3288 - Moderately Hypofractionated Proton Therapy with or without Implantable Spacer in Patients with Localized Prostate Cancer

S. Kaviraj^1,2, F. Funer¹, A. Borkowetz³, J. Thiele¹, S. Löck⁴, C. Thomas³, M. Krause^1,4, E. Troost^1,4, and T. Hölscher^1,4; ¹Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany, ²Radiation Oncology, Philipps-University Marburg, Marburg, Germany, ³Department of Urology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany, ⁴OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany

Purpose/Objective(s): Hypofractionated radiotherapy is becoming popular for the treatment of localized prostate cancer, even though the rectum remains a dose-limiting organ at risk. The value of the use of an implantable retroprostatic spacer (RPS) for the reduction of rectal toxicity has not been studied for moderately hypofractionated proton beam therapy (PBT). This study therefore aimed to assess the gastrointestinal (GI) and genitourinary (GU) toxicities in patients with localized prostate cancer treated with moderately hypofractionated PBT with or without an implantable RPS. It is hypothesised that grade > 1 GI toxicity can be reduced by the RPS. Materials/

Methods: In this single-institution analysis of prospectively collected data, patients with localized prostate cancer were recruited between October 2018 and February 2023. Men were treated with PBT up to 57-62 Gy (RBE) in 20 fractions with [PBT with RPS] or without a RPS [PBT without RPS]. Patients were initially assigned to receive PBT with RPS, but the onset of the COVID-19 pandemic led to a reprioritization of healthcare resources and patients were subsequently treated without RPS. Outcomes in this interim analysis include comparison of early (at 3 months) and late toxicity (at 1 year) per CTCAE v4.0 as well as of rectal dose-volume histograms between the two groups.
Results: In total, 58 men, aged 18 years or older, and with an Eastern Cooperative Oncology Group performance status (ECOG PS) of 0-1 were treated. Of these, 30 received PBT with RPS and 28 PBT without RPS. Baseline characteristics were well balanced in both groups. Baseline GI and GU symptoms as well as acute toxicity were not significantly different between both groups (Table 1). There was a statistical trend of a higher late grade > 1 GI toxicity in men without RPS (11%) (p=0.07). Rectal dose parameters, i.e. volume receiving 40 or 57 Gy (V40Gy, V57Gy), and the maximum dose to the rectum (rectal Dmax) were all statistically significantly lower in patients treated with PBT and RPS (p<0.001).
Conclusion: Rectal dose parameters were significantly reduced with the use of a RPS, possibly resulting in lower grade > 1 GI toxicity. Enrollment still continues and further assessment in a larger population is required.

Abstract 3288 – Table 1 Abbreviations: CTCAE = Common Terminology Criteria of Adverse Events; GI = gastrointestinal; GU = genitourinary; Mon = months; Yr = year; PBT = proton beam therapy; RPS = retroprostatic spacer

Adverse events per CTCAE	PBT with RPS (n=30)		PBT without RPS (n=28)		p-value	p-value
	Any Grade	> Grade 1	Any Grade	> Grade 1	Any Grade	> Grade 1
Baseline GU	13 (43%)	4 (13%)	10 (36%)	4 (14%)	0.55	0.92
Baseline GI	0	0	0	0	-	-
3 Mon GU	19 (63%)	7 (23%)	16 (57%)	6 (21%)	0.63	0.86
3 Mon GI	3 (10%)	0	6 (21%)	0	0.23	-
1 Yr GU	21 (70%)	3 (10%)	13 (46%)	5 (18%)	0.046	0.41
1 Yr GI	4 (13%)	0	7 (25%)	3 (11%)	0.28	0.070