Boxed out.

Top Line: IMRT reduces acute toxicity for women receiving pelvic radiation for cervical or endometrial cancer, but does it reduce late toxicity?

The Study: PARCER is one of two important trials (the other being RTOG 1203 TIME-C) that evaluated the benefits of IMRT over conventional 3D planning when delivering adjuvant pelvic RT. We had a peek at late toxicity results at ASTRO 2020, and here we have the full publication. Enrolled patients had resected cervical cancer with an indication for either postoperative RT (23%, any 2 of tumor ≥ 4 cm, deep stromal invasion, or lymphovascular invasion) or postoperative chemoradiation (77%, any 1 of parametrial invasion, positive nodes, or positive vaginal margins). Radiation consisted of 50 Gy in 25 fractions to the pelvis and 12 Gy in 2 fractions HDR brachytherapy. Three hundred women were randomized to either conventional 4-field 3D conformal with weekly portal imaging or image guided IMRT. In the IMRT arm, plans were optimized to keep the small bowel V15Gy < 190 cc and the V40Gy < 100 cc. At 3 years, IMRT halved the rate of late grade 2+ GI toxicity from 42.4→ 21.1% and reduced the rate of grade 3+ late GI toxicity from 15.5→ 2.9%. However, when it came to acute toxicity, IMRT marginally reduced acute grade 2+ diarrhea (26.1→ 17.2%)  and didn’t significantly reduce acute grade 2+ GI toxicity overall (28.8 vs 29.8%). A possible explanation for the lack of improvement in acute GI toxicity was the high rate of concurrent chemo administration compared with TIME-C. Surprisingly, there were no clear small bowel dose-volume measures that fell out as strong predictors of GI toxicity. Multiple measures were included in univariable and multivariable analyses of late GI toxicity. Only V40Gy +/- 90 cc was significant on univariable analysis and no measure of small bowel dosimetry was significant on multivariable analysis. 

TBL: Compared to 3D-conformal, IMRT reduces late grade 2+ GI toxicity by half in women receiving adjuvant pelvic radiation for cervical cancer. | Chopra, J Clin Oncol 2021