ESTRO 2021 Abstract Book

S1109

ESTRO 2021

PO-1351 Patterns of local relapse following tumor-targeted dose escalation for localized prostate cancer J. Padayachee 1 , N. Sanmamed 1 , J. Lee 1 , Z. Liu 1 , A. Berlin 1 , T. Craig 1 , B. Lao 1 , A. Rink 1 , A. Bayley 2 , C. Catton 1 , A. Sundaramurthy 3 , W. Foltz 4 , A. McPartlin 5 , S. Ghai 1 , E. Atenafu 1 , M. Gospodarowicz 1 , P. Warde 1 , J. Helou 1 , S. Raman 1 , C. Ménard 6 , P. Chung 1 1 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada; 2 Sunnybrook Health Sciences Centre, Radiation Oncology, Toronto, Canada; 3 Edinburgh Cancer Centre, Clinical Oncology, Edinburgh, United Kingdom; 4 University of Toronto, Department of Radiation Oncology, Toronto, Canada; 5 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 6 Centre Hospitalier de l’Université de Montréal (CHUM), Radiation Oncology, Montreal, Canada Purpose or Objective Tumor-targeted dose escalation may improve local control rates in patients with prostate cancer, leading to improved biochemical failure-free survival (bFFS). We report outcomes of dose escalation using a strategy of simultaneous integrated boost or HDR brachytherapy boost. Materials and Methods Eighty patients with localized prostate cancer with GTV identified on multiparametric MRI (mpMRI) were enrolled in this phase 2 non-randomized trial (2012-2016). Patients with GTV >5mm and less than 33% of total prostate volume were eligible. All patients received whole gland prostate VMAT, 76 Gy in 38 fractions. Choice of GTV dose escalation was by physician and/or patient choice and delivered by integrated boost VMAT (IB- VMAT) of 95 Gy in 38 fractions (n=40); or MRI-guided HDR boost of 10 Gy in 1 fraction (n=40). The primary end- point was 3-year local control rates determined by MRI-guided biopsy and/or MRI alone. Toxicity data was collected using CTCAE v.4.0. Risk group categorization was comparable between the arms; 5% low-, 75% intermediate-, and 20% high-risk. Three patients received 6-months of concurrent/adjuvant ADT. Results Median (IQR) follow-up was 55.2 months (48.1-71.4). The overall 5-year bFFS was 92% (95% CI, 85-99). Late G2 GU toxicity was 22.5% and 27.5% in IB-VMAT and HDR boost arms, respectively. Late G2 GI toxicity was 5% in each arm. Two G3 (1 GI, 1 GU) toxicities were seen in IB-VMAT. Local control data was available for 66 patients who agreed to the 3-year post-treatment biopsy (20) or MRI alone (46); 32 in IB-VMAT and 34 in HDR boost. Local control within the boost volume was achieved in 61 patients. One patient in the IB-VMAT arm had persistent disease on biopsy, and subsequently met criteria for biochemical relapse (BCR). At last follow-up of the 66 patients, 4 developed BCR with evidence of intraprostatic relapse outside the boost volume; 1 treated with IB-VMAT and 3 with HDR boost. The spatial distribution of these relative to the boost volume were: ipsilateral lobe (IB-VMAT), marginal/contralateral/bilateral (HDR boost). These relapses appeared to correlate to sites of known microscopic disease at original diagnosis. Conclusion Dose escalation to mpMRI-defined GTV provided high rates of local and biochemical control with a favorable late toxicity profile. The majority of local treatment failures developed beyond the tumor-targeted volume, suggesting that other treatment intensification strategies may be required to further improve outcomes. PO-1352 Rectal spacing hydrogel - significant improvements in LDR prostate brachytherapy dosimetry S. Mehta 1 , S. Khaksar 1 , C. Perna 1 , S. Otter 1 , C. Mikropolous 1 , M. Cunningham 2 , S. Uribe-Lewis 3 , J. Uribe 3 , S. Langley 3 1 Royal Surrey County Hospital, Oncology, Guildford, United Kingdom; 2 Royal Surrey County Hospital, Medical Physics, Guildford, United Kingdom; 3 Royal Surrey County Hospital, Urology, Guildford, United Kingdom Purpose or Objective Low dose rate prostate brachytherapy (LDR-PB) is an effective treatment in patients with prostate cancer. Used as monotherapy in low to intermediate risk patients, or in combination with androgen deprivation therapy (ADT) and external beam radiotherapy in patients with more advanced disease (trimodal). Adverse rectal dosimetry causing early and late rectal toxicity can significantly impact patients’ quality of life. The purpose of this study is to evaluate the impact of rectal spacing hydrogels in improving dosimetry of LDR-PB with a direct comparator arm, of which there are currently limited data. Materials and Methods Data from patients treated with I125 LDR-PB were identified from our prospective patient database. All patients included had a same day, post-implant CT scan prior to catheter removal with dosimetric analysis, as per local protocol. SpaceOAR hydrogel (SpOAR), was injected transperineally posterior to Denovillier’s fascia under U/S guidance. Patients treated with LDR-PB alone had SpOAR inserted immediately following their implant, while those treated as part of a trimodal approach had SpOAR inserted at the time of fiducial marker insertion. Consecutive patients treated with SpOAR were compared to historic control patients, without SpOAR, who were matched for age, clinical stage, Gleason score, iPSA, and prostate volume; identified from our database of >4500 patients and matched using GenMatch. The distance between the prostate and anterior rectal wall was assessed at base, mid-gland and apex in all patients. Prostate, rectal and urethral dosimetry was assessed, as per local protocol. Patients were divided into two groups for analysis: LDR-PB-mono and trimodal. Dosimetry and mean distance between the prostate and anterior rectal wall was compared between patients with and without SpOAR. Results 67 patients with SpOAR were identified, with 201 matched controls. LDR-PB-mono patients were treated to a dose prescription of 145Gy, trimodal patients to 110Gy. SpOAR resulted in a significant increase in mean distance between prostate and anterior rectal wall which was greatest at the base, with an overall mean increase of 7mm in LDR-PB-mono group and 6.5mm in the trimodal group (Table 1).