Room: AAPM ePoster Library
Purpose: The electronic portal imaging device (EPID) has been increasingly used in patient-specific QA (PSQA). With the EPID calibrated at the source-imager-distance (SID) of 100 cm, treatment beams are often QAed with EPIDs at the same SID. However, at such a close SID, there are concerns for verifying SBRT plans, which consists of high dose-rate flattening-filter free (FFF) beams with small field sizes. This work compares EPID-based PSQA results with EPIDs at 2 different distances.
Methods: At our institute, most PSQAs are performed on Varian aSi1200 EPIDs and analyzed using a commercial software (Perfraction; SunNuclear). Although the EPID is calibrated in dosimetry mode at 100 cm SID, Perfraction allows the user to deliver PSQAs at different SIDs (we chose 100 cm and 150 cm). To do so, a Perfraction calibration plan (62 fields for 100 cm SID and 43 for 150 cm SID) was delivered and verified for each SID. After each Perfraction calibration was confirmed, treatment beams were delivered twice with the EPID extended to 100 cm or 150 cm. For each beam, the delivered and Perfraction predicted EPID image were evaluated by 2D gamma index with the criteria of 2%, 2 mm, and 10% threshold.
Results: The Perfraction calibration fields were delivered on the same machine and the same day. The average Gamma passing rate between delivered and predicted EPID images was 98.7% and 98.3% for 100 cm and 150 cm SID. We selected 10 lung SBRT plans which were created with 6 MV FFF VMAT beams. The gamma passing rate significantly increased from on average of 93.3% ± 5.5% with 100 cm SID to 98.8% ± 1.7% with 150 cm SID (p < 0.001).
Conclusion: EPID-based PSQAs, higher passing rates were observed with the SID at 150 cm than at 100 cm.