The role of proton therapy in the treatment of large irradiation volumes: A comparative planning study of pancreatic and biliary tumors

Purpose: The purpose of this study was to examine the potential benefit of proton therapy for abdominal tumors. Extensive comparative planning was con ducted investigating the most up-to-date photon and proton irradiation tech nologies. Methods and Materials: A number of rival plans were generated for four pati ents: two inoperable pancreatic tumors, one inoperable and one postoperativ e biliary duct tumor. The dose prescription goal for these large targets wa s 50 Gy, followed by a boost dose up to 20 Gy to a smaller planning target volume (PTV). Photon plans were developed using "forward" planning of copla nar and noncoplanar conformal fields and "inverse" planning of intensity-mo dulated (IM) fields. Proton planning was simulated as administered using th e so called spot-scanning technique. Plans were evaluated on the basis of n ormal tissues' dose-volume constraints (Emami B, Lyman J, Brown A, et al. T olerance of normal tissue to therapeutic irradiation. Int J Radial Oncol Bi ol Phys 1990;21:109-122) and coverage of treatment volumes with prescribed doses. Results: For all cases, none of the forward calculated photon plans was abl e to deliver 50 Gy to large PTVs at the same time respecting the dose-volum e constraints on all critical organs. Nine evenly spaced IM fields achieved or nearly achieved all maximum dose constraints to critical structures for two out of three inoperable patients. IM plans also obtained good results for the postoperative patient, even though the dose to the liver was very c lose to the maximum allowed. In all cases, photon irradiation of large PTV1 s to 50 Gy followed by a 20 Gy boost entailed a risk very close to or highe r than 5 % for serious complications to the kidneys, liver, or bowel. Simpl e arrangements of 2, 3, and 4 proton fields obtained better dose conformati on to the target, allowing the delivery of planned doses including the boos t to all patients, without excessive risk of morbidity. Dose homogeneity in side the targets was also superior with protons. Conclusion: For the irradiation of large PTVs located in the abdominal cavi ty, where multiple, parallel structured organs surround the target volumes, proton therapy, delivered with a sophisticated isocentric technique, has t he potential to achieve superior dose distributions compared with state of- the-art photon irradiation techniques. IRI photon plans obtain better resul ts in the postoperative case, because the reduced volume lessens the effect of the unavoidable increase of integral dose to surrounding tissues. (C) 2 000 Elsevier Science Inc.