Dose-escalated simultaneously integrated boost photon or proton therapy in pancreatic cancer in an in silico study: gastrointestinal organs remain critical

Purpose
To compare the dosimetric results of an in silico study among intensity-modulated photon (IMRT) and robust multi-field optimized intensity-modulated proton (rMFO-IMPT) treatment techniques using a dose-escalated simultaneously integrated boost (SIB) approach in locally recurrent or locally advanced pancreatic cancer patients.

Material and Methods
For each of 15 locally advanced pancreatic cancer patients, a volumetric modulated arc therapy (VMAT), a Tomotherapy (TOMO), and an rMFO-IMPT treatment plan was optimized on free-breathing treatment planning computed tomography (CT) images. For the photon treatment plans, doses of 66Gy and 51Gy, both as SIB in 30 fractions, were prescribed to the gross tumor volume (GTV) and to the planning target volume (PTV), respectively. For the proton plans, a dose prescription of 66Gy(RBE) to the GTV and of 51Gy(RBE) to the clinical target volume (CTV) was planned. For each SIB-treatment plan, doses to the targets and OARs were evaluated and statistically compared.

Results
All treatment techniques reached the prescribed doses to the GTV and CTV or PTV. The stomach and the bowel, of the latter in particular the duodenum and the small bowel, were found to be frequently exposed to doses exceeding 50Gy, irrespective of the treatment technique. For doses below 50Gy, the IMPT technique was statistically significant superior to both IMRT techniques regarding decreasing dose to the OARs, e.g. volume of the bowel receiving 15Gy (V15Gy) was reduced for rMFO compared to VMAT (p=0.003) and TOMO (p<0.001).

Conclusion
With all photon and proton techniques investigated, the radiation dose to gastrointestinal OARs remained critical when treating patients with unresectable locally advanced or locally recurrent pancreatic cancer using a dose-escalated SIB approach.