.2 The physician placed a 1.5 cm margin around the

expanded volume to represent the block margin of the photon and electron fields. Based on the

depth of the PTV-5mm, 12MeV electron energy was chosen to pair with a 6MV photon beam.

The couch angle was rotated to 90° so that the angled beams would be parallel to the sloping

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chest wall surface. The initial weighting of 50% photon and 50% electron contributions covered

beyond the depth of PTV-5mm, adjusting of the electron contribution to 70% and the photon

contribution to 30%, with 3 mm of bolus over both fields for treatment, provided the ideal

coverage and was approved by the radiation oncologist. The treatment fractionation was 2.5

Gy/fraction for 20 fractions to a total dose of 50 Gy.

Patient three required irradiation to the left chest wall for inflammatory breast cancer. When

patients require radiation therapy to the left breast or chest wall, concern should be taken to limit

the dose to the heart. Radiation oncologists must be cognizant of the potential for adverse cardiac

effects of incidental radiation particularly in the setting of left-sided breast cancers. The radiation

oncologist requested that the medical dosimetrist construct tangent fields that would encompass

the patient’s internal mammary nodal chain and a supraclavicular field cover supraclavicular

nodes. Upon review of the tangent fields, the radiation oncologist did not approve of the amount

of lung and heart present that would allow for coverage of the internal mammary nodes (IMN).

The medical dosimetrist suggested treating the chest wall with a combination of steep tangent

fields and an abutting photon-electron mixed field that would cover the internal mammary nodes.

The photon-electron mix would be customized to deposit the desired tumor dose to the chest wall

and IMN, while limiting dose to the underlying heart and lungs. This technique would decrease

possible long-term complications of lung and cardiac issues.

The supraclavicular and tangent fields were optimized using 6MV photons and a step-and-shoot

technique. The internal mammary dose distribution was achieved by using an 80% electron and

20% photon beam mix. Often single electron beam energy can offer adequate coverage of the IM

nodes, but given the thickness of the patient’s chest wall, dual electron energies of 12 MeV and

16 MeV were used in conjunction with a 6MV photon beam. It should also be mentioned that the

ballooning effect of the lower electron isodose curves into the adjacent medial tangent photon

field usually requires additional step-and-shoot segments within the tangent fields to create a

homogenous distribution. A composite plan was reviewed and approved by the radiation

oncologist reflecting a treatment fractionation of 2.0 Gy/fraction for 25 fractions to a total dose

of 50 Gy. A clinically setup boost delivered an additional 10 Gy to the chest wall scar for a total

dose of 60 Gy to that area. Because of the patient’s inflammatory breast cancer diagnosis, chest

wall photon fields (including IMN photon field) required the use of daily 5mm bolus. This did

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cause a brisk skin reaction, but the patient was able to complete the prescribed course without a

break in treatment. Bolus usage was eventually adjusted to three times per week near the

completion of treatment to avoid further skin irritation.

Plan Analysis and Evaluation

Photon-electron mixed beam fields offer a viable treatment option for tumor volumes that have

both superficial and deep margins. The presented case studies exhibited how combinations of

photon and electron energies have a variety of uses for different anatomical regions. The

presented case studies also exhibited how varying contributions of photons and electrons to

tumor volumes can achieved the desired outcome when observing the changing isodose lines

encompassing the area of interest and evaluation of DVH data for structures that dose should be

limited to. An ideal treatment plan may not always ensure that 100% of a specific volume is

covered, but rather a satisfactory compromise of all plan objectives has been met.

For patient one, usage of a photon-electron mixed beam accomplished the radiation oncologist’s

goal of limiting dose to the oral cavity and contralateral parotid. Visual inspection of the dose

distribution proved to be of more value than the DVH analysis for tumor coverage and sparing of

the oral cavity. However, the DVH presented valuable information regarding the amount of dose

received by the contralateral parotid. When compared to an enface photon and electron beam

arrangement, it is clear that given the small size and superficial location of PTV-5mm, the

AP/PA field arrangement offered the best outcome (Figure 1). If the PTV-5mm volume were

larger, other unilateral treatment options may have been considered including a wedged pair or

the previously mention en face photon-electron mix.

Optimization of this case proved to be challenging because of the vague instructions of the

physician. Although a PTV was contoured, the radiation oncologist mentioned that there were

areas of interest at the deep margins of the volume that were not contoured. This made it difficult

for the medical dosimetrist to choose adequate energies to cover a volume that was not drawn,

also not having clearly defined doses for the skin surface proved frustrating. Clear

communication between the radiation oncologist and medical dosimetrist will always make

planning tasks easier.

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For patient two the challenge of treating a volume in such close proximity to the lungs and heart,

exhibited the valuable nature of photon-electron mixed beam treatments. The volume was clearly

defined by the radiation oncologist and his instructions regarding volume were easy to decipher.

When dose distribution of a photon-only field is compared to the mixed beam technique, it is

evident how much dose is spared to the heart and lung (Figure 2). The addition of bolus ensured

adequate dose to the skin surface. Given that bolus is available in varying thicknesses, it is

important to choose a thickness that does not exceed the desired dose to the skin express by the

radiation oncologist. This could produce an undesirable outcome such as skin breakdown which

may require a break in treatment. Given that this is the patient’s second skin cancer in the same

region, the likelihood of a future recurrence cannot be dismissed. Limiting of dose to the heart

and lung may prove of greater importance if future disease necessitates treatment to this area in

the future.

Patient three also exhibited the ideal situation for utilizing a photon-electron beam mix. Given

the inflammatory nature of her disease and positive IMN, it was the primary objective of the

medical dosimetrist to create a beam arrangement that would limit dose to the heart and lungs,

yet provide a homogenous dose distribution. This case was made more complicated by the less

than desirable cosmetic outcome of the patient’s surgery. The patient’s chest wall varied in

thickness making it a challenge to select an energy that would adequately cover the IMN. The

medical dosimetrist eventually decided to use a mix of 12MeV (prescribed to d90) and 16MeV

(prescribe to d95) electron energies coupled with a 6MV photon beam to obtain the desired

coverage. Using the step-and-shoot technique proved beneficial to creating a homogeneous dose

distribution and adequate coverage of internal mammary nodes (Figure 3 and 4). The technique

allowed for decreasing of the hot spot that was produced in the medial tangent field as a result of

the ballooning of the lower electron isodose lines of the adjacent IMN field. As the electron

beam penetrates a medium, the beam expands rapidly below the skin surface due to scattering.3

The selection of energies and beam contributions for this patient reflects the complexity of the

treatment plan (Figure 5).

As mentioned previously, the selection of thickness for bolus plays an important role in

treatment planning. Patient three was treated with 5mm bolus daily for the majority of her

treatments. In hindsight, a selection of 3mm bolus may have achieved the desired effect without

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having to adjust the amount of times per week the bolus was applied. The initial prescription

stated that bolus should be used daily for the photon field only (no bolus for electron fields), but

this was changed to three times per week for the last two weeks of treatment.

Results and Discussion:

Photon-electron mixed beam treatments are an excellent option for covering treatment volumes

having both a superficial and deep component. It is also beneficial when trying to limit dose to

underlying and adjacent structures. The presented case studies offered varying treatment

planning options and highlighted issues that could enhance and complicate planning. It is of

critical importance that both the visual isodose distribution and DVH be evaluated carefully.

Reviewing one and not the other may not allow for the ideal treatment plan. Clear

communication by the radiation oncologist to the medical dosimetrist is imperative to lessening

wasted time and subpar plan production.

Although photon-electron mixed beams proved ideal for the presented cases, it should always be

evaluated carefully when used. If a volumes’ deep margin extend beyond the limits of this

technique, but superficial aspect are present, perhaps a lone photon field with thicker bolus may

be the ideal option. Also wedged pairs with applicable bolus or unequally weighted opposed

fields with bolus may produce a better overall solution.

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Parotid doses from enface electron-photon fields vs AP/PA photons with enface electron field

Figure1: Comparison of dose distributions for Patient 1 and DVH of chosen beam arrangement.

All images data courtesy of VCU Health System.

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Dose distribution of photon-electron mix (left) versus photon only distribution (right).

Figure2: Comparison of dose distributions for Patient 2 and DVH of chosen beam arrangement.

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Comparison of steep tangent fields with abutting electron fields versus extended tangent fields

Figure 3: Composite of 6MV tangent and supraclavicular photon fields with abutting photon-

electron mixed internal mammary field.

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Figure 4: DVH for Patient 3 that reflects coverage of internal mammary nodes.

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Figure 5: Prescription doses for composite chest wall with internal mammary node irradiation

References

1. Haffty B, Buchholz T, Perez C. Early stage breast cancer. In: Perez C, Brady L, Halperin

E, eds. Principle and Practices of Radiation Oncology. 5th ed. Philadelphia, PA:

Lippincott, Williams & Wilkins; 2008:1272.

2. Stanton R, Stinson D, Shahabi S. Treatment planning. In: An Introduction to Radiation

Oncology Physics. Madison, WI: Medical Physics Publishing; 1992:244.

3. Khan F. Electron beam therapy. In: The Physics of Radiation Therapy. 3rd ed.

Philadelphia, PA: Lippincott, Williams & Wilkins; 2003:309.