Beyond surface dose: subcutaneous dose escalation underlies TomoTherapy's skin toxicity in breast radiotherapy.

Abstract

<h4>Purpose</h4>To elucidate the dosimetric mechanisms underlying the increased incidence of acute radiation dermatitis associated with Helical TomoTherapy (TOMO) compared to Volumetric Modulated Arc Therapy (VMAT) in whole-breast irradiation.<h4>Methods and materials</h4>A retrospective analysis included 40 patients with right-sided breast cancer receiving hypofractionated whole-breast irradiation (40 Gy/15 fractions), evenly matched between VMAT (Elekta Synergy) and helical TomoTherapy (TOMO) groups. Skin dose was evaluated using a multi-parametric approach: 1) Dose Surface Maps (DSM); 2) a novel 5-mm thick PTV-projected adjacent superficial tissue (P-PAST) structure; and 3) equivalent dose in 2-Gy fractions (EQD₂, α/β=10). Clinical skin toxicity was graded per RTOG criteria.<h4>Results</h4>Clinically significant (Grade 2-4) acute skin toxicity was substantially more frequent with TOMO (70%) than with VMAT (30%, p=0.026). DSM showed TOMO produced a more confined high-dose region on the skin surface compared to VMAT. However, TOMO delivered significantly higher physical and biologically effective doses to the subcutaneous P-PAST region: mean EQD₂ was 44.47 ± 1.33 Gy vs. 42.12 ± 0.76 Gy for VMAT (p<0.001). D2cc and maximum dose EQD₂ were also elevated in the TOMO group.<h4>Conclusions</h4>The heightened skin toxicity with TOMO is not predicted by conventional skin surface dose metrics but is explained by a significant dose escalation within the subcutaneous P-PAST region. Plan evaluation for TOMO should incorporate volumetric dose assessment of the dermal compartment to mitigate toxicity.