the swiss imrt dosimetry intercomparison using a thorax phantom · 2016. 1. 31. · the swiss imrt...

The Swiss IMRT dosimetry intercomparison using a thorax phantom

H. Schiefer1, A. Fogliata2, G. Nicolini2, L. Cozzi2, W.W. Seelentag1, E. Born3, F. Hasenbalg3, J. Roth4, B. Schnekenburger4, K. Münch-Berndl5, V. Vallet6, M. Pachoud6, B. Reiner7,

G. Dipasquale8, B. Krusche9, and M.K. Fix3

1 Klinik für Radio-Onkologie, Kantonsspital St.Gallen, St.Gallen, Switzerland, 2 Oncology Institute of Southern Switzerland, Medical Physics Unit, Bellinzona, Switzerland, 3Division of Medical Radiation Physics, Inselspital, Bern University Hospital, and University of Bern, Switzerland, 4 Institut für Radio-Onkologie, Universitätsspital Basel, Switzerland, 5: Radio-Onkologie, Lindenhofspital, Bern, Switzerland, 6 University Institute for Applied Radiophysics, Grand-Pré 1, CH-1007 Lausanne, Switzerland, 7 Radiation Oncology, University Hospital, Zurich, Switzerland, 8Service de Radio-Oncologie, Hôpitaux Universitaires de Genève, Geneva, Switzerland, 9Institut für Physik, Universität Basel, CH-4056, Basel, Switzerland

Corresponding Author:

Hans Schiefer

Klinik für Radio-Onkologie

Kantonsspital

CH-9007 St.Gallen

Switzerland

Tel: +41 71 494 2239

FAX: +41 71 494 2893

Email: [email protected]


Abstract

Purpose: In 2008, a national Intensity Modulated Radiation Therapy (IMRT) dosimetry

intercomparison was carried out for all 23 radiation oncology institutions in Switzerland. It

was the aim to check the treatment chain, focused on the planning, dose calculation and

irradiation process.

Methods: A thorax phantom with inhomogeneities was used, in which thermoluminescence

dosimeter (TLD), and ionization chamber measurements were performed. Additionally,

absolute dosimetry of the applied beams has been checked. Altogether, 30 plan-measurement

combinations have been used in the comparison study. The results have been grouped

according to dose calculation algorithms, classified as “type a” or “type b”, as proposed by

Knöös et al.1.

Results: Absolute dosimetry check under standard conditions: The mean ratio between the

dose derived from the single field measurement and the stated dose, calculated with the

treatment planning system, was 1.007 ± 0.010 for the ionisation chamber and 1.002 ± 0.014

(mean ± standard deviation) for the TLD measurements.

IMRT plan check: In the lung tissue of the planning target volume (PTV) a significantly better

agreement between measurements (TLD, ionisation chamber) and calculations is shown for

“type b” algorithms than for “type a” (p < 0.001). In regions outside the lungs the absolute

differences between TLD measured and stated dose values, relative to the prescribed dose,

(Dm-Ds)/Dprescribed , are 1.9 ± 0.4% and 1.4 ± 0.3%, respectively. These data show the same

degree of accuracy between the two algorithm types if low density medium is not present.

Conclusion: Our results demonstrate that the performed intercomparison is feasible and

confirm the calculation accuracies of “type a” and “type b” algorithms in a water equivalent

and low-density environment. It is now planned to offer the intercomparison on a regular basis

to all Swiss institutions using IMRT techniques.

Key words: dose comparison, radiotherapy, Intensity-Modulated, heterogeneity,

thermoluminescence dosimetry

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I. INTRODUCTION

A few years ago, IMRT (Intensity Modulated Radiation Therapy) was described as “a revolution in

the treatment of cancer”.2 Now, IMRT is practiced in most radiotherapy centers and is accepted as an

improvement on existing treatment techniques for several disease sites3-5.

Due to its complexity, IMRT is associated with a variety of uncertainties.6, 7 Uncertainties leading to

inaccurate dose delivery have implications on tumor control, treatment morbidity and toxicity. 8, 9 Thus,

comprehensive quality assurance (QA) procedures are essential to check calculation and delivery. 10 For

patient related QA, the validation of at least the data transfer and the intended dose to be delivered is

important.11

In addition to the patient and machine related QA procedures performed by the institutions

themselves, an audit organized by an independent external body is a fundamental step in any dosimetry

QA program.6 The audits provided by the European Society for Therapeutic Radiology and Oncology

(ESTRO), the Radiological Physics Center (RPC) and other institutions comprise not only basic tests of

the machine calibration, but they also check a substantial amount of the total therapy chain.12-14 Ebert et al.

presented a comprehensive view on national and international studies and intercomparisons.8

An intercomparison based on a standard patient represented by an anthropomorphic phantom enables

an end to end test, i.e. all technical steps in the treatment chain can be checked. 8 In the hierarchical

structure defined by Kron et al.15 such kind of test is classified as Level III, while an intercomparison

measuring the linear accelerator output under reference conditions in a regular phantom is referred to as

Level I.

An issue often discussed is the ability of the dose calculation algorithms to properly take into account

inhomogeneities, especially in low-density regions. Additionally, it is well known that dose calculation

errors can be enhanced due to very small fields (< 4 cm edge length) which are typical for the IMRT

technique.1, 16-21

This paper deals with the first such national intercomparison study held in Switzerland. All Swiss

radiotherapy institutions participated in the study. Dose distributions in an anthropomorphous phantom

with inhomogeneities were investigated. Calculations using different treatment planning systems were

compared with measurements using different detectors.

Since an intercomparison was considered of general interest, the SSRMP (Swiss Society of

Radiobiology and Medical Physics) decided to organize a national IMRT dosimetry intercomparison and

offer it to all Swiss radio-oncology centers. The aim was to check multiple components of IMRT

treatments using a thorax phantom provided with inhomogeneities. The intercomparison was organized

by the physics team of the Cantonal Hospital St.Gallen which is responsible for all national dosimetric

intercomparisons that have been performed in Switzerland since 2001.

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II. MATERIALS AND METHODS

All 23 radiation oncology institutions in Switzerland participated in the Swiss IMRT intercomparison

between July 2008 and February 2009. A thorax phantom together with TLDs and ionization chamber

inserts were sent to each participating center with a pre-defined time schedule. Two phantoms were used

in a two week cycle. Detailed information on how to perform plans, calculations and measurements were

also included. Each center had the possibility to perform ionization chamber measurements using their

own equipment. It is legally required in Switzerland that all ionisation chamber calibrations have to be

traceable to the National Primary Dosimetry Laboratory (METAS; Bundesamt für Metrologie und

Akkreditierung). The 95% confidence interval of the METAS calibration for photon beams is 1.4% (at

the date of the intercomparison). All errors are stated at one standard deviation of the mean of the

quantity of interest.

II.A. Detectors

TLD-100 discs (4.5 mm Ø x 0.9 mm; Harshaw Inc.) were used together with a TLD reader model

“5500” (Harshaw Inc.). The TLDs were tempered in a PTW-TLDO oven (PTW Freiburg). Reference

irradiations were performed using a “Theratron 60” cobalt unit (AECL of Canada). An evaluation of

consecutive reference irradiations has shown that the measurement reproducibility of one single TLD is

better than 0.5% (1 SD). The measurement uncertainty for a detector consisting of five discs has been

determined in the course of a preliminary study, which is described below.

Ionization chamber measurements were conducted by the individual centers using their own

equipment.

II.B. Phantom

For the IMRT intercomparison, the thorax phantom 002LFC (CIRS Inc.) has been selected. The

phantom is shown in figure 1. It consists basically of a 15 cm thick slice (slice 01, on the right side of the

picture) and 15 standard slices (each 1cm thick, on the left side). They are composed of water equivalent

material with inhomogeneities mimicking lungs (density 0.21 g/cm3) and a bony vertebra (density 1.6

g/cm3), with invariant geometry and density distribution in the longitudinal direction. Slice 01 can

accommodate different ionization chambers (using appropriate inserts allowing the usage of different ion

chamber types) in pre-defined positions.

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Figure 1. Schematic view of the CIRS thorax phantom. Right: Ionization chamber measurements

are performed in the 15 cm slice. Left: Standard slice. Sketch provided with courtesy by CIRS Inc.

A 1 cm slice has been modified with 54 holes to accommodate two TLDs each. The TLD positions

are grouped as shown in figure 2, where also different anatomical structures are overlaid: Planning Target

Volume (PTV), located both in soft tissue (11 TLD positions), and in lung tissue (11 TLD positions), lung

tissue (16 TLD positions), soft tissue (11 TLD positions, 5 of them in the ‘heart’ structure), bone (5 TLD

positions). Four TLD positions coincide with ionization chamber measurement points (Pos1: PTV in soft

tissue, Pos2: PTV in lung tissue, Pos3: bone, and Pos4: lung outside the PTV).

Figure 2. TLD and ionisation chamber (Pos1 to Pos4) measurement points.

The aim was to check the applied dose on one single transversal plane with TLDs and ionisation

chambers. This allows cross checking the measurements. In order to avoid perturbations of the dose

distribution in the measurement plane induced by the measurement equipment, measurements have been

performed in a plane outside the beam divergence, 5 cm off axis to the isocenter. In this way both the

influence of the small air gap between slices and the “shadowing” of TLD detectors are eliminated.

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II.C. Preliminary study

Dose calculations of a single 6 MV photon field laterally crossing both lungs (gantry angle: 90°; field

length: 20 cm, field width: 10 cm, dose to the isocentre position: 1.8 Gy) have been carried out by the

Division of Medical Radiation Physics, Inselspital and University of Bern. The “Voxel Monte Carlo”

(VMC++) code within the Swiss Monte Carlo Plan environment has been applied.22 The code was

explicitly commissioned for the Linac used in Berne.

Six institutions participated in the preliminary study. The tissue phantom ratios (TPR20, 10) of the

used linacs showed a very similar beam quality within 0.8%. Thus the commissioning of each individual

linac was not necessary for the purpose of this study. 24 TLD positions in a transversal plane have been

aligned on two paths, longitudinal and transverse to the field axis. Each position contained five TLDs.

II.D. Intercomparison

II.D.I. Linac calibration check

A correct machine calibration is the basis of every successful irradiation treatment. Kron et al. 15

highlighted the usefulness of undertaking simultaneous Levels I and III measurements. Following Kron et

al., a single field irradiation with 2 Gy in a water equivalent portion of the phantom was used as machine

calibration check (a special insert with 8 TLD at 10 cm depth was included for this purpose). The field

setup was: field size 10 x 10 cm2, source to surface distance SSD = 90 cm, gantry angle 0°, i.e. reference

geometry as recommended by the Swiss protocol for high energy photon beams.23 A measurement under

the same conditions was performed with an ionization chamber (“Pos 1” in figure 2). The institutions

were asked to calculate the dose with the same algorithm as used for the plan. Additionally, the

institutions stated the expected dose in water in 10 cm depth, for the same field setup and the identical

number of monitor units (MU).

From this field irradiation, factors (converting the dose in the phantom to the dose expected in water

when applying the same number of monitor units) for both TLD and ion chamber measurements are

determined. They are used to derive the expected dose in water under the same conditions (results see

table 2).

II.D.II. IMRT treatment

The computer tomography (CT) scans of the phantom were carried out by the individual institutions.

Since the phantom was already filled with TLDs when sent to the testing institutions, the applied CT dose

also had to be measured.

Preparatory measurements confirmed that the dose within the phantom can be approximated with

sufficient accuracy by the dose of the surface. Similar to the RPC (Radiological Physics Center)

procedure11, surface dose was determined by strips of three TLDs each.

Furthermore one additional strip was provided in order to measure the background contribution

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during transport and storage. Doses accumulated by the TLD in those strips were subtracted from the total

dose read from TLDs prior to analysis.

Two Perspex slices with cutouts for PTV and “heart” at the phantom longitudinal edges permitted an

easy delineation of the PTV and the heart structures for the treatment planning (otherwise not visible).

The PTV and the heart are 8 cm long, while the lung and spinal cord structures cover the entire phantom

length.

Centers were asked to prepare an IMRT plan fulfilling the following constraints:

• PTV: dose prescription (100%) to median PTV Dose = 2.00 Gy.

• Spinal cord: Maximum dose Dmax < 75% of the prescribed dose.

• Both lungs outside PTV: Less than 20% of each lung should receive more than 35% of the prescribed dose (D20% < 35%)

• Heart: Dmax < 55% of the prescribed dose.

The institutions were asked to irradiate the phantom hosting TLDs and ionization chamber according to

this plan. The dose calculated by the treatment planning system (called Ds, stated dose) at each TLD and

ion chamber measuring point were provided to the coordinating center in St.Gallen, along with the results

of the ionization chamber measurements.

IMRT was the suggested technique, but other techniques have also been accepted. Some institutions

carried out the calculation with two different calculation algorithms (five centers) or participated twice in

the intercomparison by testing different machines (one center) or irradiation techniques (one center using

IMRT or RapidArc). All plans used 6 MV beams.

Altogether 30 plan-measurement combinations were evaluated, and each is considered as independent

in the present study. Twelve evaluations related to static IMRT (using static field segments), eleven to

dynamic IMRT (using dynamic sliding window method or Rapid Arc), one to Tomotherapy and six to

3D-CRT irradiation techniques. A total of 24 machines have been tested.

For data analysis the percentage dose difference between stated and measured dose was reported:

(Dm-Ds)/Dprescribed, where Dm is the measured dose, Ds the stated dose and Dprescribed is the dose prescription

of 2 Gy (median PTV dose). The “absolute difference” is defined by the expression (Dm-Ds)/Dprescribed .

For the evaluation, the algorithms used by the institutions have been classified as “type a” and “type

b” algorithms, as defined by Knöös et al.1 and Fogliata et al.18: “Type a” algorithms are primarily based

on equivalent path length for inhomogeneity correction. “Type b” algorithms account for electron

transport in an approximate way as well as the secondary photon transport in the medium, thus

accounting for density changes along all dimensions. The treatment planning systems (TPS), calculation

algorithms and irradiation techniques used in the intercomparison are summarized in table 1. No

institution performed the dose calculation with a MC algorithm.

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Table 1. Applied TPS, calculation algorithms and irradiation techniques.

TPS calculation algorithm and version3D-CRT

dyn. IMRT

static IMRT all

type

a (

12) Eclipse PBC + mod.Batho 7.5,7.6,8.0,8.1,8.2,8.5 7 2 9

KonRad PBC 2.2 1 1

Precise Plan Clarkson 2.12 1 1

Eclipse PBC + eqTAR 8.1 1 1

type

b (

18) Pinnacle CCC 8.0 2 4 6

Eclipse AAA 7.5 / 8.1 / 8.2 / 8.5 4 1 5

Oncentra MasterPlan CCC 3.0 3 1 4

XiO multigrid conv./superp. 4.3 / 4.34 2 2

Hi-Art (Tomo) conv./superp. 3.1 1 1

all 6 12 12 30

III. RESULTS

III.A. Preliminary study

Apart from measurements close to the field edges, the absolute dose difference between MC dose

calculations and TLD measurements is 0.9±0.6% in the normal tissue and 1.1±1.2% in lung, respectively

(1 SD). These values are in the same range as the measurement uncertainty of 0.6% to 0.9% observed in

previous dosimetry intercomparisons in a water tank.24

III.B. IMRT dosimetry intercomparison

III.B.I. Linac calibration check

Table 2 shows the ratio between the dose derived from the phantom measurement, Dm, and the stated

dose under standard conditions, Ds. It is a measure of the linac calibration.

Table 2. Results of the absolute dosimetry measurements (28 evaluations)

Dm/Ds ion. chamber Dm/Ds TLD

“type a” 1.005 ± 0.006 1.000 ± 0.014

“type b” 1.008 ± 0.012 1.004 ± 0.014

All 1.007 ± 0.010 1.002 ± 0.014

The maximum deviation of the TLD measured dose to the stated dose is 3.2%. 24 of 28 TLD

measurements (85%) agree better than 2% with the stated doses, which is an excellent result. No relevant

differences between ionization chamber and TLD measurements have been found (p = 0.19) although a

0.5% higher measured value (on average) for ion chamber is present with respect to TLD. The TLD

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measurements show a larger standard deviation than the ionization chamber measurements. This is likely

to be due to the lower precision of the TLD measurements compared to ionisation chamber measurements

and due to linac calibration by the institutions, where the ionisation chamber system used for calibration

can correspond to the one used in the IMRT intercomparison. No information about the uncertainty of the

ionisation chamber measurements is available.

III.B.II. IMRT planning and treatment

A visual example of the results in terms of percentage dose differences is shown in figure 3, for a

“type a” (Eclipse, PBC) and a “type b” (Eclipse, AAA) algorithm for the same unit. Differences occur

especially in the lung region with high doses (PTV), and in particular for the “type a” algorithm, as is to

be expected.

Figure 3. Graphical representation of the dose difference values for an IMRT case, planned with 7

fields (gantry angles of 40, 90, 125, 180, 235, 285 and 345 degree). Deviations larger than 5% are

highlighted in dark gray (Electronic version: Overdosed points in comparison with the calculation in dark

red, underdosed points in dark blue). Deviations within 1% are colored in white. Left: “Type a” algorithm

(Eclipse PBC), right: “Type b” algorithm (Eclipse AAA), for the same treatment unit and measurement

set.

To show the accuracy of different dose calculation algorithms, especially related to the calculations in

low density media, the relative difference between the (Dm-Ds)/Dprescribed values in the lung and the soft

tissue regions within the PTV are calculated. Since the dose level is comparable in the lung and normal

tissue regions of the PTV, differences between these media are independent of systematic deviations

originating from the planning process or machine calibration, but depend on the dose calculation

algorithm. Averaged values derived from ionization chamber and TLD measurements for 30 plan-

measurement combinations are shown in table 3. TLD values are averaged over 11 single positions

whereas the ionization chamber value is based on a single measurement point.

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Table 3. Ion chamber and TLD measurements in the soft tissue and lung region within the PTV: Mean

(Dm-Ds)/Dprescribed values and associated uncertainties in %. The number of cases per algorithm is added in

parentheses. For the Tomotherapy machine, the ionization chamber measurements are missing.

PTV soft PTV lung difference

Ion ch. TLD Ion ch. TLD Ion ch. TLD

type

a

Eclipse PBC (9) -1.8 ± 1.2 -1.4 ± 2.3 -3.3 ± 2.8 -5.4 ± 2.1 -1.5 ± 2.3 -4.0 ± 1.4

KonRad PB (1) -2.3 -0.4 -10.7 -8.7 -8.5 -8.3

PrecisePlan Clarkson (1) 0.0 -0.5 -6.5 -4.9 -6.6 -4.4

Eclipse PBC eTAR (1) 0.5 0.5 -2.3 -3.8 -2.7 -4.3

type

b

Pinnacle CCC (6) 0.4 ± 0.9 0.6 ± 0.6 0.2 ± 0.8 -0.5 ± 1.0 -0.1 ± 0.4 -1.1 ± 0.8

Eclipse AAA (5) -1.8 ± 0.9 -1.8 ± 1.6 -1.0 ± 1.3 -1.5 ± 1.3 0.7 ± 0.9 0.4 ± 0.8

MasterPlan CCC (4) 0.2 ± 0.9 2.1 ± 0.6 1.6 ± 3.6 2.1 ± 2.8 1.4 ± 3.1 -0.1 ± 2.5

XiO-CMS (2) -2.2 ± 0.7 0.0 ± 1.5 0.0 ± 1.4 -0.2 ± 1.4 2.2 ± 0.8 -0.1 ± 0.1

Tomotherapy CS (1) - -1.3 - -4.2 - -2.9

Mann-Whitney-tests for both TLD and ionization chamber measurements in the PTV prove that

“type b” algorithms are superior to “type a” in calculating the dose in the lung region within the PTV,

with p < 0.001.

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Table 4 shows the percentage dose difference in regions outside the PTV: left and right lungs, normal

soft tissue, heart and spinal cord (embedded in the bony-like structure).

Table 4. Mean percentage (Dm-Ds)/Dprescribed values in the regions outside the PTV

Considering all TLD measurements (all points) and institutions, the mean absolute difference

between measured and stated dose values, relative to the prescribed dose, (Dm-Ds)/Dprescribed , is 2.5 ±

2.0% for the “type a” and 1.4 ± 1.1% for the “type b” algorithms. In regions outside the lungs the values

are 1.9 ± 0.4% and 1.4 ± 0.3%, respectively, while for points inside lung densities the figures are 3.3 ±

1.7% and 1.4 ± 0.3%, respectively. The data show the same degree of accuracy between the two

algorithm types as long as no low density medium is present.

Figure 4 shows the mean of the (Dm-Ds)/Dprescr. absolute values for the PTV lung and normal tissue

regions, categorized by the algorithm type and the irradiation technique. The data give no hint about an

influence of the irradiation technique.

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mean TLD measurement: (Dm-Ds)/Dprescribed [%]

Algorithm left lung right lung normal tissue heart spinal cord

type

a

Eclipse PBC -1.7 ± 2.5 -0.7 ± 1.3 -0.3 ± 2.1 -0.8 ± 1.7 -2.3 ± 2.4

KonRad PB -4.7 -2.4 -1.4 -3.5 -2.3

PrecisePlan integr.

alg.

-4.5 -3.3 -1.5 -0.4 1.7

Eclipse PBC eTAR 1.2 -1.5 1.0 1.6 0.4

mean “type a” -1.9 ± 2.6 -1.1 ± 1.4 -0.4 ± 1.9 -0.8 ± 1.8 -1.7 ± 2.4

type

b

Pinnacle CCC 0.8 ± 1.3 0.1 ± 1.0 0.7 ± 1.2 0.5 ± 0.9 -0.2 ± 2.1

Eclipse AAA -0.9 ± 2.1 -0.5 ± 0.6 -0.5 ± 1.1 -0.4 ± 1.1 -1.4 ± 1.9

MasterPlan CCC 0.0 ± 1.7 1.2 ± 0.6 0.7 ± 0.8 0.4 ± 0.6 1.5 ± 1.1

XiO-MSC 1.1 ± 0.4 0.4 ± 2.1 -0.6 ± 2.5 -1.7 ± 2.4 2.9 ± 2.7

Tomotherapy CS 0.8 -1.2 -0.6 0.4 -0.7

mean „type b“ 0.2 ± 1.7 0.2 ± 1.2 0.2 ± 1.5 0.0 ± 1.4 0.1 ± 2.3


Figure 4. Mean absolute (Dm-Ds)/Dprescr. values (error bars for 1 SD) grouped according to the

irradiation technique and the applied algorithm type. The number of comparisons is quoted in

parentheses.

Figure 5 shows the graded (Dm-Ds)/Dprescr. absolute values for the PTV normal and lung tissue (30

evaluations), derived from the TLD measurements. For the normal tissue, all values except one (97%) are

smaller than 5%. 24 values (80%) are smaller than 3%. The corresponding values for the lung are 77%

and 50%. As stated earlier, it can be observed that “type b” algorithms show a better agreement with the

measurements in the lung region than “type a” algorithms. This statement is not valid for the normal

tissue region. An influence of the irradiation technique cannot be observed.

Figure 5. Graded (Dm-Ds)/Dprescr. values for PTV lung and normal tissue. Black symbols: “type a”

algorithms, white symbols: “type b” algorithms. Irradiation techniques are indicated with different

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symbols.

Criteria for failure rates have not been defined. But these results underline again the highly satisfying

dosimetry performed by the institutions in Switzerland.

IV. DISCUSSION

A first IMRT plan intercomparison project was held in Switzerland in 2008-2009, aiming to check

dose distributions mainly from IMRT plans, as calculated by treatment planning systems and as measured

(with TLD and ion chamber) in an anthropomorphic thorax phantom.

The preliminary study, checking one single field against Monte Carlo simulations, proved the

accuracy of the TLD measurements both in water and lung equivalent media to be sufficient. They were

reliable and therefore suitable for the intercomparisons.

Additionally, ionisation chamber measurements were performed by the individual institutions, giving

further and immediate information supporting the TLD results.

The specific check of the plan dose calculation and delivery chain gave valuable information to the

participating centers. Additionally, the results from the intercomparison showed some interesting features,

mainly differences between “type a” and “type b” dose calculation algorithms: With a multicenter

intercomparison, where several and different TPS were used as well as different techniques, we

confirmed that “type b” algorithms take inhomogeneities into account better than “type a”. This finding

coincides with other groups25.

Some “type a” algorithms show deviations between calculations and measurements of more than 5%

in the PTV region located in the lung area, with both TLDs and ionization chamber. The same pattern is

detected in lung regions outside the PTV. A useful additional outcome of this dosimetry intercomparison,

is that it gives the participating centers the opportunity to evaluate the degree of accuracy of their dose

calculation algorithm when used in near clinical conditions. In particular two main points could be

addressed. The first point concerns the cases where the target partially includes a low density medium: in

this case a center running a “type a” algorithm is informed that the dose delivered to the PTV in the lung

tissue could be about 5% lower than planned (and expected), with possible issues in terms of treatment

outcome. The second point relates to the dose delivered to healthy lung tissue: the user needs to consider

the issue of the tolerance dose level stipulated for the lung and the dose computed by their TPS,

depending on whether the algorithm is a type “a” or “b”. These dosimetric tests permit users a better

understanding of their algorithm response in certain conditions and the possible consequences.

Within soft tissue, both “type a” and “b” algorithms present dose calculations which agree with the

measurements within 2% of the prescribed dose, with no significant difference between the two types.

This finding coincides with other published results.1,18,26. The irradiation technique (3D-CRT, static or

dynamic IMRT) has no influence on the agreement between measurements and calculations.

No information is available on the time needed for planning, irradiation and documentation of the

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intercomparison. But our own experiences show that the effort has been about three times larger than for

a clinical IMRT plan.

The intercomparison focused on the currently available dose calculation algorithms. In 2008, MC was

not available for routine planning in the institutions. Due to the rapid adoption of MC algorithm in the

community with the increasingly powerful computing, the inclusion of MC will be an issue for future

dosimetric intercomparisons.

Finally, it can be stated that all the treatment plans carried out by the Swiss radiotherapy centers

fulfilled the requirements in terms of planning objectives. However, it has to be pointed out that relevant

parts of the overall treatment chain cannot be checked in the frame of a dosimetry intercomparison. For

this a more general intercomparison would be necessary including e.g. the diagnosis and the therapy

concept, the delineation of the planning structures, the positioning of the patient and treated volume, etc.

It is the responsibility of the institutions to arrange and participate in advanced education and QA

concepts including such tasks. These elements are subjects of ongoing investigations.27-30

V. CONCLUSION

The intercomparison procedure has turned out to be feasible and yields valuable convincing results.

In the future, the IMRT intercomparison will be repeated on a three years regular interval with modified

objectives adapted to the current demands.

VI. ACKNOWLEDGMENTS

The persons responsible for the IMRT intercomparison - Hans Schiefer and Wolf W. Seelentag -

thank all the participants for the excellent cooperation in the planning process of the intercomparison and

in the intercomparison itself. They express their sincere thanks especially to the participants of the pilot

study which served additionally as co-authors of this article. A special thank due to the members of the

AMS team of the University of Berne, above all to Michael Fix, Federico Hasenbalg and Ernst Born for

the MC calculation which confirmed the accuracy of the TLD measurements. A thank goes to Nicci

Lomax for the final article review.

Tables:

See text

Figure Captions:

See text

Author to whom correspondence should be addressed: Electronic address: [email protected];

Telephone: (0041) +71 494 2239; Fax: (0041) +71 494 289.3

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mailto:[email protected]


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