Session Item

The acute toxicity risks associated with MR guided prostate stereotactic body radiotherapy with daily online plan adaptation (MRg-A-SBRT) compared to fiducial or CT-guided non-adaptive prostate SBRT (CT-SBRT) remain unknown.

We performed a PRISMA-compliant systematic review and meta-analysis of prostate stereotactic body radiotherapy (SBRT) studies that were published between 1/1/18-8/31/22. Inclusion criteria were: prospective study, samples >= 10 patients, prostate SBRT evaluated (4-5 fraction regimen), acute toxicity rates documented. Included studies were dichotomized into 2 groups: treatment delivery platform utilizing either MRg-A-SBRT or CT-SBRT as no studies meeting inclusion criteria of MR guided non-adaptive or CT/fiducial guided adaptive therapy were identified. Retrospective analyses were excluded as were studies of SBRT boost, SBRT re-irradiation or post-operative SBRT. Grade 2 or higher genitourinary (GU) and gastrointestinal (GI) acute toxicity rates were extracted from each study. A random effects model was utilized to estimate toxicity rates and meta-regression was performed to compare toxicity between MRg-A-SBRT and CT-SBRT.

Twenty-nine unique prospective studies (9 MRg-A-SBRT, 20 CT-SBRT) were identified that met the inclusion criteria and included a total of 2547 patients (329 MRg-A-SBRT, 2218 CT-SBRT). MR-g-A-SBRT studies included 3 studies evaluating a 0.35T system and 6 evaluating a 1.5T system. Proportions of patients in the MRg-A-SBRT studies were higher for high risk prostate cancer (41% vs. 11%, p<0.01), use of androgen deprivation therapy (70% vs 22%, p<0.01), use of pelvic lymph node irradiation (16% vs 6%, p<0.01) and use of rectal spacers (10% vs 4%, p<0.01). The median posterior and non-posterior PTV margins were similar between groups (3mm vs 3mm posterior, p=0.41, and 5mm vs 5mm non-posterior, p=0.36). The median prescription dose (EQD2, a/b = 10) was similar between groups (MRg-A-SBRT 52.1 Gy vs. CT-SBRT 52.25, p=0.09). The ranges of acute G2+ GU toxicity were 5.0%-33.3% in the MRg-A-SBRT studies and 9.1%-46.7% in the CT-SBRT studies. The ranges of acute G2+ GI were 0%-8.3% in the MRg-A-SBRT group and 2.0%-23.3% in the CT-SBRT group. The pooled estimates for acute G2+ GU and GI toxicity for MRg-A-SBRT were 16% (95%CI 10-24%) and 4% (95%CI 2-7%) and for CT-SBRT were 28% (95%CI 23-33%) and 9% (95%CI 6-12%), respectively. On meta-regression, the odds ratios for acute G2+ GU and GI toxicities comparing MRg-A-SBRT and CT-SBRT were 0.47 (95%CI 0.27-0.79, p<0.01) and 0.40 (95%CI 0.17-0.94, p=0.04), respectively.

MRg-A-SBRT is associated with reduced risk of both acute GU and acute GI toxicity compared to CT-SBRT. Further prospective studies are indicated to evaluate the clinical benefits of MR guided radiotherapy and adaptive radiotherapy for prostate SBRT. Longer follow up will be needed to evaluate late toxicity and disease control outcomes.