Study of out-of-field dose in photon radiotherapy: A commercial treatment planning system versus measurements and Monte Carlo simulations

Abstract

Purpose: An accurate assessment of out-of-field dose is necessary to estimate the risk of second cancer after radiotherapy and the damage to the organs at risk surrounding the planning target volume. Although treatment planning systems (TPSs) calculate dose distributions outside the treatment field, little is known about the accuracy of these calculations. The aim of this work is to thoroughly compare the out-of-field dose distributions given by two algorithms implemented in the Monaco TPS, with measurements and full Monte Carlo simulations.

Methods: Out-of-field dose distributions predicted by the collapsed cone convolution (CCC) and Monte Carlo (MC_Monaco ) algorithms, built into the commercially available Monaco version 5.11 TPS, are compared with measurements carried out on an Elekta Axesse linear accelerator. For the measurements, ion chambers, thermoluminescent dosimeters, and EBT3 film are used. The BEAMnrc code, built on the EGSnrc system, is used to create a model of the Elekta Axesse with the Agility collimation system, and the space phase file generated is scored by DOSXYZnrc to generate the dose distributions (MC_EGSnrc ). Three different irradiation scenarios are considered: (a) a 10 × 10 cm² field, (b) an IMRT prostate plan, and (c) a three-field lung plan. Monaco's calculations, experimental measurements, and Monte Carlo simulations are carried out in water and/or in an ICRP110 phantom.

Results: For the 10 × 10 cm² field case, CCC underestimated the dose, compared to ion chamber measurements, by 13% (differences relative to the algorithm) on average between the 5% and the ≈2% isodoses. MC_Monaco underestimated the dose only from approximately the 2% isodose for this case. Qualitatively similar results were observed for the studied IMRT case when compared to film dosimetry. For the three-field lung plan, dose underestimations of up to ≈90% for MC_Monaco and ≈60% for CCC, relative to MC_EGSnrc simulations, were observed in mean dose to organs located beyond the 2% isodose.

Conclusions: This work shows that Monaco underestimates out-of-field doses in almost all the cases considered. Thus, it does not describe dose distribution beyond the border of the field accurately. This is in agreement with previously published works reporting similar results for other TPSs. Analytical models for out-of-field dose assessment, MC simulations or experimental measurements may be an adequate alternative for this purpose.

Keywords: out-of-field dose; peripheral dose; second cancer; treatment planning systems.

Fig. 1

Semiflex IC and MC in‐plane lateral profiles (left vertical axis) and local differences (right vertical axis). See Fig. S1 for the in‐plane profiles in the Varian linac. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 2

Average energy of a 6 MV photon beam of an Elekta Axesse linac at 5 cm depth in water as a function of the distance to the central axis for a 10 × 10 cm² field. The increase of the energy at the farthest point has also been described in other works, ²⁰ , ²¹ and it can be explained by the increase of head leakage and collimator scatter. ²² An SSD = 95 cm setup was simulated. See Fig. S2 for the average energy obtained for the Varian linac. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 3

Half in‐plane dose profiles calculated with Monaco and experimental data for the Elekta linac. Profiles calculated with Eclipse's PBC algorithm and experimental data obtained for the Varian linac are shown in Fig. S3. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 4

Local differences between calculated (Monaco’s CCC and MC_Monaco) and experimental (EBT3, TLD‐100, Semiflex IC and MC_EGSnrc) in‐plane dose profiles, relative to the algorithm. Negative values indicate that the algorithm underestimates the measured dose. The values corresponding to the Eclipse’s PBC algorithm and the experimental data obtained for the Varian linac are shown in Fig. S4. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 5

IMRT dose distribution analysis for MC_Monaco: (a) dose distributions calculated by the TPSs; (b) dose distribution measured with the Octavius Detector 1500; (c) dose distribution measured with the EBT3 film; (d) local differences between (a) and (b), with respect to (a); (e) local differences between (a) and (c), with respect to (a). Note: the lower part of (e) presents “noisy” blue‐to‐yellow regions that are a consequence of an error in the scanning process of the film and should not be taken into account in the analysis. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 6

Color bars represent relative differences (left vertical axis) in mean absorbed dose between Monaco and MC_EGSnrc relative to the latter, for different organs. d_50%‐CoM represents the distance from the CoM of each organ to the field edge (right vertical axis). These distances are plotted as solid black squares. [Color figure can be viewed at wileyonlinelibrary.com]