Multi-Institutional Dosimetric Analysis of Toxicity from Definitive SBRT in Cervical Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Brachytherapy is standard of care in locally advanced cervical cancer. In patients unable to undergo brachytherapy due to complex medical comorbidities, technical reasons, or refusal, treatment with stereotactic body radiotherapy (SBRT) is an option. We report a retrospective analysis of dosimetric parameters and toxicities for definitive cervix patients who received SBRT boost across three institutions. <h3>Materials/Methods</h3> Eligible patients had FIGO IB2-IVB cervical cancer and did not receive brachytherapy. Patient were treated with concurrent chemotherapy and pelvic external beam (EBRT) followed by SBRT with patient immobilization and fiducial image guidance. GTV and HR-CTV were defined according to EMBRACE II guidelines for brachytherapy. Clinicopathological data, dosimetric data, toxicity (CTCAE V 4.0), and outcome data was collected. Spearman correlations completed in R v4.2.1. <h3>Results</h3> 95 patients were included. All patients received EBRT to the pelvis with doses ranging from 45- 51.75Gy in 25-29 fx. SBRT doses to HR-CTV ranged from 16.8-40 Gy given in 4-6 fractions. Median (IQR) HR-CTV volume was 72.6 (58.7). The median total EQD2 were 104.25 (20.3) and SBRT EQD2 60 (20.3). Total EQD2 to 2cc of rectum and bladder were 65.67 (11.9) and 75.8 (21.5). Rates of acute and late grade 1-2 and grade 3 toxicities are reported in Table 1; there were no grade 4+ toxicities. Low grade urinary frequency was the most common reported toxicity and decreased over time. GU stricture (ureteral) was the most common acute grade 3 toxicity and was correlated with EQD2 of EBRT, SBRT, and total dose (P < 0.001), in addition to bladder 2cc SBRT (p = 0.04) and total EQD2 (p=0.02). Fistulas were rare, though some grade 3 fistula occurred. Acute grade 1-2 recto-vaginal fistulas were correlated with EQD2 of EBRT (P < 0.01), SBRT (p= 0.02), and total EQD2 (p=0.02). Late grade 3 fistulas were significantly correlated with HR-CTV volume, EQD2 of EBRT and SBRT (p=0.01) in addition to rectum 2cc SBRT dose (p=0.012), SBRT EQD2 (p=0.01), and total EQD2. <h3>Conclusion</h3> These data suggest that higher bowel/bladder 2cc SBRT and higher total EQD2 is associated with greater rates of acute and late grade 3 bowel and bladder toxicity in a cohort of women receiving SBRT for cervical cancer. Overall rates of ≥ grade 3 toxicities were small and SBRT can be a safe option for patients who cannot receive standard of care brachytherapy, especially when careful attention is given to D2cc volumes.