# Patterns of prostate recurrence after focal salvage prostate brachytherapy for radiorecurrent prostate cancer

## TL;DR

This study finds that many prostate cancer recurrences after focal brachytherapy happen outside the treated area, suggesting current treatment margins may be too small.

## Contribution

The study identifies the spatial patterns of prostate cancer recurrence after focal salvage brachytherapy and questions the adequacy of current treatment margins.

## Key findings

- 52% of prostate recurrences after focal salvage brachytherapy occur outside the treated area.
- Outfield recurrences are at least 11.9–13.4 mm away from the treated region.
- A 5 mm margin around the tumor may not be sufficient for effective treatment.

## Abstract

•40–52 % of prostate recurrences after focal salvage brachytherapy are classified as outfield recurrences.•The outfield recurrences occur at a minimal median distance of 11.9–13.4 mm from the treated GTV with salvage brachytherapy.•A CTV margin of 5 mm in focal salvage brachytherapy may be inadequate for optimal disease control.

40–52 % of prostate recurrences after focal salvage brachytherapy are classified as outfield recurrences.

The outfield recurrences occur at a minimal median distance of 11.9–13.4 mm from the treated GTV with salvage brachytherapy.

A CTV margin of 5 mm in focal salvage brachytherapy may be inadequate for optimal disease control.

Focal high-dose-rate (HDR) salvage brachytherapy has emerged as a treatment for radiorecurrent prostate cancer. This study aims to evaluate patterns of recurrence after focal salvage brachytherapy and to assess the adequacy of current treatment margins.

Between March 2015 and December 2021, 39 patients with radiorecurrent prostate cancer underwent focal HDR brachytherapy. All patients had biopsy-confirmed local recurrence and were staged using Choline- or PSMA-PET/CT and multiparametric MRI. A 5 mm margin around the GTV was applied to define the CTV. Post-treatment recurrences were analyzed using rigid image registration of PET/CT and MRI to assess spatial relationships among the initial recurrence (Rec1), the recurrence following salvage brachytherapy (Rec2), and the brachytherapy dose distribution. The recurrences were categorized into infield, marginal, and outfield based on overlap of relapse with the treated CTV and based on dose received on the site of the relapse. Additionally, spatial analysis measured minimal distances between Rec1 and Rec2.

Nineteen of 39 patients experienced clinical recurrence, with 12 exhibiting 25 local lesions. Based on spatial overlap, 20 % of Rec2 lesions were infield, 28 % marginal, and 52 % outfield. Dose-based classification indicated 52 % infield, 8 % marginal, and 40 % outfield recurrence. The median distance between Rec1 and Rec2 in outfield cases was 11.9–13.4 mm.

A substantial proportion of local recurrences after focal salvage brachytherapy occur outside the treated volume. Current 5 mm margins may be insufficient.