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

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.