Aims: The aim of this study was to compare the plan results that were obtained by using different calculation grid sizes ranging from 3mm to 10mm, and the same dose calculation algorithm Pencil Beam (PB), in Intensity Modulated Radiotherapy (IMRT) for different treatment sites Head-And –Neck, Pelvis (Carcinoma Cervix) And Brain Cancers. Introduction: Ever since the advent and development of treatment planning systems, the uncertainty associated with calculation grid size has been an issue. Even to this  day , with highly sophisticated 3D conformal and intensity-modulated radiation therapy (IMRT) treatment planning systems (TPS), dose uncertainty due to grid size is still a concern. Methods and Material: Twelve patients in which four patients of Head-And –Neck, Pelvis And Brain tumours respectively were considered for the study. IMRT Plans were generated for a 6,600cGy, 5,000cGy & 5,400cGy prescribed doses for Head-and–Neck,  Pelvis and  Brain  tumours respectively using Oncentra v 4.3 TPS . For each patient, dose calculation with PB algorithms using dose grid sizes of 3.0 mm, 5.0 mm, and 10.0 mm were performed. Results:  The plans were evaluated as per the ICRU guidelines and dose constraints were maintained as per the  Quantec  guidelines. The dose differences for the varying grid sizes in  Tumour Volumes and Organs at Risk were analyzed and tabulated. Conclusions: Overall, the effect of varying grid size on dose variation appears to be insignificant. However, 3 mm is recommended to ensure acceptable dose calculations, especially in high gradient regions.

Keywords: Dose grid; 2D array; Organs at risk; Intensity-modulated radiotherapy; Dose-volume changes; Head-and-neck cancers.