Session Item

The aim of the current study is to compare surface dose while varying the distance between beam spoiler and phantom surface as well as commissioning of compensator based Total Body Irradiation (TBI).

Elekta Synergy®Platform™ treatment unit with 6MV photon beam, gantry angle 270°, collimator angle 45° and field size 40x40 cm², SSD at 385cm was set up for this study. The TBI commissioning was performed for the bi-lateral extended SSD treatment technique. Beam spoiler made by Acrylic sheet of 1.0cm thickness used for percentage depth dose (PDD) measurement at 10cm, 20cm, 30cm and 40cm from the phantom surface to evaluate dose at maximum depth (Dmax). All the measurements were carried out with the acrylic phantom size of 30x30x30cm³ using a Parallel plate (PPC40) and Farmer (FC65) ion chamber for depth ranging from 0.1cm to 1.5cm and 1.0cm to 24.0cm with water cans on both sides to account for lateral scatter. As a part of TBI commissioning, beam profile measurements and HVL were performed using both Perspex (Prpx) and Aluminium (Al) material. An in- house Rice flour (RF) phantom was customized with different anatomical regions such as head, neck, shoulder, umbilicus, thigh, knee and ankle for quality assurance purpose. The separation of each anatomical region was measured using a vernier calliper for calculating Monitor units and thickness of the compensator. The calculated MU was delivered while keeping Prpx or Al compensators to compensate for varying thickness of the phantom body. In addition, In-Vivo Diodes (IVD) were placed on the different regions of the RF phantom to measure the delivered measured dose which is compared with the calculated dose for validation. 

The results of PDD revealed that the measured surface dose varies with the distance of the beam spoiler from the phantom surface. An increased Dmax and surface dose was observed when the beam spoiler moved from the phantom surface. The D_max was observed 0.2cm at 10cm and 20cm distance. Similarly, the D_max was found to be 0.3cm, 0.4cm at 30cm and 40cm. The flatness and symmetry of the beam profile were observed at 104.8% and 100.2%. In addition, a half-value layer of Prpx and Al was 17cm and 8cm respectively. The percentage of deviation between the measured and calculated dose of each anatomical region was less than ±4.7% for both Prpx and Al.

The present study concluded that to achieve a uniform dose to the whole body during TBI, the beam spoiler can be kept at an optimum distance ranging from 10cm to 20cm to get the desired skin dose. After successful commissioning and measurement validation, we could establish a compensator based TBI technique for Elekta Synergy®linac.