Abstract

TIMING OF REPLANNING IN HEAD AND NECK CANCERS ON RADIATION BY IMRT AND ITS SIGNIFICANCE IN TUMOUR VOLUME AND CRITICAL STRUCTURE VOLUME AND DOSE

Author(s): Ashwini Gopal

BACKGROUND Radiation to head and neck cancers by Intensity Modulated Radiotherapy (IMRT) is associated with high-dose conformality to target volume, which results in reduced dose to the critical normal tissues. Average duration of radiation will be 6 to 7 weeks. During the course of treatment, anatomical variations like loss of weight or tumour shrinkage may lead to interfractional variation. This variation might result in inadequate coverage of target volume or over dosage of critical structures. Adaptive radiotherapy using replanning in between treatment is one of the ways to reduce such uncertainty. In this study, timing of replanning was analysed with respect to tumour and critical structure (parotid), volume change and dose received by parotids. The aim of the study is to determine the timing of replanning in head and neck cancer treated by IMRT and its significance to parotid volume and mean dose and tumour volume and Planned Target Volume (PTV). MATERIALS AND METHODS From October 2014 to January 2016, 40 patients of head and neck squamous cell carcinoma treated in our institution by IMRT were grouped into ARM1 (A1) replanning at 15 fractions and ARM2 (A2) replanning at 20 fractions of radiation. Paired t-test was applied to analyse the difference and level of significance between initial scan and repeat scan. The difference was considered significant, if p-value was less than 0.05. RESULTS In A1 group, volume comparison showed statistically significant difference in target volume- PTV 703.05 cm3 vs. 554.71 cm3 (p=0.0005), GTV T (tumour) 52.86 cm3 vs. 36.47 cm3 (p=0.0001), and GTV N (node) 106.28 cm3 vs. 62.95 cm3 (p=0.0124) and left parotid volume 22.97 cm3 vs. 17.61 cm3 (p=0.0001) and right parotid volume 21.02 cm3 vs. 17.27 cm3 (p=0.0000). Mean dose to left parotid 38.57 Gy vs. 37.92 Gy (p=0.6776), mean dose to right parotid 40.60 Gy vs. 41.49 Gy (p=0.5733). In A2 PTV (cm3) 751.03 vs. 645.98, p=0.0000; GTV T (cm3) 52.51 vs. 28.01, p=0.0005; GTV N (cm3) 44.38 vs. 23.03, p=0.0001; left parotid volume 23.04 cm3 vs. 15.94 cm3 (p=0.0000); and right parotid volume A2-25.21 cm3 vs. 16.22 cm3 (p=0.0000). Mean dose to left parotid 37.18 Gy vs. 35.86 Gy (p=0.5930) and mean dose to right parotid 34.89 Gy vs. 36.15 Gy (p=0.6558). CONCLUSION Both treatment arms showed significant difference in volume though did not have benefit in dose to critical structures. Concluding mid treatment replanning is beneficial in all cases of head and neck cancers treated by IMRT.