Tomoharu Sato

General ion recombination effect in a liquid ionization chamber in high-dose-rate pulsed photon and electron beams

Abstract Liquid ionization chambers (LICs) are highly sensitive to dose irradiation and have small perturbations because of their liquid-filled sensitive volume. They require a sensitive volume much smaller than conventional air-filled chambers. However, it has been reported that the collection efficiency has dependencies on the dose per pulse and the pulse repetition frequency of a pulsed beam. The purpose of this study was to evaluate in detail the dependency of the ion collection efficiency on the pulse repetition frequency. A microLion (PTW, Freiburg, Germany) LIC was exposed to photon and electron beams from a TrueBeam (Varian Medical Systems, Palo Alto, USA) linear accelerator. The pulse repetition frequency was varied, but the dose per pulse was fixed. A theoretical evaluation of the collection efficiency was performed based on Boag’s theory. Linear correlations were observed between the frequency and the relative collection for all energies of the photon and electron beams. The decrease in the collected charge was within 1% for all the flattened photon and electron beams, and they were 1.1 and 1.8% for the 6 and 10 MV flattening filter-free photon beams, respectively. The theoretical ion collection efficiency was 0.990 for a 10 MV flattened photon beam with a dose rate of 3 Gy·min−1. It is suggested that the collected charge decreased because of the short time intervals of the beam pulse compared with the ion collection time. Thus, it is important to correctly choose the pulse repetition frequency, particularly when flattening filter-free mode is used for absolute dose measurements.

Optimal Parameter Decision Method in VMAT for Lung Tumors with Respiratory Motions

When performing lung cancer treatments using volumetric modulated arc therapy (VMAT) technique, dose error related to respiratory motion of tumors and multi leaf collimator (MLC) movement may occur. The dose error causes daily dose variation in multiple fractionations irradiation. The purpose of this study is to verify the influence of the respiratory motion and the MLC movement on the daily dose variation, and to confirm the feasibility of deciding robust planning parameter against the dose variation. We prepared 5 VMAT plans for imitating lung tumor in thorax dynamic phantom. Dose calculations of these plans were done taking into account the respiratory motions. We examined the relation between dose variation and two parameters that were number of respiration in an arc and MLC gap width. We presented the relationship between the dose variation and each parameters using regression analysis, and we could derive the approximation formula for estimating the dose variation using these parameters. We could estimate dose variation in another VMAT plans using the approximation formula and another plans parameters. By confirming dose variation in planning procedure using this estimation method, we may decide planning parameter taking the dose variation into account. So, we could establish the estimation method to decide adequate planning parameters in VMAT.