Room: AAPM ePoster Library
To quantify the impact of range shifter parameters on plan quality of intra cranial proton beam therapy
12 patients with intracranial tumor were chosen for this study. Clinical plans generated with range shifter (RS) in place to cover the superficial part of the target. These plans were re-optimized with range of all possible air gap combinations. This group of plans generated for both 4 cm (RS4) and 7.5 cm (RS7.5) thick RS and with Monte carlo (MC) and pencil beam convolution (PBC) algorithms. Conformity index (CI), gradient index (GI) and integral dose (ID) were calculated and compared with baseline plan.
Plans calculated with PBC and RS4 shows better CI and vice versa for MC and RS7.5. Most cases CI steadily decreases as the AG increases. In terms of CI, PBC and RS4 are the better choice.
In all the patients GI increases with increase in the gap. Increased GI makes the plan quality poorer. Mean increase in GI per cm of AG is found to be 0.44±0.19%, 0.68±0.2% for RS4, RS7.5 respectively. Most cases PBC shows marginally higher GI values than MC. RS4 shows lesser GI values than RS7.5.
Integral dose is higher for MC calculations compare to PBC. Mean increase in ID per cm of AG is 0.34±0.14%, 0.51±0.21% for RS4, RS7.5 respectively. RS7.5 shows 52% Mean increase in ID per cm of AG in comparison with RS4
Range shifter is an integral component PBT of shallow extended tumors. AG between patient and range shifter plays a vital role in plan quality. The impact of AG on the plan quality is estimated in this study. Though PB shows better CI it overestimates the dose. MC, RS4 and lesser AG are the better choice for the plan quality in terms of CI, GI and ID.