ESTRO 2021 Abstract Book

S1103

ESTRO 2021

their baseline levels nor RT intent, independently predicted clinically significant ANX and DEP at radiotherapy end or their acute worsening.

PO-1343 Dose-escalated stereotactic radiotherapy for prostate cancer: A feasibility study using a rectal spacer and urethral sparing

R. Lewin 1 , I. Bar-Orian 1 , N. Honig 1 , L. Tsvang 1 , M. Ben-Ayun 1 , I. Weiss 1 , Z. Symon 1 1 Sheba Medical Center, Radiation Oncology , Tel Hashomer, Ramat-Gan, Israel

Purpose or Objective Dose escalation harbors potential for both improved cancer control and worse toxicity. We investigated the feasibility of delivering stereotactic body radiation therapy (SBRT) beyond the standard 36.25 Gy in 5 fractions (fx) in men with prostate cancer using a rectal spacer and urethral sparing. Materials and Methods An IRB approved clinical database was reviewed to extract patients with prostate cancer treated with SBRT to 40 Gy. Initial workup included MRI, Fusion biopsies and Ga68 PSMA PET scans. Patients with severe urinary symptoms at baseline were excluded. All patients had fiducial marker placement and rectal separation with a hydrogel spacer prior to treatment. VMAT plans were designed to deliver 40 Gy (8 Gy/fx) in 5 every-other-day fractions reducing urethral dose to 32.5 Gy. Elective nodal irradiation (ENI) was 25 Gy in 5 fx. Treatment was delivered with a full bladder and empty rectum using cone-beam CT (CBCT). Biochemical failure was according to the Phoenix definition. The worst Common Toxicity Criteria Adverse Events V4.0 were recorded, acute < 3 months and late > 3 months. Results Sixty men treated between 2017 and 2020 with a minimum follow-up of 12 months were eligible. Median age was 71 (range: 51-81). Median pre-SBRT PSA was 7 ng/dl. ISUP grading was 1 (n=10), 2 (n=32), 3 (n=15), and 4 (n=3). NCCN risk group distribution was low-risk (n=8), favorable intermediate risk (n=28), unfavorable intermediate risk (n=16), high risk (n=3), very high risk (n=3) and two patients with oligo-metastatic disease. Eighteen (30%) patients received concomitant androgen deprivation therapy (ADT), 15 short-term (3-6 months) and 3 long-term (12-18 months). ENI was given in 9 patients. Median follow-up was 16 months (range: 6-33). There were no complications of spacer implantation. Average spacing achieved was 1.3 cm (range: 0.53-2.14). Median rectal V100%, V90%, and V80% were 0% (range: 0-6%), 3% (range: 0-14%), and 6% (range 0.1-16%), respectively. Only 1 patient (1.6%) experienced biochemical failure after 15 months. Acute gastrointestinal toxicity grades 1, 2, and 3 were 13% (n=8), 8% (n=5), and 2% (n=1, diarrhea), and acute genitourinary toxicity was 67% (n=40), 10% (n=6), and 0%, respectively. Late gastrointestinal toxicity grades 1, 2, and 3 were 12% (n=7), 5% (n=3), and 1.6% (n=1, fecal incontinence), and late genitourinary toxicity was 30% (n=18), 15% (n=9), and 0% (n=0) respectively. Of note, the grade 3 fecal incontinence observed in one patient developed after resection of a fibrous nodule in anal ring. Conclusion Dose-escalated SBRT for prostate cancer using a rectal spacer and a urethral sparing technique was feasible with a very low rate of a greater than grade 2 gastrointestinal or genitourinary toxicity. Longer follow-up and a matched standard dose SBRT cohort will be performed for comparison. PO-1344 Preliminary results of Phase II trial on Carbon-ion boost followed by IMRT for high risk PCa F. Patti 1 , G. Marvaso 2 , M. Pepa 1 , B. Vischioni 3 , M. Zaffaroni 4 , S. Comi 5 , D. Zerini 4 , M. Augugliaro 4 , S. Russo 3 , F. Valvo 6 , T. Giandini 7 , B. Avuzzi 8 , R. Valdagni 9 , O. De Cobelli 10 , F. Cattani 5 , E. Orlandi 3 , B.A. Jereczek-Fossa 11 , R. Orecchia 12 1 IEO - European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy; 2 Department of Oncology and Hemato-Oncology, University of Milan; IEO - European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy; 3 National Center for Oncological Hadrontherapy (CNAO), Clinical Department, Pavia, Italy; 4 IEO - European Institute of Oncology, Division of Radiotherapy, Milan, Italy; 5 IEO - European Institute of Oncology, Medical Physics Unit, Milan, Italy; 6 National Center for Oncological Hadronterapy (CNAO), Clinical Department, Pavia, Italy; 7 Fondazione IRCCS Istituto Nazionale dei Tumori, Unit of Medical Physics, Milan, Italy; 8 Fondazione IRCCS Istituto Nazionale dei Tumori, Department of Radiation Oncology 1, Milan, Italy; 9 Department of Oncology and Hemato-Oncology - University of Milan; Fondazione IRCCS Istituto Nazionale dei Tumori, Department of Radiation Oncology 1, Milan, Italy; 10 Department of Oncology and Hemato-Oncology, University of Milan; IEO - European Institute of Oncology IRCCS, Division of Urology, Milan, Italy; 11 D epartment of Oncology and Hemato-Oncology, University of Milan; IEO - European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy; 12 IEO - European Institute of Oncology IRCCS, Scientific Directorate, Milan, Italy Purpose or Objective The definition of the optimal treatment for patients affected by high-risk prostate cancer (PCa) still represents a debated topic. Systemic- and radio-therapies (RTs) are subjected to resistance mechanisms developed by tumour cells and PCa is not an exception. In addition, carbon ion radiotherapy (CIRT) offers the advantage of a steep dose gradient due to the inverted profile of in-depth dose deposition compared to photons, which permits a greater sparing of organs at risk. The aim of this Phase II trial was to evaluate the feasibility, in terms of acute toxicity, of this mixed beam approach for patients with high-risk prostate cancer (PCa). The treatment consists of a CIRT boost followed by whole-pelvis intensity-modulated RT (IMRT). Materials and Methods Patients with localized high-risk PCa (NCCN classification) were enrolled at three different Cancer Centers. The primary endpoint was the evaluation of safety and feasibility with acute toxicity scored up to 1 month after the end of RT. Secondary endpoints were treatment early (3 months after the end of RT) and long-term tolerability, quality of life (QoL), and efficacy. At the end of RT, clinical assessment and prostate-specific antigen (PSA) measurements were performed every 3 months for at least 2 years and gastrointestinal (GI) and