Role of Overall Treatment Time When Estimating TCP & NTCP of Head & Neck Radiotherapy Treatment Plans in Altered Fractionation

Introduction: The present study demonstrated role of overall treatment time when estimating tumor control probability (TCP) and normal tissue complication probability (NTCP) for moderately hypofractionated and accelerated fractionation schedules in head & neck treatment plans. Repopulation effect in the squamous cell carcinoma is an influencing factor that should be considered when evaluating TCP and NTCP in early responding tissue. This effect can be incorporated by the means of overall treatment time in days. Material and Methods: The proposed study separated in two parts. In the first case, we assumed four moderately hypofractionated schedules for demonstration, including conventional fractionation schedule (CFS) (۷۰Gy/۳۵ #), fractionation schedule ۱ (۶۶Gy/۳۰#), fractionation schedule ۲ (۶۰Gy/۲۴#) & fractionation schedule ۳ (۵۵Gy/۲۰#). Four independent volumetric modulated arc treatment plans were generated at different fractionation schedules for ۱۵ patient’s data set and therefore led to a total of ۶۰ treatment plans. The treatment plan created for CFS is the reference plan for comparison of calculated TCP & NTCP amongst the four plans. The rest three plans for each patient were created simply by changing the dose prescription for FS۱, FS۲ & FS۳, the mean total dose and dose per fraction. In the second scenario, conventional fractionation schedule (۶۶Gy/۳۳# with five fractions per week) compared against accelerated fractionation schedule (۶۶Gy/۳۳# with six fractions per week). The cumulative dose volume histogram for all treatment plans were used for TCP/NTCP estimation by Niemierko EUD, Poisson model and LKB model. The TCP/NTCP calculated in two different way for tumor & oral mucosa of head & neck site. Contrary to the second case, the overall treatment time (OTT) in days not accounted in the first case. Results: It was statistically significant difference (p<۰.۰۵) obtained between calculated TCP/NTCP in both moderately hypofractionated and accelerated fractionation schedules. Conclusion: There is significant impact of OTT and it should be considered when evaluating TCP/NTCP for early responding tissue.