Robust intensity-modulated proton therapy with dose-escalated simultaneous integrated boost reduces the low-dose to surrounding tissues in pancreatic cancer patients

Purpose
This in-silico study on simultaneous integrated boost dose-escalation in non-metastatic pancreatic cancer patients dosimetrically compared robust multi-field optimized intensity-modulated proton therapy (IMPT) with volumetric modulated arc therapy (VMAT).

Material and Methods
For five patients, both treatment plans were optimized on free-breathing CTs using RayStation. For VMAT, at least 95% of the prescribed doses of 66Gy and 51Gy to the boost (GTV) and PTV (CTV+5mm), respectively, were to cover 95% of the targets. For IMPT, robust optimization with a setup uncertainty of 5mm and a density uncertainty of 3.5% was applied to the GTV and CTV, with the aforementioned dose levels (RBE) again covering 95% of the targets. The OAR dose constraints adhered to local guidelines and QUANTEC.

Results
All treatment plans reached the prescribed doses to the targets. Doses to the bowel, stomach and/or liver exceeded at least one constraint in all treatment plans, since those OARs were next to or within the targets. While VMAT reduced the median V50Gy of the stomach, doses to the remaining gastrointestinal organs, e.g. liver and kidneys, were lower for IMPT (Fig. 1). Overall, IMPT deposited less low dose outside the CTV (Fig. 2, median integral V20Gy: 1483.4ccm vs. 756.2ccm).

Conclusion
Disregarding inter- and intra-fractional organ motion, dose escalation with IMPT and VMAT is possible. IMPT reduced the dose to surrounding normal tissues, except for OARs overlapping with the target volume, in which the dose was higher due to the robust optimization approach. Additional patients will be included in this study.