PPP05 Presentation Time: 11:06 AM

Purpose

Brachytherapy (BT) boost to the whole-gland (i.e., clinical target volume [CTV]) improves oncological outcomes in prostate cancer patients, albeit with higher likelihood of genitourinary (GU) and gastrointestinal (GI) toxicities. Magnetic resonance (MR) imaging can unveil the gross tumour volume (GTV) with high sensitivity and specificity (i.e., 86% (CI 82-89%) and 99% (CI 98-99%), respectively) in patients with localized disease, which also correlates with the most common area of local recurrence after radiation treatment. To improve the therapeutic index of BT boost, one potential approach is circumscribing it to the MR-depicted GTV (i.e., focal BT boost). Another approach is the use of rectal spacers, which significantly decrease the occurrence of radiation-related toxicities. However, their use is limited in ultrasound-guided BT given their echogenic noise limiting visualization of prostate and catheters. An MR-guided BT setting can converge the benefits of improved soft-tissue resolution and rectal spacers in the absence of device-related imaging artifacts. Herein, we present the results of a prospective study using rectal spacers for MR-guided focal BT boost combined with stereotactic body radiotherapy (SBRT) to the prostate.

Materials and Methods

Patients with localized prostate cancer and visible GTV on MR were enrolled in a prospective study (NCT00913939). All patients underwent insertion of rectal spacer under ultrasound guidance. Subsequently, patients received MR-guided HDR BT boost (15Gy in 1 fraction), followed by SBRT to prostate (33Gy to CTV, 30Gy to PTV, in 5 fractions). The present study reports on the first 73 patients enrolled.

Results

Seventy-three patients are included in this analysis, with a median follow-up of 42 months (IQR 30-57). Most patients (47/73; 64.4%) had Grade Group 2 disease; while 37 (50.7%) and 27 (37%) were respectively categorised as NCCN favourable and unfavourable intermediate-risk, and 9 (12.3%) high-risk disease. Thirty percent of patients received combinatorial ADT for a medial duration of 6 months (IQR 6-20). Median number of BT catheters were 5 (IQR 4-7). Acute Grade 1 GI and GU toxicities were seen in 22 (30.1%) and 66 (90.4%) of patients, respectively. Only one case reported Grade 2 acute GU toxicity (dysuria). Late (i.e., 3-months after treatment) Grade 1 GI and GU toxicities were observed in 4 (5.5%) and 29 (39.7%) cases, respectively. There was a single case of late Grade 2 GU toxicity (haematuria). No incidence of acute or late Grade 3-4 toxicities was observed. Seven (9.6%) biochemical recurrence events were recorded, with an associated BCR-free survival of 83.7% at 5 years. Among those with BCR, 6 cases had recurrence localization by imaging: two local recurrences alone (both intra-prostatic outside the focal boost volume), while four had regional and/or distant recurrences.

Conclusions

MR-guidance allows the incorporation of rectal spacers to the BT treatment paradigm, which together with the use of focal BT boost combined with SBRT renders a favourable therapeutic index for men with localized prostate cancer.