Dosimetric Importance of the Implementation of Daily Image Guidance in Radiotherapy Practice

The purpose of this study is to show the dosimetric importance of using daily image guidance in radiotherapy treatment.

A sample of 30 patients with various head and neck cancers were retrospectively selected for the studies. The prescribed dose, 66 Gy/33 fractions was used for all patients who received 7 to 9 beams, 6 MV Intensity Modulated Radiation Therapy (IMRT) plan delivered by Varian Truebeam STx. Before the first fraction of the treatment, the patient's shift corresponds to isocenter was noted and corrected. Subsequently, the images were taken daily for all 30 patients and the maximum, mode, median and mean of all shifts were recorded and applied to the base plan and recalculated for quantitative analysis of tumour coverage and Organ at Risk's (OARs) doses using various dosimetric parameters such as Homogeneity Index (HI), Conformation Number (CN), Conformity Index (CI), Coverage Index (COVI), Dose Gradient Index (DGI) and Unified Dosimetry Index (UDI) of shift plan.

The results showed the Planning Target Volume (PTV) and the OARs values deteriorated from its base plan values in the various shift plans created by applying the patient setup errors analyzed using image guidance. Mean dose of maximum shift plan with a significant P value of 0.002, D2% of maximum shift plan with a significant P value 0.028, the D98% Values of maximum and mode plans with the significant P value 0.004 each, the D50% and D80% values of the maximum shift plans with their significant P values 0.001 and 0.002 respectively. Also, HI95%, CN95% and COVI values of the maximum shift plan showed much variation with significant P values of 0.004, 0.040 and 0.0004 respectively from their base plan values. There were significant changes observed in OARs values between base plan and shift plans.

Implementation of daily image guidance in radiotherapy is mandatory taking into account of its dosimetric importance and to achieve the goal of radiotherapy practice of sparing the critical organs without compromising the target coverage.