3258 - Study on the Focal Boost to the Dominant Intraprostatic Lesions Using Proton Therapy in High-Risk Prostate Cancer Patients with Pelvic Lymph Node Irradiation: Assessing Toxicity Using the Accumulated

A. Ong^1,2, K. Knight², V. Panettieri^2,3, M. Dimmock^2,4, J. K. L. Tuan¹, H. Q. Tan¹, and C. Wright²; ¹Division of Radiation Oncology, National Cancer Centre, Singapore, Singapore, ²Department of Medical Imaging and Radiation Sciences, Monash University, Melbourne, Australia, ³Department of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Australia, ⁴Keele Univeristy, staffordshire, United Kingdom

Purpose/Objective(s): This feasibility study investigated focal boosts to the DILs and assessed the clinical impact based on physical dose-volume (DV) and biological metrics compared to the non-boost plans. The biological parameters were derived from the planned (D_P) and accumulated (D_A) DV data of 150 patients with high-risk prostate cancer (HR-PCa). Materials/

Methods: Ten HR-PCa patients with multiparametric MRI-defined DILs were investigated. Two intensity-modulated proton therapy (IMPT) plans were generated for each patient: one with and the other without focal boost. Evaluation metrics included DV metrics, TCP for DILs and the remaining prostate tissue, and NTCP for the rectum, bladder, and urethra. Data on TCP, NTCP and DV metrics were analysed using the Wilcoxon signed-rank test (p<0.05). NTCP from QUANTEC (QT) was also used for comparison. A constant relative biological effectiveness (RBE) of 1.1 was assumed for proton therapy.
Results: A significantly higher Dmean was estimated to the DILs when comparing IMPT-boost (B) to IMPT-non-boost (NB) (IMPT-B: 68.1 GyRBE vs IMPT-NB: 60.8 GyRBE, p<0.01). TCPs for IMPT-B of the prostate-DILs and DILs were higher than for IMPT-NB for a/ß of 1.5 and 3 Gy (p<0.001). The Rectal D_A-NTCP corresponded to the highest GI toxicity risk of 5.4 % (IMPT-NB) -5.6% (IMPT-B) compared to D_P and QT fitted LKB models. The differences in NTCPs were similar for both plans (Table 1).
Conclusion: Focal dose escalation using IMPT can achieve a higher TCP while keeping the NTCP comparable to that for IMPT-NB plans. The Rectal D_A-NTCP resulted in a greater predicted GI toxicity compared to that for D_P. The inclusion of D_A to predict NTCP accounts for interfraction organ motion and could facilitate the reliability of the predicted toxicity while devising dose escalation strategies in patients with HR-PCa.

Abstract 3258 – Table 1

	IMPT-B median (IQR), %	IMPT-NB median (IQR), %	% Diff median	p-value
TCP for a/ß:1.5 Gy
Prostate-DIL	94.3 (94.0-94.4)	93.3 (92.9-93.6)	0.9	<0.01
DIL	99.3 (99.1-99.4)	94.3 (93.6-95.0)	4.9	<0.01
TCP for a/ß:3 Gy
Prostate-DIL	86.0 (85.5-86.1)	84.0 (82.9-84.4)	1.9	<0.05
DIL	97.0 (96.5-97.4)	84.2 (82.6-85.7)	12.7	<0.01
NTCP
Rectal
QT	4.2 (2.8-4.7)	4.1 (2.8-4.5)	0.0	0.59
DP	2.7 (1.8-3.3)	2.7 (1.7-3.1)	0.1	0.17
DA	5.6 (4.1-4.5)	5.4 (4.6-6.2)	0.2	0.09
Bladder
DP	10.4 (9.7-13.4)	10.5 (9.6-13.5)	0.0	0.68
DA	9.4 (8.5-12.4)	9.5 (8.5-12.6)	0.0	0.92
Urethra	4.3 (4.1-4.5)	4.3 (4.1-4.4)	0.1	<0.05