Determination of Setup Error and Planning Target Volume Margin With Daily Onboard Imaging in Patients of Head and Neck and Rectal Cancers Receiving Imrt : A Single Institution Study From North East India.

Introduction :The concept PTV ( planning target volume ) was introduced in ICRU 50 which states that PTV is a geometrical concept and is dened to select appropriate beam sizes and arrangements taking into consideration the net effect of all possible geometrical variation , in order to ensure that the prescribed dose is actually absorbed in the CTV. Planning target volume (PTV) in order to deliver the desired dose to the clinical target volume (CTV) accounts for systematic (Σ) and random ( σ) errors during conformal radiotherapy. Methods and Material: A total of 30 patients with head and neck( 15 in nos.) and rectal cancer (15 in nos.) undergoing radiotherapy with IMRT technique treated in Varian TRUEBEAM linac were included. A total no of 915 KV CBCTs were taken and daily shifts in all three directions for each patient were recorded. The Results: shifts in all the directions were recorded into a tabular format and using SPSS software the systematic ( Σ ) and random ( σ ) error were calculated for all the three directions and further the PTV margin were calculated using Van Herk's equation which came out to be : 0.5 cm for head and neck and 1.6 cm for rectal cancer. Conclusion: The study revealed isotropic PTV3D margins for head and neck to be 0.5 cm and for rectal cancers it came out to 1.6 cm .