U‐E‐T‐620: Radiobiological Implication of Margin for Target Expansion in Head and Neck IMRT with Daily IGRT

Abstract

Purpose: To account for organ motion and set up uncertainties a margin is added to the clinical target volume (CTV) to form the Planning Target Volume (PTV). There exists significant institutional variability of margins employed between CTV to PTV. The margin used has significant implication for extra tissue (PTV‐CTV) dose and directly related with normal tissue complication. This study quantifies the setup uncertainties to optimize the PTV margin and its radiobiological implication for target expansion in H&N cancer. Methods: Nine Nine H&N patients treated with IMRT and daily IGRT with on‐board‐imaging (OBI) were chosen for this study. Using Eclipse treatment planning system (TPS), treatment plans were generated for different margins ranging from 0 to 10 mm subject to the same optimization criterion for PTV and OAR using 6 MV photon beam. The DVH, extra tissue volume and total MU were recorded for all these IMRT plans. NTCP was calculated using the Lyman Kutcher Burman model from DVH data. The daily setup errors from OBI for the entire treatment were also analyzed. Results: Analysis of the 9 patients setup with over 800 data points showed that 98% of the points are within ±5mm using daily IGRT. With increasing margin, the PTV volume increases linearly. There is a 4‐fold relative increase in extra tissue volume for margin increase from 0–10 mm With increasing PTV margin the NTCP also increases linearly for the parotid glands. Similar patterns were noticed for all other OARs. Conclusion: With OBI the setup uncertainty could be easily achieved within ±5 mm for 98% of the H&N treatments. Increase in PTV margin increases NTCP. It is concluded that PTV margin should be limited to ±5mm to reduce extra volume irradiation and also reduces NTCP of OARs.