Analysis of Cone Beam CT based inter Fractional Set up Errors in Intensity Modulated Radiotherapy and Need for Safety Margins in Head and Neck Cancer Patients: An Institutional Experience

Abstract

Background: The efficacy of IMRT depends on the patient’s set up during each fraction, ideal treatment set up results in better tumor volume coverage and minimal toxicities to surrounding normal structures. random set up errors are the prevalent ones. Cone beam CT is a method of developing a volumetric image guiding system allowing better assessment of treatment set up errors.

Purpose: The aim is to evaluate and quantify inter-fraction set up errors in the IMRT treatment and to decide adequate margins for the target volumes in head and neck cancer patients Method: A total of 300 CBCTs were performed and analysed in 50 patients of head and neck carcinoma. Set up verifications were done using KV-CBCT, taken on first 3 consecutive days of treatment; followed by weekly CBCTs for 3 weeks. Systemic corrections were done on day 3 after treatment initiation. The mean and range for random and systemic errors, as well as safety margins were calculated.

Results: The shifts were reduced after systemic correction to 1.46mm, 1.46mm and 1.70mm respectively. Transitional shifts more than 2.5 mm were seen in 46%, 42% and 34% of cases in ML, AP and SI directions, whereas it was seen in 12%, 14% and 8% of cases after systemic corrections. After systemic correction, the adequate PTV margin was found to be 3 mm, to compensate for set up errors.

Conclusion: This study suggested that CBCT for initial 3 consecutive days is sufficient to overcome set up errors, provided that systemic corrections are done if mean set up errors are >2.5 mm.