Trend analysis of the dosimetric impact of anatomical changes during proton therapy for maxillary sinus carcinoma.

PURPOSE: Anatomical changes, such as shrinkage and aeration, can affect dose distribution in proton therapy (PT) for maxillary sinus carcinoma (MSC). These changes can affect the dose to the target and organs at risk (OARs); however, when these changes occur during PT is unclear. This study aimed to investigate the dosimetric impact of anatomical changes during PT. MATERIALS AND METHODS: Fifteen patients with MSC were enrolled in this study. Initial PT plans were generated based on initial computed tomography (CT) images. Several repeat CT images were obtained to confirm anatomical changes during PT. Evaluation PT plans were generated by copying initial PT plans to repeat CT images. The dose differences of the target and OARs were evaluated by comparing both the plans. RESULTS: At 3-4 weeks after the initiation of PT, the target volume reduced by approximately 10% as compared with the initial volume. Consequently, the target volumes gradually varied until the end of treatment. The value of V95 (volume that received 95% of the prescription dose) in the clinical target volume of the evaluation PT plan was similar to that of the initial PT plan. However, the dose to OARs, such as the contralateral optic nerve, contralateral eyeball, brainstem, and optic chiasm, increased significantly from the middle to the later phases of the treatment course. In contrast, there was a slight dose difference in the ipsilateral optic apparatus. CONCLUSION: The trend analysis in this study showed that anatomical changes appeared 3-4 weeks after the start of PT, and the dose to the OARs tended to increase. Therefore, it is recommended to check the status of tumor 3-4 weeks after the start of treatment to avoid the deterioration of dose distribution due to these changes.

The impact of different setup methods on the dose distribution in proton therapy for hepatocellular carcinoma.

PURPOSE: To investigate the impact of different setup methods, vertebral body matching (VM), diaphragm matching (DM), and marker matching (MM), on the dose distribution in proton therapy (PT) for hepatocellular carcinoma (HCC). MATERIALS AND METHODS: Thirty-eight HCC lesions were studied retrospectively to assess changes in the dose distribution on two computed tomography (CT) scans. One was for treatment planning (1st-CT), and the other was for dose confirmation acquired during the course of PT (2nd-CT). The dose coverage of the clinical target volume (CTV-D98 ) and normal liver volume that received 30 Gy relative biological effectiveness (RBE) (liver-V30 ) were evaluated under each condition. Initial treatment planning on the 1st-CT was defined as reference, and three dose distributions recalculated using VM, DM, and MM on the 2nd-CT, were compared to it, respectively. In addition, the relationship between the CTV-D98 of each method and the distance between the center of mass (COM) of the CTV and the right diaphragm top was evaluated. RESULTS: For CTV-D98 , significant differences were observed between the reference and VM and DM, respectively (P = 0.013, P = 0.015). There were also significant differences between MM and VM and DM, respectively (P = 0.018, P = 0.036). Regarding liver-V30 , there was no significant difference in any of the methods, and there were no discernable difference due to the different setup methods. In DM, only two out of 34 cases with a distance from right diaphragm top to COM of CTV of 90 mm or less that CTV-D98 difference was 5% or more and CTV-D98 was worse than VM were confirmed. CONCLUSION: Although MM is obviously the most effective method, it is suggested that DM may be particularly effective in cases where the distance from right diaphragm top to COM of CTV of 90 mm or less.