EMBRACing the numbers.

Top Line: What dose constraints are associated with late toxicity when performing image-guided adaptive brachytherapy?

The Study: EMBRACE-I was a prospective observational study with a particular focus on modern, image-guided brachytherapy after concurrent chemoradiation for > 1400 patients with locally advanced cervical cancer. It showed that volumetric prescribing (as opposed to prescribing to a point) both increased target coverage and reduced normal tissue dose. Here is an analysis of dosimetric predictors of late toxicity from the study. The overall rate of grade 3+ GI events was 6.5%, and these were broken down into specific subsites (i.e. anus/rectum, colon, etc.) for further analysis. Unfortunately, there wasn’t much treatment plan data for the EBRT portion of treatment. Only the V57Gy was reported for cases that had an EBRT pelvic node boost. The cumulative D2cc and D0.1cc of the rectum, sigmoid, and bowel, and dose to the ICRU recto-vaginal reference point were reported and normalized to EQD2(3) for normal tissues. From the EBRT portion, larger lymph node boost volumes and para-aortic nodal coverage were associated with increased toxicity. V57 (surrogate for LN boost volume) ≥ 165 cc increased proctitis risk 5→ 14% and cramping 28→ 40%. While technique (3D, IMRT, VMAT) was not associated with any toxicity outcome, the use of simultaneous integrated boost had lower risk of late GI toxicity. D2cc for rectum and bowel were associated with various outcomes as was dose to the ICRU RV-RP. An increase in rectal D2cc from < 65 Gy to > 70 Gy increased grade 2+ rectal bleeding from <5% → 16% and increased the rate of heavier rectal bleeding from 14→ 26%. An increase in rectal D2cc from <65 Gy to > 65 Gy increased the rate of grade 2+ proctitis from <5% to 13% and the rate of difficulty with bowel control from <22% → 29%. Bowel D2cc < 60 Gy was associated with low toxicity, but no clear bowel constraint was found.

TBL: In the EMBRACE-I experience, the use of 45 Gy EBRT prescription with SIB to nodes to minimize V57 and attempting to keep the rectum D2cc < 65-70 Gy and the bowel D2cc < 60 Gy reduces late GI toxicity. | Spampinato, Int J Radiat Oncol Biol Phys 2021