KEY POINTS
- This planning study generated beam-specific planning-target-volume single-field uniform dose and robustly optimized intensity-modulated proton therapy plans for 10 patients previously treated for prostate cancer.
- Both approaches prescribed 36.25 Gy equivalent in five fractions to the prostate and proximal seminal vesicles using four fields. All patients had implanted gold markers and a 10 mL hydrogel spacer.
- Robustness was assessed using 20 scenarios combining setup shifts of ±3 mm and range variations of ±3.5%, together with accumulated-dose estimates from five cone-beam-computed-tomography-derived virtual computed tomography datasets.
- Worst-case clinical target volume D99 decreased by a median of 1.26 Gy equivalent with SFUD versus 1.61 Gy equivalent with IMPT (p = 0.0059), but remained above 95% of prescription in every patient with both techniques.
- SFUD showed slightly smaller worst-case deviations in target homogeneity, while IMPT showed smaller deviations for selected organ-at-risk metrics, including rectum V18.1 and bladder V37.
- Across nominal, worst-case and accumulated-dose comparisons, IMPT consistently reduced rectum V18.1, rectum V29 and bladder V18.1. The largest accumulated-dose difference was a median 4.39-percentage-point reduction in rectum V18.1.
- Target-dose differences under accumulated anatomical changes were small and not statistically significant. The analysis used a pencil-beam algorithm, weekly images acquired during a 21-fraction course and incomplete cone-beam computed tomography anatomy, rather than prospectively delivered five-fraction treatments.
CLINICAL TAKEAWAY
Robustly optimized IMPT provided modest low- and intermediate-dose sparing without a clinically important loss of target robustness. The comparison does not establish inherent superiority over SFUD because the approaches used different uncertainty-management strategies, and the ten-patient virtual study provides no evidence of reduced gastrointestinal or genitourinary toxicity.