The effect of high energy photon beam in dose- volume parameters and integral dose of normal structures in Head and neck IMRT

Introduction: The aim of this study is to investigate the effect of partially used high photon energies on dosimetric parameters and integral doses of nasopharynx IMRT plans.   Materials and Methods: Two 7 coplanar IMRT plan were created for eleven patients (6 MV only IMRT plan in both phase1 and phase2 and 6 MV phase1-mixed energies (6 MV and 15 MV) phase2) with the equivalent dose constraints. The plans comparison was based on dose- volume histogram. Organ at risk sparing and the integral dose of normal structures were analyzed for target coverage, D95% to PTV50 and PTV70, V77 for PTV70, HI, and CI for both PTVs. Results: High energy IMRT plan (6 MV phase1- mixed energy phase2) had better outcome for D95% of PTVs compared to 6 MV only IMRT plan (68.84 and 71.5 for PTV70 in 6MV, and 6MV-mixed, respectively). In addition, D95% for PTV50 were 48.11and 49.2, respectively. Despite energies, HI and CI of the two plans were identical with each other. There were somewhat increase in V77 and Dmean of PTV70 for 6 MV-mixed compared to 6 MV plan but still in the tolerance limit. The 6 MV-mixed plan successfully reduced the Dmean, Dmax, D50% of organ at risks were compared to 6MV only technique. Moreover, volume getting 1,5,10, 30, 50 and 70 Gy and total integral doses of OARs and normal tissue plus IDs of OARs receiving low, mid and high dose regions were reduced by high energy plan. Conclusion: Our Results stated that using partial high energy photon beams can make an improvement dosimetry results in the head and neck IMRT treatment.