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Adjuvant pelvic radiotherapy can lower the risk of pelvic recurrence in patients with cervical cancer and endometrial cancer who have undergone hysterectomy. Intensity-modulated radiotherapy (IMRT) is associated with significantly lower gastrointestinal (GI) toxicities. For successful pelvic IMRT, accurate clinical target volume (CTV) definition based on precise location of at-risk nodal regions is essential. The current RTOG guideline suggests a 7-mm uniform margin around great vessels to cover nodal regions. However, the small bowels are frequently adjacent to the great vessels, leading to substantially inclusion of small bowels within the RTOG CTV. Based on the anatomic distribution of pelvic nodal regions, a modified margin-based delineation was hypothesized (Figure 1). This study aimed to compare outcomes and GI toxicities between RTOG- and modified margin-based adjuvant pelvic IMRT in patients with gynecological cancer.

The data of 590 patients treated with post-operative pelvic IMRT for gynecologic cancer between 2010 and 2020 were analyzed. Pelvic nodal regions were delineated by RTOG atlas or modified margin-based delineation definitions (Figure 1). Common Terminology Criteria for Adverse Events (CTCAE) was used to assess GI toxicity. Vx indicated the volume (mL) of small bowel that received a radiation dose of x Gy. 

Median follow-up time was 6.4 years (IQR: 3.7–9.6 years). Overall, 352 (59.7%) and 238 (40.3%) patients underwent RTOG and modified margin-based IMRT, respectively.  The median age was 56 years; 176 (29.8%) and 414 (70.2%) patients had cervical cancer and endometrial cancer, respectively. The patient and tumor characteristics were not different between delineations. The V45, V30, and V15 of small bowel were significantly lower in the modified margin group than RTOG group (V45: 117.6 ± 36.9 vs. 169.0 ± 44.1, p<0.001; V30: 526.2 ± 110.8 vs. 601.9 ± 133.0, p<0.001; V15: 977.5 ± 188.1 vs. 1116.6 ± 245.4, p<0.001, respectively). Patients who received modified margin-based IMRT experienced significantly less CTCAE grade ≥2 acute GI toxicity than patients who received RTOG-based IMRT (16.4% vs. 33.5%; p<0.001). The CTCAE grade ≥3 late GI toxicity was significantly less in the modified margin group than RTOG group (0.8% vs. 4.8%, p<0.001). The patterns of failures were not different between groups. In the overall cohort, the 5-year overall survival (OS) and progression-free survival (PFS) for RTOG vs. modified margin groups were 85.1% vs. 87.2% (p=0.36) and 80.7% vs. 83.5% (p=0.21), respectively. In the subgroups analysis based on cancer types, the OS and PFS were not significantly between delineations (Figure 2).

Modified margin pelvic nodal delineation reduced the dose-volume to the small bowel, and lowered the GI toxicity in adjuvant pelvic radiotherapy. The survival outcomes were similar between delineations. Modified margin delineation may achieve optimized therapeutic ratio in patients with gynecological cancer.