Comparative Analysis of an Image-Guided Versus a Non-Image-Guided Setup Approach in Terms of Delivered Dose to the Parotid Glands in Head-and-Neck Cancer IMRT

Purpose: To assess the impact of interfractional variations of shape and setup uncertainties on the dose to the parotid glands (PGs) in head-and-neck cancer intensity-modulated radiotherapy and image-guided radiotherapy (IGRT). Methods and Materials: Two scenarios were analyzed retrospectively for 10 head-and-neck cancer patients, treated with helical TomoTherapy (TomoTherapy Inc., Madison, WI): the IGRT scenario and the non-IGRT scenario. The initial dose-volume histograms derived from the planning computed tomography (PCT) scan and 120 recalculated dose-volume histograms of the PGs of each scenario and of corresponding fractions were compared. Setup errors, cumulative median doses (CMDs) for 6 fractions, overall volumes of the PGs, and volumes that received less than 1 Gy or more than 1.6 Gy per fraction were analyzed. Results: The mean decrease in the PG volume was 0.13 cm³/d. There was a significantly higher CMD than initially predicted (mean increase for 6 fractions, 1.13 Gy for IGRT and 0.96 Gy for non-IGRT). The volume that received less than 1 Gy per fraction decreased (mean difference to PCT, 1.36 cm³ for IGRT [p = 0.003] and 1.35 cm³ for non-IGRT [p = 0.003]) and the volume that received more than 1.6 Gy per fraction increased with increasing fraction number (mean difference to PCT, 1.14 cm³ for IGRT [p = 0.01] and 1.16 cm³ for non-IGRT [p = 0.006]). There was no statistically significant difference between the two scenarios (CMD, p = 0.095; volume that received <1 Gy per fraction, p = 0.896; and volume that received >1.6 Gy per fraction, p = 0.855). Conclusions: In the analyzed group the actual delivered dose to the PGs does not differ significantly between an IGRT and a non-IGRT approach. However, IGRT in head-and-neck cancer intensity-modulated radiotherapy is strongly recommended to improve patient setup.