Comparison of different treatment planning approaches for intensity-modulated proton therapy with simultaneous integrated boost for pancreatic cancer

Background
Neoadjuvant radio(chemo)therapy of non-metastasized, borderline resectable or unresectable locally advanced pancreatic cancer is complex and prone to cause side-effects, e.g., in gastrointestinal organs. Intensity-modulated proton therapy (IMPT) enables a high conformity to the targets while simultaneously sparing the normal tissue such that dose-escalation strategies come within reach. In this in silico study, we compared four IMPT planning strategies including robust multi-field optimization (rMFO) and a simultaneous integrated boost (SIB) for dose-escalation in pancreatic cancer patients.

Methods
For six pancreatic cancer patients referred for adjuvant or primary radiochemotherapy four rMFO-IMPT-SIB treatment strategies of two or three (non-)coplanar beam arrangements were calculated. Dose values for both targets, the elective clinical target volume (CTV) and the SIB, and the organs at risk as well as target conformity and homogeneity indexes, derived from the dose volume histograms, were statistically compared.

Results
All treatment plans of each strategy fulfilled the prescribed doses to the targets (D95%≥95%, D2%≤107%). No significant differences for the conformity index were found (p>0.05), however, treatment plans with a three non-coplanar beam approach were most homogenous to both targets (p<0.045). Dose constraints for large and small bowel as well as for the liver and the spinal cord were met with all beam arrangements. Irrespective of the planning strategies, the dose constraint for the duodenum and stomach were not met. Using the three-beam arrangements, the dose to the left kidney could be significant decreased when compared to a two-beam strategy (p<0.045).

Conclusion
Based on our findings we recommend a three-beam configuration with at least one non-coplanar beam for rMFO-IMPT-SIB in advanced pancreatic cancer patients achieving a homogeneous dose distribution in the target while simultaneously minimizing the dose to the organs at risk.