Dosimetric assessment of TomoDirect radiotherapy and TomoHelical radiotherapy in the clinical implementation of total skin irradiation

Haiyang Wang^1*Yifei Pi¹Chunbo Liu¹Panfeng Wang²Chengzhang Long²Yaping Qi³Fanyang Kong¹Bin Han¹Fangna Wang¹Fei Jia¹Lele Liu¹Dandan Xu¹Tengfei Ji¹Huijuan Wu¹Yuexin Guo^1*

• ¹The First Affiliated Hospital of Zhengzhou University, Zhengzhou, China
• ²Henan Provincial Institute of Metrology and Testing Sciences, Zhengzhou, China
• ³Beijing Institute of Metrology, Beijing, China

Purpose: This study aims to compare the technical characteristics of tomodirect (TD) radiotherapy and tomohelical (HT) radiotherapy in total skin irradiation (TSI) applications. We conducted a comprehensive evaluation of dosimetric parameters and delivery efficiency in TD-based treatment planning to establish clinical guidelines for modality selection in mycosis fungoides. Materials and Methods: This retrospective study analyzed eight mycosis fungoides patients treated with TSI between June 2020 and June 2023, utilizing a 5-mm-thick diving suit bolus to enhance skin dose distribution with a prescription of 24 Gy delivered in 20 fractions (5 fractions/week). Comparative treatment planning employed both TD and HT techniques, with TD plans utilizing 7, 9, and 11 equally spaced coplanar beams (0°starting angle) , Ring0, Ring1, Ring2, Ring3, and Ring4 of 1 cm thick away from the planning target volume (PTV) at the distances of 0, 1, 2, 3, and 4 cm and other normal tissues (NT) were generated, the auxiliary structures were completely blocked during planning. The other plan parameters remained consistent. Plan quality assessment compared target mean dose (PTVmean), homogeneity index (HI), conformity index (CI), and organ-at-risk (OAR) doses between techniques, with additional evaluation of treatment delivery efficiency through time comparisons. Results: When using NT, Ring4, and Ring3 auxiliary structures in complete-block mode, TD plans with more 9 beams demonstrate comparable PTVmean, HI, and CI to HT plans, whereas TD plans less than9 beams exhibit inferior target coverage. Neither HT nor TD plans meet clinical requirements when Ring2, Ring1, or Ring0 structures are fully blocked. However, both techniques fail to satisfy OAR constraints when Ring2/Ring1/Ring0 are blocked. Treatment delivery times remain similar between modalities with NT/Ring4/Ring3 blocking, but efficiency significantly decreases for both when deeper structures (Ring2/Ring1/Ring0) are included in the blocking protocol. Conclusion: When employing complete-block mode for NT, Ring4, and Ring3 structures, TD plans utilizing more 9 beams demonstrate comparable dosimetric performance to HT plans in terms of target coverage, OAR sparing and treatment delivery efficiency. However, both modalities fail to meet clinical dosimetric requirements when deeper-seated structures (Ring2/Ring1/Ring0) are included in the blocking protocol.