Dosimetric Verification of Different Volumetric Arcs of Total Body Irradiation

Abstract

Main aim of this study is to find the difference between calculated and measured doses in volumetric arc radiation therapy (VMAT) of total body irradiation (TBI) technique and to find the uniformity of doses delivered at the junction region of adjacent arcs using Gafchromic films and silicon diodes. The study contains twenty (20 no.) VMAT TBI patients treated between Mar,2014 to Feb,2020 on Varian Trilogy for retrospective dosimetric analysis and verification. The clinically deliverable plans using pair of arcs (CW&CCW) at each iso-center along the length of body from cranial to caudal direction uptomid thigh were made using eclipse treatment planning system (TPS) from varian. The verification plans for each isocenter were made and delivered on arc phantom. The pre-treatment verification of plans is done using PTW octavious phantom Fig:1a&b) Arc Phantom with EDP-15 p-type Diode and gafchromic film using large virtual CT images of phantom. The verification of surface doses (In-vivo dosimetry) at different arc junctions were measured using silicon diodes, gafchromic films. The Gaf-chromic films of 4cm in longitudinal direction (more than junction overlapping region 3cm) are used for dose measurement while doses measured by diode as per the arrangement in fig 1a and point doses using 0.6cc FC65G (Sr No. 2757 from Rosalina). Films were scanned using EPSON 1100 flat bed scanner and corresponding values were noted. The Gamma analysis and average dose difference at the skin surface between TPS calculated and measured with different dosimeters were found. TBI using VMAT is a new approach to our center for the patients who undergo BMT. In order to cover whole body from cranial to caudal height of patient up to mid-thigh, multiple successive isocenters just like cranio spinal irradiation. The position of anterior posterior and right left coordinates is same for all isocenters and change is only in longitudinal shifts.