ESTRO 37 Abstract book

S979

ESTRO 37

The EPID was used for dose plane distribution verification. An 'in house” development program was used to convert the portal image to a dose distribution. Then, the VeriSoft program was used to calculate a 2D gamma index (local dose, suppress dose below 10% of max dose of calculated volume) for 3%3mm and 2%2mm criteria. Also, the accuracy of dose computation by Monaco TPS is independently checked by independent secondary check software (PTW-Diamond version 6.0). Diamond performs calculation based on modified Clarkson method, which integrates the primary and the scatter components of the radiation dose to a point from all individual and segmental subfields. Results Table 1 shows the gamma passing rates and the MU differences, for both type of treatment. For the Octavius 4D, the mean 3D gamma passing rate for all prostate cases was slightly superior to the SIB cases. The same happens for the 2D gamma passing rate. Furthermore, the mean for the MU calculation is lower for the prostate cases. Figure 1 shows an example of a dose plane distribution from the TPS and EPID and the gamma evaluation (2%2mm and 3%3mm), for both cases.

Conclusion The TSET beams for two TB linacs were commissioned and the data compared with previous data used for Varian Clinacs. A good level of agreement was found for the fundamental beam data, with the main difference arising due to the difference in sensitivity of the monitor chambers within the linac, causing an increase in the set MU by a factor of 2.8. Verification of the technique on the new linacs was provided by delivering a sample treatment with TLDs used to confirm the dosimetry. The baseline values were established and quality control tests were implemented. EP-1817 Quality assurance analysis for VMAT prostate treatments E.M. Ambroa Rey 1 , D. Navarro Jiménez 1 , R. Gómez Pardos 1 , J. García-Miguel 1 , T. Ramirez Muñoz 1 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Radiation Oncology, Terrassa, Spain Purpose or Objective Advanced radiation therapy techniques, such as volumetric modulated arc therapy (VMAT), requires a patient-specific pre-treatment quality assurance (QA). This can be performed by dose calculations, direct dose measurements, or a combination of both. Simple point measurements however are insufficient for dose verification of complex dose depositions with high dose gradients such as for VMAT . The aim of this study is to evaluate three different pre- treatment QA methods for prostate VMAT treatments: 3D dose calculation using the Octavius 4D; dose plane verification using an electronic portal imaging device (EPID) from an Elekta Synergy machine and an independent monitor unit (MU) calculation. Material and Methods A total of 180 patients were examined: 75 with localized prostate cancer and 105 with prostate and pelvic irradiation using the simultaneous integrated boost (SIB) technique. VMAT plans calculations were carried out using the Monaco treatment planning system (TPS) version 5.10. The Octavius 4D was used for 3D dose verification. In order to reconstruct and analyze the measured 3D dose from each plan, the PTW VeriSoft 6.2 patient plan verification software was used and a volumetric 3D gamma index analysis (max dose of calculated volume, suppress dose below 10% of max dose of calculated volume), for dose difference and distance to agreement (DD/DTA) of 3%3mm and 2%2mm, was performed.

Conclusion Despite the fact that SIB cases have a more heterogeneous dose distribution compare to prostate cases, QA results are only slightly lower. In general, we can conclude that both cases achieve a satisfactory QA