Pancreas SBRT requires online adaptive treatment to adequately manage motion

Hypofractionated stereotactic body radiotherapy (SBRT) for pancreatic cancer has shown improved local control but is challenging due to the proximity of several organs at risk (OARs). This study investigated possible dosimetric variations over a simulated SBRT course for pancreatic cancer using daily MR images considering breath hold (BH) and internal target volume (ITV) motion management techniques for three treatment regimes. Ten healthy volunteers were scanned on a Siemens Skyra 3T MRI. Each volunteer had 2 sets of scans acquired per day, 3 h apart, over five days, to represent a daily simulation planning scan and a pre-treatment scan. A hypothetical GTV was generated, with a BH GTV and ITV expanded to generate two different planning target volumes (PTV). Treatment plans for each were generated using the Raystation TPS on each bulk density corrected scan. Plans were propagated and accumulated to simulate conventional, daily adaptive and online adaptive treatment regimes, with DVH comparison of PTV and OAR doses. The BH PTV D95% varied on average - 2.9% ± 3% and - 5.1% ± 5.3% for conventional and daily adaptive regimes, while the ITV PTV D95% varied on average - 8.1% ± 7.2% and - 8.5% ± 5.4% respectively. For the online adaptive approach, the BH PTV D95% varied on average 1.5% ± 2.4% while the ITV PTV D95% varied on average 3% ± 4.9%, achieving improved agreement of target dose. In conclusion, pancreas SBRT shows significant dosimetric variations from positional variations of target and OARs, but accuracy may be improved with online adaptive treatment planning.