Dosimetric comparison of CyberKnife and conventional linac prostate SBRT plans: analysis of the PACE-B Study

Ragu Ratnakumaran; Archana Sasitharan; Asadullah Khan; Hira Mayet; Jonathan Mohajer; Victoria Hinder; Douglas H Brand; Ryan Fullarton; Andrew Loblaw; Emma Hall; Nicholas van As; Alison C Tree; PACE trial investigators

doi:10.1016/j.ijrobp.2025.01.014

Dosimetric comparison of CyberKnife and conventional linac prostate SBRT plans: analysis of the PACE-B Study

Int J Radiat Oncol Biol Phys. 2025 Jan 23:S0360-3016(25)00066-5. doi: 10.1016/j.ijrobp.2025.01.014. Online ahead of print.

Affiliations

¹ The Royal Marsden NHS Foundation Trust, London SM2 5PT, UK; Radiotherapy and Imaging Division, Institute of Cancer Research, London SM2 5NG, UK. Electronic address: ragu.ratnakumaran@icr.ac.uk.
² Radiotherapy and Imaging Division, Institute of Cancer Research, London SM2 5NG, UK.
³ The Royal Marsden NHS Foundation Trust, London SM2 5PT, UK.
⁴ The Institute of Cancer Research, Clinical Trials and Statistics Unit, London SM2 5NG, UK.
⁵ Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, UK.
⁶ Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada.
⁷ The Royal Marsden NHS Foundation Trust, London SM2 5PT, UK; Radiotherapy and Imaging Division, Institute of Cancer Research, London SM2 5NG, UK.

PMID: 39862896
DOI: 10.1016/j.ijrobp.2025.01.014

Abstract

Purpose: In the PACE-B study, a non-randomised comparison of toxicity outcomes between stereotactic body radiotherapy (SBRT) platforms revealed fewer urinary side-effects with CyberKnife (CK) compared to conventional linac (CL) SBRT. This analysis compares baseline characteristics and planning dosimetry between the CK-SBRT and CL-SBRT cohorts in PACE-B, aiming to provide insight into possible reasons for differing toxicity outcomes between the platforms.

Methods: Dosimetric parameters for the surrogate urethra (SU), contoured urethra, bladder, bladder trigone (BT), and rectum were extracted from available CT planning scans of PACE-B SBRT patients. The SU and BT were retrospectively delineated. Dose levels analysed included Dmax, D2, D50, and D95, where D(n) represents the dose (Gy) to (n)% of the structure. Baseline characteristics and planning dosimetry between CK-SBRT and CL-SBRT cohorts were compared using Mann-Whitney U, t-test, and chi-squared tests.

Results: Of the 414 patients who received SBRT, 169 (41%) were treated with CK-SBRT and 245 (59%) with CL-SBRT, with dosimetric parameters available for 94% of patients (390/414). There was a non-statistically significant trend towards more low-risk prostate cancer in the CK-SBRT cohort (12% vs. 6% p=0.02 (ns)). Margins were similar between platforms, except posteriorly where CK-SBRT had smaller margins. CK-SBRT plans had significantly higher median SU Dmax (45.9Gy vs. 42.8Gy, p<0.0001), D2% and D50% compared to CL-SBRT plans. Additionally, CK-SBRT plans had significantly higher median BT Dmax (43.4Gy vs. 41.6Gy, p<0.0001), D2% and D95%, as well as higher median bladder Dmax, D50% and D95%. CK-SBRT plans had lower median rectal D2% (35.5Gy vs. 36.0Gy, p<0.0001), but higher rectal D50% and D95%.

Conclusions: Although the CK-SBRT cohort showed lower urinary toxicity, the planned doses to urinary substructures were actually higher, likely due to heterogeneous dose planning. Factors like intrafraction tracking or other confounding variables may explain the differences in toxicity outcomes between the treatment platforms.