A Quality Index for Total Skin Irradiation (TSI-QI): Validation of Total Skin Electron Irradiation and Total Skin Helical Irradiation Techniques.

Abstract

Total skin irradiation (TSI) techniques vary widely in dosimetric performance, yet no standardized metric exists to evaluate their overall quality. Current guidelines address parameters such as dose homogeneity, depth-dose fall-off, and x-ray contamination, but lack a comprehensive quantitative tool. This study introduces and validates the Total Skin Irradiation Quality Index (TSI-QI), a novel composite metric for assessing TSI techniques based on these 3 critical parameters. Radiographic films were placed in transverse sections of an anthropomorphic phantom and irradiated using full-treatment TSI to generate entrance-to-exit dose profiles (EEDP). From these profiles, 3 key metrics were derived: dose uniformity within the target, dose fall-off beyond the target, and internal x-ray contamination. Each factor was normalized against established clinical thresholds and combined to calculate the TSI-QI using the formula TSI-QI = H^f + D^f + X^f, where H^f represents homogeneity, D^f denotes dose fall-off, and X^f indicates x-ray contamination. The index's efficacy was evaluated on 2 widely adopted techniques: Stanford total skin electron irradiation (TSEI) and total skin helical irradiation (TSHI). TSEI yielded a TSI-QI score of 0.554, within the acceptable quality range, whereas TSHI scored 1.893, exceeding the threshold. Thresholds were defined according to AAPM Report No. 23 and European Organization for Research and Treatment of Cancer recommendations (EORTC). Detailed analysis showed that TSEI provided superior homogeneity, sharper fall-off, and lower x-ray contamination compared with TSHI. These dosimetric parameters are directly related to clinical safety and toxicity, because dose fall-off and photon contamination reflect healthy tissue protection, whereas homogeneity ensures effective skin tumor coverage. The TSI-QI, thus, offers a reliable tool for evaluating and comparing TSI techniques, supporting institutional quality assurance and cross-center standardization while helping identify protocols unsuitable for safe clinical implementation.