The effect of immobilisation devices on radiotherapy dose distributions
Creators: name: Gray, Alison
Publisher: name: RMIT University
Year: 2024

description: In radiotherapy, when an x-ray beam passes through an immobilisation device, the dosimetric effects are generally ignored or a single transmission factor is applied to correct the dose calculations. When the immobilisation device is not of uniform density or thickness or is not made of radio-translucent material, large inaccuracies in the dose calculation may occur. By including the physical characteristics of the immobilisation device in the dose calculation by the treatment planning system (TPS), a more accurate dose calculation may be obtained. <br><br>For some patient setups utilising an immobilisation device, the beam may also traverse a large air gap (up to 15 cm) prior to entering the patient. Previous studies have investigated the ability of TPSs to calculate the dose beyond small air gaps, such as those which occur within the body, but little work has been reported for large air gaps created by immobilisation devices.<br><br>The aim of this study was (i) to determine the accuracy of the Eclipse™ pencil beam convolution algorithm (when using the equivalent tissue air ratio method of inhomogeneity correction) to calculate the dose distribution and linear accelerator monitor units (MUs) required for a 6 MV x-ray beam passing through an immobilisation device which is included in the dose calculation and (ii) to determine the accuracy of the algorithm to calculate the dose beyond large air gaps.<br><br>Treatment plans were created using Eclipse for four different commercial immobilisation devices. The dose distribution and MUs for the plans were calculated with and without the immobilisation device included. A simple phantom geometry using solid water slabs and a complex geometry using an anthropomorphic phantom were studied for each immobilisation device.<br><br>When the immobilisation device was not included in the dose calculation, the maximum difference between the measured and calculated dose was -8.4% and -7.7% for the simple and complex cases respectively. The results were within an acceptable clinical tolerance level of 2.5% for all cases when the immobilisation device was included in the dose calculation.<br><br>An air gap arrangement was simulated by supporting water equivalent slabs (0.2 to 4 cm thickness) above a water phantom. The depth dose was measured beyond a range of air gaps (1 to 15 cm) and the results compared to Eclipse. The attenuation coefficient for the water equivalent slabs was measured in air and the scattered and primary dose components for the experimental setup were also derived.<br><br>The results indicated that as the air gap increases, the dose reduces at the water surface. For larger air gaps, the dose beyond the air gap is also reduced at depth. Eclipse™ computed the same dose for all air gap dimensions for a given slab thickness and did not predict the reduction in dose after the air gap. In the specific case where 2 cm thick water equivalent material was placed before a 15 cm air gap, Eclipse over-predicted the dose by 34% at the surface of the water phantom and by 3%, 3% and 2% at depths of 5, 10 and 15 cm respectively<br>