ESTRO 36 Abstract Book

S429 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective Current clinical portal dosimetry from Varian evaluates dose using calibrated units (CU). This work assesses the quality of the first Varian implementation of the MAASTRO algorithm for pre-treatment absolute portal dosimetry (in Gy) of 6X-FF fields. Material and Methods To achieve the proposed goal, a comparison was made between the gamma analysis results obtained using both Varian’s clinical portal dosimetry (PDIP v10.0.28) and the MAASTRO algorithm [1] implementation made available for the authors in the Portal Dosimetry (PD) application accessible through the Varian Citrix Research Environment (CRE). For this study, 10 breast IMRT breast plans and 10 VMAT prostate plans were chosen from the patients’ database. In total, 71 IMRT fields and 40 VMAT arcs were compiled for analysis. Each plan was recalculated with gantry zero on a water-equivalent slab phantom, for later comparison of absolute dose at 5cm depth. Verification plans were created for irradiation with 6X-FF beams at the Varian Edge LINAC in order to measure the doses at the Electronic Portal Imaging Device (EPID) level. For each field/arc, the measured doses and the calculated doses were compared by gamma analysis in CU for PDIP and in absolute dose values (Gy) for the PD system on the CRE. [1] Nijsten SM et al, 'A global calibration model for a-Si EPIDs used for transit dosimetry”, Med. Phys. 34(10): 3872-84, 2007 Results Table 1 presents the summary of the gamma analysis results obtained in the comparison between the measured dose at the EPID and the calculated dose using the MAASTRO algorithm implementation and PDIP. The results show that the analyzed IMRT plans using the MAASTRO algorithm obtained, on average, a higher gamma pass rate, lower mean gamma values and lower dose differences than while using PDIP. The same is observed for VMAT plans. Figure 1 shows the graphical comparison between the gamma passing rate obtained using the MAASTRO algorithm and PDIP, where the black circles represent the comparison of the gamma passing rates for IMRT plans (averaged over all beams) and the open triangles represent the comparison of the gamma passing rates for VMAT plans (averaged over all arcs). One can see that the gamma pass rate obtained using the MAASTRO algorithm is consistently higher than the one obtained using PDIP.

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Conclusion It is shown that the MAASTRO algorithm implementation for gamma analysis based on absolute dose comparison is reliable and provides very good results for both types of plans tested. When compared with the results obtained with PDIP v10.0.28, the MAASTRO algorithm presents at least as good results for the pre-treatment portal dosimetry as the currently available PDIP, while reporting absolute dose results, making it a viable, and even desirable, alternative. PO-0811 Monte Carlo simulation of peripheral dose for Gamma Knife treatments B. Sanchez-Nieto 1 , E. Doerner 1 , A.M. Cardona 1 , F. Bova 2 1 Pontificia U-dad Catolica de Chile, Insitute of Physics, Santiago, Chile 2 Gamma knife Chile, Radiotherapy, Santiago, Chile Purpose or Objective Induction of second cancers after external beam radiotherapy (RT) is associated to the dose deposited outside the treatment field (Peripheral Dose -PD) [1] . New advances in radiation oncology have increased the survival of patients beyond the period of latency of the occurrence of secondary cancer (> 5 years), so that the estimation of PDs has become particularly relevant. Commercial treatment planning systems present a great uncertainty in the dose calculation outside the treatment field (differences up to 50%) [2] ; therefore, alternative methodologies for estimation of PD to radiosensitive organs are needed. There are previous studies [3,4] applicable to external RT with linear accelerators. However, no such a model exists for Gamma Knife. The aim of this study was to estimate the peripheral dose associated to radiosurgery treatments using Monte Carlo (MC) and experimental measurements with TLDs. Material and Methods A Leksell Gamma Knife 4C radiosurgery equipment was modeled using the set of C++ libraries Egspp, part of the MC platform EGSnrc. The model includes the entire set of 201 Cobalt-60 sources, along with their respective beam channel. Validation was performed by comparing profiles and dose deposited in depth during irradiation of the Lucy QA Phantom with all sources opened. Then, the photon spectrum and absorbed dose were calculated and measured with TLD-100 pairs, for the plan above, at 14 points of a pseudo-anthropomorphic phantom. TLD-100 had calibration factors for 6 MV nominal energy. TLD readings were corrected by an energy response correction factor due to the change in response from the 6 MV calibration beam to the softened spectrum at the measurement points. Results The simulated geometry was tested using a raytracing method, included in Egspp, which allowed visualization of geometrical details to be compared with the available

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