Target Coverage Improvement With Dose Matching in Carbon-Ion Radiation Therapy for Pancreatic Cancer.

[PURPOSE] Although carbon-ion radiation therapy (CIRT) creates a sharp dose distribution, inaccurate irradiation positioning may reduce the tumor dose. In pancreatic CIRT, interfractional tumor motion is a factor causing tumor dose reduction. This motion is typically accounted for in the planning target volume, and it cannot provide sufficient margin because the tumor is surrounded by the gastrointestinal tract. Online adaptive radiation therapy (ART) can solve this problem, but other problems such as equipment design and excessive time consumption remain in CIRT. The purpose of this feasibility study was to evaluate the effectiveness of dose matching (DM), which is more convenient than ART in pancreatic CIRT.

[MATERIALS AND METHODS] On the in-room computed tomography images, search isocenters were placed 3 dimensionally around the isocenter determined by target matching (TM) at 0.2 cm intervals. The fractional dose distributions were then calculated at each isocenter. The coordinate with the best clinical target volume coverage (CTV V95%) was determined as the DM isocenter. In actual treatment, the use of couch shifting is assumed for irradiation in accordance with the DM isocenter. To evaluate the effectiveness of DM, variations from the initial plan for CTV V95% (ie, ΔCTV V95%) and organ-at-risk (OAR) dose (ΔD) in bone matching (BM), TM, and DM were compared.

[RESULTS] The median ΔCTV V95% values in BM, TM, and DM were -2.18%, -1.39%, and -0.36%, respectively. DM significantly improved CTV V95%. OAR doses were within their limits. Toxicity in DM was considered equivalent to that in BM because the maximum ΔD in DM was similar to the BM results.

[CONCLUSION] DM significantly improved CTV V95% in pancreatic CIRT within dose constraints of OARs. However, DM should be properly applied by considering treatment efficacy and efficiency. The appropriate use of TM, DM, and online/offline ART is required for each treatment site to improve the target coverage.