3254 - Comparison of Adaptive Magnetic Resonance Guided and Computed Tomography Guided Stereotactic Body Radiation Therapy for Prostate Cancer

Purpose/Objective(s):

Adaptive stereotactic body radiation therapy (SBRT) for prostate cancer has shown to be safe and effective, while shortening overall treatment time. This study aims to compare the dosimetry and toxicities in localized prostate cancer patients treated with either adaptive MR-guided SBRT (MRgSBRT) or CT-guided SBRT (CTgSBRT).

Materials/Methods:

Following IRB approval, a retrospective review was conducted for patients with localized prostate cancer who had either received prostate MRgSBRT or CTgSBRT from 8/2021 to 1/2025 at a single institution. Doses ranged from 36.25 – 40 Gy in 5 fractions to the entire prostate with optional simultaneous dominant intraprostatic lesion boost to 40 – 42.5 Gy. Planning margins were 2 mm and 1 mm posteriorly (MR-arm) or 4 mm and 3 mm posteriorly (CT-arm). Patients with locally recurrent prostate cancer, distant metastatic disease, or concurrent treatment to pelvic lymphatics were excluded. Kruskal-Wallis test or Fisher’s exact test were used to compare patients’ baseline characteristics. Wilcoxon rank sum test was used to identify associations in initial planned dosimetry (CTV, bladder, urethra, rectum) and patient reported outcomes via American Urological Association (AUA) Symptom Score questionnaire. Fisher’s exact test was used to compare associations in GU and GI toxicities, graded by RTOG Common Toxicity Criteria.

Results:

A total of 67 patients (median age 68) were included; 25 patients received MRgSBRT without rectal spacer, 10 patients received MRgSBRT with rectal spacer, and 32 patients received CTgSBRT with rectal spacer (median follow-up: 13, 14, and 3 months respectively). Most patients (94%) had 2 mm urethral expansion avoidance structure. There was no significant association between SBRT guidance method and prostate volume (p = 0.334). MRgSBRT patients were adapted more often (median 5.0 without spacer, 4.0 with spacer), compared to CTgSBRT patients (median 2.0). MRgSBRT plans were less homogeneous and associated with lower CTV minimum and higher CTV maximum (p < 0.001). CTgSBRT plans were associated with higher maximum bladder dose (p = 0.005), mean bladder dose (p = 0.024), minimum urethral dose (p < 0.001), and mean urethral dose (p = 0.021) but lower maximum urethral dose (p < 0.001). There were no significant associations in minimum rectal dose (p = 0.950), maximum rectal dose (p = 0.274), and mean rectal dose (p = 0.219). Furthermore, there were no significant associations between treatment modality and GU toxicity (p = 0.077), GI toxicity (p = 0.321), or changes in AUA scores (p = 0.108). When comparing MRgSBRT patients with and without rectal spacers, there were no significant associations in GI toxicity (p = 0.770), maximum rectal dose (p = 0.506), or mean rectal dose (p = 0.760).

Conclusion:

This study found no significant difference in physician-scored toxicities or patient-reported outcomes between adaptive MRgSBRT and CTgSBRT. Additional prospective studies are warranted to study the differences in adaptive technologies.