Determining optimal GTV-to-PGTV margins for CT-guided dose-escalated radiotherapy with daily image guidance in locally advanced rectal cancer.

BACKGROUND: Neoadjuvant chemoradiotherapy for locally advanced rectal cancer yields pathological complete response rates of only 10%-20%. Dose-escalation strategies may improve outcomes, but optimal GTV-to-PGTV margins for CT-guided radiotherapy with daily IGRT remain undefined. METHODS: Twelve LARC patients undergoing CT-guided daily IGRT with a simultaneous integrated boost were included. Daily diagnostic-quality fan-beam CT (FBCT) scans were acquired for IGRT. GTV and CTV were delineated on planning CT and all FBCTs. Target coverage margins were assessed by isotropically expanding the planning GTV until more than 95% of the voxels of the sequential GTVs were covered. A margin with a coverage probability threshold of 90% was defined as adequate. An independent validation cohort of 30 patients who underwent weekly FBCT-guided image guidance was further analyzed. Overlap volumes between PGTVs and organs-at-risk (OARs; bladder and small bowel) were calculated to assess OAR sparing. RESULTS: Analysis of 286 FBCT scans showed that a 6 mm isotropic GTV-to-PGTV margin achieved>95% coverage in>90% of fractions. Compared with 10 mm expansion, a 6 mm PGTV reduced the overlap volumes with the bladder and small bowel by 68.5% and 68.4%, respectively. A 6 mm isotropic expansion achieved>95% coverage in 91.3% of fractions in the validation cohort. CONCLUSION: A 6 mm isotropic GTV to PGTV margin provides adequate target coverage for most middle- and lower-rectal tumors while reducing OAR overlap. This finding could facilitate safer dose escalation while maintaining target coverage. However, larger margins may be necessary for smaller tumors or those located in the high rectum.