Author: Hajee Reyaz Ali Sahib, Sangeeta Hazarika, Muthukumaran Manavalan, Jaswin Raj, Daya Nand Sharma, Seema Sharma, Subramani Vellaiyan 👨🔬
Affiliation: AIIMS NCI Jhajjar, PTW North America Corporation, All India Institute of Medical Sciences 🌍
Purpose:
Purpose is to investigate the effect of extended source-to-imager distances (SID) on the resolution of VMAT patient-specific quality assurance (PSQA) in portal dosimetry (PD).
Methods:
20 VMAT PSQA treatment plans were evaluated for the study. Treatment plans were generated using Eclipse treatment planning system with the Acuros XB algorithm (ver. 15.2) and dose calculation grid size was 2.5mm. Plans were delivered using 6 MV flattening filter (FF) and flattening filter-free (FFF) beams on a Varian True Beam system equipped with aS1200 (EPID) electronic portal imaging device. The pre-treatment PSQA was performed using EPID. The PSQA plans were delivered at SIDs of 100 cm, 120 cm, and 140 cm. The active pixel area (1190 × 1190) corresponded to resolutions of 0.0336 cm, 0.0280 cm, and 0.0240 cm for SIDs of 100 cm, 120 cm, and 140 cm. Also, measurements were carried out with PTW OCTAVIUS-4D 1500 detector at actual gantry angle. To compare the planned and delivered dose gamma passing criteria of 3%/2mm was applied and the maximum threshold was kept as 10.0%. Also time taken to deliver the Meter exposure (MU) was also observed.
Results:
Measurements showed the gamma passing rate 98.39±0.75, 98.30±0.77 and 97.86±0.63 respectively for SIDs of 100 cm, 120 cm, and 140 cm. The gamma passing rate for with PTW OCTAVIUS-4D 1500 detector was 98.15±0.21. The results showed that Meter exposure (MU) was proportional to exposure time (second), which increased (0.1sec) with extended SID, respectively. From the results, it was observed that increased SID resulted in comparable gamma pass rate compared to 100cm and OCTAVIUS-4D.
Conclusion:
The EPID demonstrated comparable results with different SIDs, without requiring any distance correction. Extended SIDs results are in good agreement with OCTAVIUS-4D detector. Therefore, study suggest the reliability of EPID-based portal dosimetry for VMAT-PSQA at extended source-to-imager distances.