Effect of Diode-Based Transmission Detector Measurement on Dose Perturbation during Delivery of 6MV Photon

Abstract

<h3>Purpose/Objective(s)</h3> Delivery dose quality assurance (QA) during treatment planning execution is the most direct and effective way to ensure that the prescribed dose is delivered accurately and the patient is irradiated safely. The purpose of this article is to evaluate the stability and sensitivity of the diode-based transmission detector (TD) installed on the beam path during radiotherapy, and analyze the effect of TD on the photon beam fluence and dose distribution. <h3>Materials/Methods</h3> The beam fluence with and without TD was measured for 6MV on a linear accelerator. The point doses at different depths (0.0cm∼15.0cm), different dose values (100MU∼500MU), different dose rates (100MU/min∼600MU/min) and different irradiation fields (3cm × 3cm∼20cm × 20cm) were measured by cylindrical three-dimensional water tank, and the transmission factors (TF) were calculated and analyzed. The percentage depth dose (PDD) and beam profile under different parameters were collected to analyze the effect of TD on ray quality and dose distribution. The effect of different irradiation fields on the γ passing rate was evaluated by a planar dosimeter. <h3>Results</h3> The diode-based TD has the high stability of 0.1% (1SD) and excellent position monitoring performance (≤0.2mm). The TF varies with the depth, dose, dose rate and irradiation fields. When the depth is 0cm, the TF has maximum value of 99.55%. For the depth of 0cm∼5cm, the TF increases with the increase of dose and dose rate (<i>R</i>=0.91, 0.75). For the depth of 5cm, the TF is least affected by dose and dose rate (1SD: 0.01%, 0.07%), and decreased with the increase of irradiation fields, 98.94% for 3cm × 3cm. When the depth is >5cm, it decreases with the dose increasing (<i>R</i>=−0.88∼−0.26), and increases with the dose rate increasing (<i>R</i>=0.78). The average TF under all the above conditions is 98.46%, that is, beam attenuation is 1.54%. The movement of the position of maximum dose depth is 0.124cm for 5cm × 5cm and 0.250cm for 3cm × 3cm to the surface of the water (0cm), otherwise ≤0.003cm. TD increases dose in build-up region, 1.49% for fields of 20cm × 20cm. TD effects ray quality (≤0.0028), beam symmetry and flatness (≤0.68%). The γ passing rate does not change under the commonly used indexes of 3%/3mm and 2%/2mm. <h3>Conclusion</h3> The diode-based TD has high stability and excellent monitoring performance. Although it has little effect on ray quality, beam symmetry, flatness and γ passing rate, it will cause beam attenuation and increase surface dose.