SU‐E‐T‐632: Study the Effect of Grid Size On Head and Neck IMRT Dosimetry

Abstract

Purpose: In IMRT, ICRU‐83 requires dose prescription to be at D50% also as mean dose. Dosimetry is impacted by the calculational grid size that is studied for head and neck IMRT with special consideration on the tumor control probability (TCP) and normal tissue complication probability (NTCP). Methods: Five head and neck patients treated with IMRT and daily IGRT were chosen for this study. Using Eclipse treatment planning system (TPS) treatment plans were generated for different grid sizes in the range 1.0 mm−5.0 mm for the same optimization criterion with a 6MV photon beam. The desired dose volume constraints were kept the same to provide uniform dose to the PTV. The dose volume histogram (DVH) was recorded for all IMRT plans and dosimetric data were compared. ICRU‐83 dose points such as D2%, D50% and D98% were also calculated. In addition, TCP and NTCP were calculated using LQ Poisson model and Lyman Kutcher Burman model respectively from DVH data. Results: The PTV mean dose decreases with increasing grid size from 1mm to 5mm with an average decrease of 1.5%. It is also found that TCP decreases with increase in grid size with an average decrease of 1.6%. Mean dose and NTCP for the right parotid increases with increase in grid size with an average increase in mean dose by 1.6% and NTCP by 4.8%. Similar patterns were observed for all other OARs: left parotid, mandible, brainstem and spinal cord. Conclusion: It is concluded that smallest possible grid size should be used for accurate dose calculation in head and neck IMRT planning. Smallest calculational grid also improves TCP for target volumes and reduces NTCP for normal tissues. The grid size effect is most important for smaller structures such as cochlea, optic chiasm and lens.