# Dosimetric Comparison of VMAT Alone and VMAT with HDR Brachytherapy Boost Using Clinical and Biological Dose Models in Localized Prostate Cancer

## TL;DR

This study compares different radiation therapy combinations for prostate cancer to find the best balance between tumor control and healthy tissue protection.

## Contribution

The study provides a quantitative radiobiological comparison of combined EBRT and HDR brachytherapy regimens for prostate cancer.

## Key findings

- HDR-BT regimens with 2 × 15 Gy combined with specific EBRT schedules showed favorable dose distributions for bladder and rectum.
- EBRT-only regimens were better for sparing the urethra compared to higher HDR doses.
- All regimens met clinical dose constraints, but biological modeling revealed key differences in organ protection.

## Abstract

Radiotherapy is a key treatment for prostate cancer, and combining external beam radiotherapy (EBRT) with high-dose-rate (HDR) brachytherapy can increase treatment effectiveness. However, it remains unclear which dose combinations best balance tumor control and protection of healthy organs. In this study, we retrospectively analyzed ten prostate cancer patients who were actually treated with combined EBRT and HDR brachytherapy. Using their original treatment plans, we applied radiobiological models to compare how different combinations affected the prostate and nearby organs such as the bladder, rectum, and urethra. We found that certain combinations may offer more favorable dose distributions to the tumor while limiting exposure to critical structures. These results do not define a single best regimen but provide quantitative insights that may help guide treatment planning in clinical practice and support future research on individualized prostate cancer radiotherapy.

Background: Combining external beam radiotherapy (EBRT) with high-dose-rate (HDR) brachytherapy (BT) enables biologically effective dose escalation in prostate cancer. However, comparative evaluation of such regimens using radiobiological modeling remains limited. Methods: Dose regimens based on clinical practice were analyzed using α/β values of 1.5 and 3 Gy for the prostate. Ten patients with available planning CT, pelvic MRI, and ultrasound-guided BT plans were retrospectively evaluated. Physical and biological dose distributions were recalculated for various EBRT and HDR-BT combinations. Biological effective dose (BED) values were determined for the prostate and organs at risk (OARs: anterior rectal wall, bladder base, urethra). Regimens yielding the highest ΔBED between prostate and OARs were considered most favorable. Results: All regimens met clinical dose constraints. The most favorable ΔBED profiles for bladder and rectum were observed with HDR-BT regimens (2 × 15 Gy) combined with either 23 × 2 Gy or 15 × 2.5 Gy EBRT, independent of the assumed α/β value. EBRT-only regimens achieved superior urethral sparing, while higher HDR doses led to increased urethral exposure. Conclusions: This study underscores the value of radiobiological modeling in differentiating and optimizing prostate cancer radiotherapy strategies. While the trade-offs between dose escalation and OAR sparing are clinically known, our biologically driven analysis provides a more quantitative foundation for selecting and tailoring combined EBRT/HDR-BT regimens in practice.