P. Oh1, J. H. Laird Jr2, S. E. Gueble3, and K. L. Johung4; 1Department of Therapeutic Radiology, Yale School of Medicine, New Haven, CT, 2New York University School of Medicine, New York, NY, 3Yale School of Medicine, New Haven, CT, 4Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT
Purpose/Objective(s): Similar rates of local control (LC) and overall survival (OS) have been demonstrated between intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) for patients with unresectable or medically-inoperable pancreatic cancer. Therefore, it remains unclear which modality is most appropriate for consolidation therapy following an initial course of chemotherapy in both neoadjuvant and palliative settings. We previously demonstrated that LC may be improved in SBRT patients with a higher internal target volume (ITV) dose covering 90% of the volume (D90). We hypothesized that SBRT would have noninferior clinical outcomes contingent upon having a similar biologically effective dose (BED) for the ITV D90, however, may provide a dosimetric advantage over IMRT by sparing organs at risk. Materials/
Methods: This was a retrospective study including patients who received either IMRT (50.4Gy in 28 fractions) concurrent with chemotherapy vs SBRT (33Gy in 5 fractions) for pancreatic adenocarcinoma at a single institution between 2014-2023. All patients were initially diagnosed as having either borderline resectable (BR), locally advanced (LA), or potentially resectable disease but unable to undergo surgery due to comorbidities. Dosimetric parameters related to ITV coverage (Dmin, Dmax, D90) and also Dmax for large bowel, small bowel, and stomach were collected and converted to BED based on an a/ß ratio of 10Gy. Primary endpoints were LC and OS. Chi-square, Wilcoxon rank-sum, and Fisher’s exact test were used to identify baseline differences. Cox regression analysis was used to identify dosimetric variables associated with clinical outcomes. Kaplan-Meier method was used for survival analysis. Results: Among 159 patients included in the study, 72 (45%) patients received IMRT (resectable 3%, BR 30%, LA 67%, while 87 (55%) patients received SBRT (resectable 28%, BR 31%, LA 41%). The median follow-up time for the whole cohort was 12 months. IMRT was associated with a higher median ITV D90 BEDa/ß10Gy compared to SBRT (60.3Gy vs 56.5Gy, p<0.001). However, there was no statistically significant difference in LC (p=0.64) or OS (p=0.23) between IMRT and SBRT. ITV D90 BEDa/ß10Gy was the only dosimetric parameter significantly associated with LC in the whole cohort (HR 0.94; 95% CI 0.88-0.99; p=0.046). No dosimetric parameters were associated with OS. SBRT was associated with lower Dmax BEDa/ß10Gy values for the large bowel (42Gy vs 62Gy, p<0.001), small bowel (53Gy vs 63Gy, p<0.001), and stomach (47Gy vs 62Gy, p<0.001). Conclusion: ITV D90 BEDa/ß10Gy was found to be associated with local control irrespective of RT modality. Given that SBRT was found to be associated with more sparing of organs at risk acutely, this analysis suggests that SBRT may confer a dosimetric advantage that may also allow for dose escalation that could potentially improve local control.
