ESTRO 2024 - Abstract Book

S4700

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

of the clinical factors analyzed were associated with the risk of toxicities higher then G1 (p<0.05). Among all the dosimetric factors tested, only bladder V 60Gy(RBE) was correlated with a higher chance of experiencing GU toxicities > G1 (p=0.04). The HD-PTV coverage goal was not met for 30 (71.4%) patients, for bladder sparing (21 (70%), Fig.1) or due to rectum and bowel proximity to PTV for the remaining 9 (30%) patients. HD-PTV ΔD95% was -5.7% and -9.3% for LEM optimized and mMKM recalculated plans, respectively. However, no correlation was found between D 95% (both for LEM and mMKM) and oncological outcomes (biochemical relapse or local recurrence).

Conclusion:

CIRT for intermediate and high risk pCa at our institute proved safe with low GI and GU toxicity rate reported in the cohort analyzed. The lower BRFS and LRFS compared to the Japanese reported data might be justified by: i) the sub optimal HD-PTV coverage, mainly due to an over-conservative approach to bladder sparing; ii) the worsening of target cold regions when converted to mMKM. This analysis led to a new planning approach to maximize target coverage, based on robust optimization combined with new bladder dose constraints (D 1cc <105% of the prescription dose). To limit GU toxicity, we routinely minimize bladder V 60Gy(RBE) , while maintaining PTV coverage. Furthermore, a direct mMKM optimization, now available in commercial TPS, is being considered to overcome the uncertainties related to different biological models when treating pCa patients with CIRT.

Keywords: CIRT, RBE Modeling, Prostate Cancer

References:

1) Okada T, Tsuji H, Kamada T, Akakura K, Suzuki H, Shimazaki J, Tsujii H; Working Group for Genitourinary Tumors. Carbon ion radiotherapy in advanced hypofractionated regimens for prostate cancer: from 20 to 16 fractions. Int J Radiat Oncol Biol Phys. 2012 Nov 15;84(4):968-72. 2) Nomiya T, Tsuji H, Kawamura H, Ohno T, Toyama S, Shioyama Y, Nakayama Y, Nemoto K, Tsujii H, Kamada T. A multi-institutional analysis of prospective studies of carbon ion radiotherapy for prostate cancer: A report from the Japan Carbon ion Radiation Oncology Study Group (J-CROS). Radiother Oncol. 2016 Nov;121(2):288-293.