Room: ePoster Forums
Purpose: Field patching technique in spot scanning proton therapy (SSPT) has been shown to potentially increase linear energy transfer within the target. However, the related dosimetric uncertainties have not been investigated. This study aims to evaluate the dose calculation accuracy of pencil beam (PB) algorithm for field patching treatment plans.
Methods: A prostate patient treated with 2 contralateral fields covering the scanning target volume (STV), was selected for the study. We created three field patching plans mimicking dose distributions of treated plan in Eclipse with 2, 4, and 6 beams stopped at the edge of clinical target volume (CTV). For each plan, dose distribution was recalculated by an in-house developed Monte Carlo (MC) calculator. Target coverage and the doses to the critical organs were compared between PB and MC algorithms. Î³-index analysis (3%/3mm) for dose distribution in STV was performed.
Results: MC recalculations showed a degradation in D98 of CTV at 0.36%, 11.80%, and 9.49% for field patching plans using 2, 4 and 6 beams respectively. D2 of CTV calculated by PB was severely underestimated by 28.95%, 11.18% and 12.38% for the three field patching plans. However, dose calculated by MC and PB for conventional beam arrangement was similar, with 0.26% and 1.38% deviation in D98 and D2 of CTV. In addition, Î³ passing rate in STV region dropped from 99.8% to 57.16%. The dose difference in D1cc of the rectum between PB and MC was as large as 575 cGy in 2-field patching plan.
Conclusion: Slight deviation of PB in dose calculation at the distal region of SOBP would lead to significant mismatch with dose calculated by MC in the presence of field patching and the heterogeneous body. MC or a more accurate PB algorithm is recommended in field patching planning for SSPT.
Funding Support, Disclosures, and Conflict of Interest: This study is partly funded by National Key Research and Development Program of China (Grant 2016YFC0105409)