A calorimeter has been designed and constructed to measure absorbed dose directly in a radiotherapy clinic.

Introduction

The quantity of interest in the clinic is absorbed dose to water (i.e. soft tissue) and the calorimeter is constructed from graphite because the radiation properties of graphite are very similar to those of water. The calorimeter consists of a front end (the calorimeter itself), means for thermal isolation and temperature control, and a measurement system. The temperature control system must be sensitive enough to allow measurements of temperature rises of the order of 1 mK and control of the temperature of the calorimeter to within ± 0.2mK over several hours has been achieved. The system is portable, opening up the possibility of making routine measurements in user radiation facilities.

Technical considerations

One of the major areas of concern was heat transfer within the calorimeter itself during irradiation. A 3-D dose distribution requires 3-D modelling and a finite element package (Pafec) was used. Pafec is a software package which can be used for a wide range of finite element problems such as transient and static stress analysis, vibrational analysis, heat transfer, etc. For a thermal analysis, the user supplies the problem geometry, material properties and initial temperature distribution, and Pafec calculates the temperature distribution at any future time.

Results

The figures below show the results of the simulation of a calorimeter irradiation - a calculated 16 MV X-ray dose distribution is used as the "input". The images show the initial calculated heat distribution within the calorimeter (assuming an instantaneously delivered dose) and the development over time. Red indicates higher temperature and blue lower temperature. The dark bands indicate interfaces between the various graphite plates that make up the calorimeter.

Click on any of the following images to view an enlarged image.