Bag and loop small bowel contouring strategies differentially estimate small bowel dose for post-hysterectomy women receiving pencil beam scanning proton therapy.

Background Small bowel (SB) dose-volume relationships established during initial computed tomography (CT) simulations may change throughout therapy due to organ displacement and motion. We investigated the impact of organ motion on SB dose-volume histograms (DVHs) in women with gynecologic malignancies treated with pencil beam scanning (PBS) proton therapy and compared PBS SB DVHs to intensity-modulated radiation therapy (IMRT). Material and methods Post-hysterectomy patients (n = 11) treated for gynecologic cancers were enrolled on an image-guided proton therapy protocol involving CT simulation with full (CTF) and empty (CTE) bladders and weekly/biweekly on-treatment scans. IMRT plans were generated for comparative analysis. SB was contoured as bowel loops or bowel bag. Wilcoxon signed-rank tests were used for matched-pair comparisons of SB, bladder, and rectum dose-volumes between CT scans and between PBS and IMRT plans. Results In PBS loops analysis, on-treatment DVH was significantly higher than CTF for doses <45 Gy (p < 0.05), and not significantly different than CTE. Specifically, V15 for loops was higher on-treatment (median 240 cm(3)) compared to CTF (median 169 cm(3), p = 0.03). In PBS bag analysis, on-treatment DVH was not significantly different from CTF across all dose ranges. Bowel bag V45 was not significantly different between on-treatment (median 540 cm(3)) and CTF (median 499 cm(3), p = 0.53). Decreasing bladder volume was associated with increasing V15 for loops and V45 for bowel bag (p < 0.005, both). Comparing PBS and IMRT, PBS resulted in significantly lower DVHs at low dose regions (<38 Gy) and higher DVHs at high dose regions (42.5-45.5 Gy) in both loops and bag analysis. IMRT plans demonstrated higher on-treatment SB loop DVHs and only minimal differences in bowel bag DVHs compared to CTF. Conclusions SB DVHs were well estimated by CTF bowel bag and underestimated by CTF loops in the setting of inconsistent bladder filling. Verifying bladder filling prior to treatment or using CTE for planning may more conservatively estimate SB dose-volume relationships.