Evaluation of the efficacy of plan adaptation in stereotactic body proton therapy for pancreatic cancer

PURPOSE: This study aimed to quantitatively evaluate the efficacy of plan adaptation in stereotactic body proton therapy (SBPT) for pancreatic cancer using daily CT-based dose evaluation, investigate the appropriate adaptation frequency. METHODS: This retrospective planning study included 10 patients previously treated with X-ray stereotactic body radiation therapy for pancreatic cancer. An initial SBPT plan was created for each patient, using robust optimization with two posterior oblique beams. The prescribed dose was D90 ≥ 40 Gy (RBE) to the CTVEVAL (the CTV excluding the gastrointestinal GI tract plus a 5-mm margin), delivered in five fractions. Daily dose evaluation (DDE) was performed using five daily CT (dCT) images per patient. An adaptive plan (AP) was generated and applied to subsequent fractions if target coverage or organs at-risk (OARs) constraints were not met; otherwise, the initial plan (IP) was employed. Non-adaptation (NA) and daily adaptation (DA) strategies were compared in terms of dose-volume metrics, accumulated doses, tumor control probability (TCP), and normal tissue complication probability (NTCP). RESULTS: Across 50 fractions, plan adaptation was necessary in 21 fractions (42%), with a mean of 2. 1 adaptations per patient. DA significantly improved target coverage on DDE compared with NA (p < 0. 05). Doses to the OARs, particularly the GI tract, were significantly reduced with DA. In the accumulated dose, DA led to a significantly higher D90 of CTVEVAL (p < 0. 05), whereas doses to the OARs demonstrated no significant differences between strategies. TCP was significantly improved with DA, whereas NTCP demonstrated no significant differences for any OARs. CONCLUSIONS: The DA strategy, according to DDE, improved target dose coverage while maintaining or reducing doses to the OARs. These findings indicate that appropriately frequent plan adaptation-an average of 2. 1 adaptations per patient-may improve SBPT safety and efficacy for pancreatic cancer.