63. Quantification of skin dose changes during Tomotherapy for head-neck cancer by dose-of-the-day recalculation

Abstract

Purpose An increase of skin dose during RT for head-neck (HN) cancer due to anatomy deformation may potentially translate into an increased risk of skin toxicity, suggesting a potential benefit from planning adaptation (ART) to counteract this effect. Current study applied an original method for dose-of-the-day calculation in quantifying changes of skin dose during Tomotherapy for head and neck (HN) cancer in order to identify those patients that could benefit of ART. Methods Planning CTs of 32 patients (SIB: 54/66 Gy, 30 fractions) were deformable registered to MVCT images taken at fraction 15 and 30 using a constrained-intensity-based algorithm. Dose-of-the-day were calculated on the deformed images; this approach was previously validated by comparing dose distributions of deformed CT and diagnostic kVCT of the same day. Superficial body layers (SL) were considered as a surrogate of skin, considering a 2 mm thickness. Skin dose variation during therapy ( Δ SL) in terms of fraction of SL receiving at least 95%–105% of the prescribed PTV dose (66 Gy) were considered. Based on the previous validation, a change ≥ + 1 cc was assessed to define patients that may benefit of ART. Results Δ SL occurred during treatment with individually variable patterns, depending on several factors like the PTV location/dimension/shape/shrinkage. Only 3 patients experienced >1 cc change of SL95%-105% during treatment (maximum variation: 6 cc) while negative Δ SL95%-105% > 1 cc, up to 17 cc, were seen in 15/33 patients. A negative change was correlated with the presence of some overlap between PTV and SL at planning (p = 0.03). Δ SL 95% are shown for all patients in Fig. 1. Download : Download high-res image (301KB) Download : Download full-size image Conclusions Most patients treated with a SIB approach with Tomotherapy for HN cancer showed negative or irrelevant changes to the dose delivered to the skin. Less than 10% of patients seem to be candidate to ART to counteract any significant skin dose increase during therapy.