# An X‐ray and γ‐ray combination strategy to exploit steep dose gradients for improved prostate cancer radiotherapy: A dosimetric comparison of Taichi Pro and Halcyon

## TL;DR

This study compares a new radiotherapy system, Taichi Pro, with Halcyon for prostate cancer, showing better tumor dose delivery and organ sparing.

## Contribution

A novel hybrid radiotherapy system combining X-ray and γ-ray for improved dose gradients in prostate cancer treatment.

## Key findings

- Taichi Pro achieved higher PGTV Dmean and Dmax compared to Halcyon.
- Taichi Pro reduced rectum and testis doses significantly.
- Halcyon had shorter beam-on time but less effective OAR sparing.

## Abstract

This study aimed to evaluate a novel technological platform, Taichi Pro—which integrates a 6 MV flattening filterfree linear accelerator with a 18 source Rotating Gamma System (RGS) to generate steep dose gradients via multisource focused γ‐rays and noncoplanar arcs—for precision radiotherapy in prostate cancer. The work provides evidence to support the clinical adoption of hybridmodality radiotherapy devices.

Fifteen prostate cancer patients were enrolled. For each patient, a dual‐modality Taichi Pro plan (RGS focused on the planning gross tumor volume (PGTV) + Linac covering the planning target volume (PTV)) and a Halcyon photon plan (three‐arc VMAT) were designed while maintaining clinically tolerable dose to organs at risk (OAR). Comparative assessments included planning target volume (PTV and PGTV) metrics (D95%, Dmean, homogeneity index HI, conformity index CI), OAR doses (rectum V40/V60/D2cc, bladder V40, testis D2cc, etc.) and delivery efficiency to evaluate the ability to escalate target dose while sparing adjacent OARs.

All plans met institutional clinical constraints. Taichi Pro significantly increased PGTV Dmean (79.48 Gy ± 1.75 Gy) compared to Halcyon (73.25 Gy ± 0.55 24 Gy, P < 0.001) and Dmax (122.74 Gy ± 8.69 Gy) compared to Halcyon (76.15 Gy ± 0.79 Gy, P < 0.001), albeit poorer homogeneity (HI: 0.50 ± 0.09 for Taichi Pro vs. 0.06 ± 0.01 for Halcyon, though within clinically acceptable limits). Taichi Pro significantly reduced rectum V60 (3.97% ± 3.25% vs. Halcyon 7.46% ± 4.78%, P = 0.016), and D2cc (61.61 Gy ± 5.01 Gy vs. Halcyon 65.29 Gy ± 4.52 Gy, P = 0.040). Taichi Pro also significantly reduced testis D2cc (2.39 ± 1.99 Gy) compared to (3.17 Gy ± 1.40 Gy, P = 0.006). Halcyon demonstrated significantly shorter beam‐on time (1.81 ± 0.23 minutes vs. 5.05 ± 1.59 minutes for Taichi Pro, P < 0.001).

Utilizing the steep dose gradient characteristic of the RGS, the Taichi Pro dual‐modality system effectively achieved target dose escalation while simultaneously improving sparing of adjacent OARs. This approach holds the potential for enhancing patient treatment outcomes and quality of life.