Submandibular function recovery after IMRT in head and neck cancer: A prospective dose modelling study.

PURPOSE: To estimate the dose response relationship for submandibular gland (SMG) recovery using salivary scintigraphy in patients diagnosed with head and neck cancer treated with curative image guided chemoradiation. MATERIAL AND METHODS: Ninety newly diagnosed head and neck cancer patients (T1-3, N0-2c, M0) treated with intensity modulated radiotherapy on a prospective clinical trial were assessed for salivary toxicity at predefined intervals using dynamic salivary scintigraphy. The SMG function was measured using salivary excretion fraction (SEF) ratios at baseline and 6 monthly. Tolerance dose (TD) 50 for submandibular gland was estimated from dose response curves. RESULTS: The mean SEF ratio of 180 SMGs decreased at 6 months with a nadir at 12 months after treatment (SEF ratio 15%) and progressively recovered over time reaching 38% over 24 months. There was significant inverse correlation between SEF ratio and mean SMG dose at 6 months (r = -0.18, p = 0.04); 12-months (r = -0.36, p < 0.001); 18-months (r = -0.48, p < 0.001); 24-months (r = -0.42, p < 0.001); and more than 24-months (r = -0.56, p < 0.001). The estimated TD 50 values at 1 year and 2 year post treatment were 36 Gy and 44 Gy respectively with SEF ratio of ≤45% used to define severe xerostomia. For every 1 Gy reduction in mean dose below 54 Gy, there is 2-2.5% reduction in the probability of severe xerostomia. CONCLUSION: The submandibular gland function declines after radiotherapy with a nadir at 12 months and there is incomplete recovery over time with continued improvement over 24 months. The TD 50 at 1 year and 2 year was 36 Gy and 44 Gy with a 2-2.5% reduction in the probability of severe xerostomia for every 1 Gy reduction in mean dose.

Effect of imaging frequency on PTV margins and geographical miss during image guided radiation therapy for prostate cancer.

BACKGROUND: The relationship between frequency of imaging during image guided radiation therapy (IGRT) and planning target volume (PTV) margin remains unclear. This issue is of practical significance given resource and time intensive nature of IGRT. The purpose of this study was to evaluate PTV margins with predefined and commonly used less-than-daily IGRT schedules using data obtained from patients treated with daily IGRT for prostate cancer. METHODS AND MATERIALS: Daily setup error and 3-dimensional daily alignment data for a total of 108 consecutive patients with prostate cancer treated with 2700 fractions of daily image guidance on tomotherapy were retrospectively analyzed. Five IGRT scenarios were simulated: alternate day, twice weekly, once weekly, first 3 days only, and no image guidance. The daily alignment data were modeled to simulate the 5 predefined scenarios by applying appropriate corrections to determine the PTV margin for each image guidance scenario. The data were also analyzed to predict possible geographical miss in any direction using 2 frequently used PTV margins of 7 and 5 mm for all the scenarios. RESULTS: Decreasing the frequency of image guidance increased the mean systematic error and the standard deviation of the systematic error. With decreased image guidance frequency, an increase in PTV margins was required to achieve adequate coverage of the clinical target volume. With reduction in image guidance from 50% to 12%, a gradual increase in percentage of fractions with predicted geographical miss using an isotropic PTV margin of 7 or 5 mm was seen. With every 15% decrease in imaging, a 5% increased risk of geographical miss was estimated. CONCLUSIONS: The use of less-than-daily IGRT requires larger PTV margins for patients treated with intensity modulated radiation therapy for prostate cancer. With every 15% reduction, a 5% increased risk of geographical miss was estimated.