Percentage depth dose evaluation in heterogeneous media using thermoluminescent dosimetry

Abstract

The purpose of this study is to investigate the influence of lung heterogeneity inside a soft tissue phantom on percentage depth dose (PDD). PDD curves were obtained experimentally using LiF:Mg,Ti (TLD-100) thermoluminescent detectors and applying Eclipse treatment planning system algorithms Batho, modified Batho (M-Batho or BMod), equivalent TAR (E-TAR or EQTAR), and anisotropic analytical algorithm (AAA) for a 15 MV photon beam and field sizes of 1 x 1, 2 x 2, 5 x 5, and 10 x 10 cm 2 . Monte Carlo simulations were performed using the DOSRZnrc user code of EGSnrc. The experimental results agree with Monte Carlo simulations for all irradiation field sizes. Comparisons with Monte Carlo calculations show that the AAA algorithm provides the best simulations of PDD curves for all field sizes investigated. However, even this algorithm cannot accurately predict PDD values in the lung for field sizes of 1 x 1 and 2 x 2 cm 2 . An overdosage in the lung of about 40% and 20% is calculated by the AAA algorithm close to the interface soft tissue/lung for 1 x 1 and 2 x 2 cm 2 field sizes, respectively. It was demonstrated that differences of 100% between Monte Carlo results and the algorithms Batho, modified Batho, and equivalent TAR responses may exist inside the lung region for the 1 x 1 cm 2 field.

Figure 1

Schematic figure of the soft tissue/lung tissue phantom.

Figure 2

Geometric phantom (a) with dimensions of $38\times 38\times 30.5{\text{cm}}^3$, and (b) TL detector holder used for TL detector calibration.

Figure 3

PDD curves calculated through the radiotherapy Eclipse planning system algorithms Batho, modified Batho, equivalent TAR, and anisotropic analytical algorithm (AAA); measured with thermoluminescent dosimetry and simulated using EGSnrc Monte Carlo code for a irradiation field of (a) $10\times 10{\text{cm}}^2$, (b) $5\times 5{\text{cm}}^2$, (c) $2\times 2{\text{cm}}^2$, and (d) $1\times 1{\text{cm}}^2$.

Figure 4

Ratio PDD results obtained with treatment planning system Eclipse version 8.1 to PDD results obtained with Monte Carlo, for $1\times 1{\text{cm}}^2$ field size.

Figure 5

Ratio PDD results obtained with treatment planning system Eclipse version 8.1 to PDD results obtained with Monte Carlo, for $2\times 2{\text{cm}}^2$ field size.