Room: ePoster Forums
Purpose: The merit of charged particle pencil beam scanning therapy (PBST) is that ability to deliver dose to complexly shaped tumors. Dose conformity can be improved using narrow pencil beam, however, localization of the dose in the Bragg peak of narrow beam makes dose distributions sensitive to lateral tissue heterogeneity. The aim of this study is to identify the appropriate beam spot size by quantifying an impact of beam spot size on the dose distributions in heterogeneous region.
Methods: The data of three head and neck cancer patients were used in this study. The lateral tissue heterogeneity surrounding each beam path was quantified with the heterogeneity number representing the variation of the Bragg peak depth across the cross section of the beams using effective density data from patient. We planned and simulated with spot sizes varied from 1 to 8 mm in sigma. Heterogeneity numbers and dose distribution were derived. Nominal dose distributions and that robustness against setup and range errors were evaluated by worst-case scenario for all tested beam spot sizes.
Results: Variety of heterogeneity numbers were observed due to air cavities and bony structures. The average heterogeneity number in treatment fields varied between 1.2 and 7.8 mmâ?»Â¹ depend on applied spot size. The smaller the spot size, the higher the heterogeneity number, and inherently more sensitive to setup and range error. Dose perturbation, the variation of dose to 95 % in the target (Î”D95), was improved from 14.8 to 4.5 % as spot size was increased. The conformity index (CI) got worse from 1.01 to 1.22 as spot size was increased. Heterogeneity number identified the range of spot size which keeps the balance between dose conformity and robustness of PBST plan for each patient.
Conclusion: This study showed the method to identify required beam spot size for PBST.