Evaluation of positioning accuracy and determination of the optimal margin for the planning target volume during radiation therapy on the Tomotherapy HD system

Relevance of research: The accurate positioning of patients during radiotherapy plays a crucial role in ensuring effective tumor control while minimizing the risk of overdosing normal tissues. The result of errors during patient’s setup may be target for underexposure of the cancer or overexposure of normal tissues. Therefore, Planning Target Volume (PTV) contains Clinical Target Volume (CTV) plus margin, which considers geometrical uncertainty due to organ motion, breathing and changing of patient position. Purpose of study: The purpose of this work is to calculate the margins using the van Herk formula and compare them with the values used in the International Oncological Center of Tomotherapy "UMIT". Methods: There were 212 patients in this investigation: 47 patients with brain tumors, 52 patients with pelvic tumors, 50 patients with breast cancer, 34 patients with head and neck cancer, 29 patients with esophageal cancer. The 5305 patient setup displacements were analyzed. The systematic and random errors were calculated for each case, and the optimal margins from CTV to PTV were calculated using the van Herk et al formula. Results: The results showed that in the case of the head and neck, brain, esophagus and pelvis the obtained values were within acceptable limits. The x and y offsets for the breast are outside the margins used in the clinic. Outcomes: In this work, the offsets from CTV to PTV were calculated and analyzed, taking into account systematic and random errors. The margins used in the clinic are sufficient for safe and high-quality radiation therapy. Exceeding values for the case of the breast may be due to respiration, which requires special monitoring and is the subject of further research. The use of IGRT and immobilization devices makes it possible to reduce dose delivery errors to the target, thereby increasing the safety of treatment