Determination of optimal clinical target volume to planning target volume margins for conformal radiotherapy planning using image guidance system in rectal cancer in prone position

Context: In advanced radiotherapy techniques such as three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT), geometrical uncertainties are very crucial as they may lead to under dosing of tumor and over dosing of the nearby critical structures and hence, it is important to determine planning target volume (PTV) margins which are specific for every center. Aims: The aim of this study is to determine adequate clinical target volume (CTV) to PTV margins specific to our radiotherapy center. Settings and Design: To calculate CTV to PTV margins for rectal cancer patients in prone position using kV cone beam CBCT data sets. Materials and Methods: With the Patient immobilized in prone position using thermoplastic mask, a CT simulation was done and a comprehensive 3DCRT plan was generated. Daily kV CBCT was done to check the patient setup error. Daily setup errors were recorded and evaluated retrospectively. Results: CTV-PTV margin calculated for pelvis in the prone position was calculated using van Herk Formula and were found to be 0.5, 1.8, 0.7 cm in the lateral, longitudinal, and vertical directions, respectively. Conclusions: Image guidance is an effective method to evaluate patient setup errors. Good quality immobilization devices and stringent patient setup policies can help to reduce PTV margins further