Comparison of dose calculation algorithms in slab phantoms with cortical bone equivalent heterogeneities

Abstract

To evaluate the dose values predicted by several calculation algorithms in two treatment planning systems, Monte Carlo (MC) simulations and measurements by means of various detectors were performed in heterogeneous layer phantoms with water- and bone-equivalent materials. Percentage depth doses (PDDs) were measured with thermoluminescent dosimeters (TLDs), metal-oxide semiconductor field-effect transistors (MOSFETs), plane parallel and cylindrical ionization chambers, and beam profiles with films. The MC code used for the simulations was the PENELOPE code. Three different field sizes (10 x 10, 5 x 5, and 2 x 2 cm2) were studied in two phantom configurations and a bone equivalent material. These two phantom configurations contained heterogeneities of 5 and 2 cm of bone, respectively. We analyzed the performance of four correction-based algorithms and one based on convolution superposition. The correction-based algorithms were the Batho, the Modified Batho, the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system (TPS), and the Helax-TMS Pencil Beam from the Helax-TMS (Nucletron) TPS. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. All the correction-based calculation algorithms underestimated the dose inside the bone-equivalent material for 18 MV compared to MC simulations. The maximum underestimation, in terms of root-mean-square (RMS), was about 15% for the Helax-TMS Pencil Beam (Helax-TMS PB) for a 2 x 2 cm2 field inside the bone-equivalent material. In contrast, the Collapsed Cone algorithm yielded values around 3%. A more complex behavior was found for 6 MV where the Collapsed Cone performed less well, overestimating the dose inside the heterogeneity in 3%-5%. The rebuildup in the interface bone-water and the penumbra shrinking in high-density media were not predicted by any of the calculation algorithms except the Collapsed Cone, and only the MC simulations matched the experimental values within the estimated uncertainties. The TLD and MOSFET detectors were suitable for dose measurement inside bone-equivalent materials, while parallel ionization chambers, applying the same calibration and correction factors as in water, systematically underestimated dose by 3%-5%.

Dosimetric comparison of Acuros XB with collapsed cone convolution/superposition and anisotropic analytic algorithm for stereotactic ablative radiotherapy of thoracic spinal metastases

“…The AAA algorithm was shown to underestimate, while the CCC algorithm overestimates, the dose inside targets (including the vertebral body and the bone marrow) compared to the AXB calculations. This is in agreement with the results of Carrasco et al (34) where five dose calculation algorithms of Helax‐TMS and Varian‐Eclipse were compared with MC calculations and TLD measurements in a slab phantom geometry using a cortical bone‐equivalent heterogeneity. In that study, CCC was shown to overestimate the dose inside the bone‐equivalent material by $3\%-5\%$ for 6 MV beams.…”

“…Much of the esophagus is located at a lung/soft tissue interface, and the ${\mathrm{D}}_{5\text{cc}}$ shows both AAA and CCC overestimate the dose. This is similar to observations in lung tumor cases, where kernel‐based models often overestimate the dose due to its incapability to model the electron‐photon coupled transport across the interface to account for forward/backward scattered photons and loss of electronic equilibrium 11 , 34 . Dose‐to‐medium was chosen to be the dose reporting methods in this case.…”

Improving accuracy for stereotactic body radiotherapy treatments of spinal metastases

“…While most of the PerFraction gamma fail points were in peripheral tissue and neither consistently higher nor lower in dose, target and CNS doses were generally greater than the TPS. This may just indicate differences between the two dose computation algorithms’ treatment of bone — differences which have been demonstrated previously for other TPSs 38, 39. PerFraction's log‐file analysis thus shows promise as a QA tool to detect plans that cannot be delivered accurately, even for complicated treatments such as SBRT spine.…”

Comparative evaluation of modern dosimetry techniques near low‐ and high‐density heterogeneities

“…The average dose difference (calculated data ‐ measured data) for all the tested cases in this study were 4.3%. Another study by Carrasco et al (24) involved comparing five TP dose calculation algorithms against MC simulation, MOSFET and TLD measurements in multilayer slab phantom with cortical bone used as high‐density heterogeneity. In that study, TLD measurements underestimated MC calculations by $5.7\%\pm 1.1\%$ near the exit interface.…”