SU-E-T-401: On the Use of Onboard Portal Dosimetry for Patient-Specific QA of RapidArc Plans.
PURPOSE
The goal of this project is to conduct a systematic investigation on the utility of an onboard portal dosimetry (PD) system for patient-specific QA of RapidArc plans. This abstract reports the preliminary results of this ongoing investigation.


METHODS
The PD system of Varian Medical Systems has been used in many centers as a convenient and viable tool for patient-specific QA of fixed-beam IMRT plans. Because the onboard portal imager rotates with the x-ray source, portal dose obtained at each pixel from a complete RapidArc delivery is a sum of portal doses delivered at different gantry angles of the arc. As a Result, the sensitivity of PD in detecting deviations of photon fluence delivered at different gantry angles is inherently low. We investigated the potential sensitivity loss and its implications for patient-specific QA by 1) dividing the arc into multiple smaller arc-segments, 2) analyzing the dose concordance as a function of arc-segments, and 3) comparing the concordance of PD QA results with those obtained from 3D dose verification using the Delta4 system.


RESULTS
Preliminary studies were conducted using RapidArc plans for prostate and head & neck cases. For each case, three verification plans (using single arc (300°), five 60° arc-segments and ten 30° arc-segments) were examined. All PD QA plans passed the QA criteria based on the gamma index. Detailed comparison of portal-°dose profiles indicated general concordance between the measured and predicted, with increased local variations for plans with smaller arc segments. The QA results using PD for all patients were concordant with that obtained by Delta4 based on 3D dose comparison.


CONCLUSION
Although the preliminary results shown concordant QA results between PD and Delta4, this does not constitute a sufficient test of PD for RapidArc QA. Further investigation is ongoing and updated results will be reported in the meeting.