Construction of a large graphite calorimeter for measurements in small fields used in radiotherapy

Abstract

Abstract. As the national metrology institute for ionization radiation the LNE-LNHB develops, maintains and transfers dosimetric references for high energy x-ray radiotherapy beams. Through the European project MetrexRT, a new approach has recently been developed to extend the dosimetric references to small fields (less than 2 cm wide). This method is based on the measurement of an absorbed dose on a surface larger than the beam. A new primary dosimeter (a graphite calorimeter) with a large surface has been built and characterized as well as a large surface ionization chamber. Both dosimeters can now be considered operational for measurements in small beams.

Feasibility of using a dose-area product ratio as beam quality specifier for photon beams with small field sizes

“…Considering that the absolute measurement of DAP at LNE-LNHB refers to an integration area with radius 1.5 cm [25,26] Table 8 and Table 6).…”

“…The accuracy of Monte Carlo calculation of absorbed dose in regions extending laterally beyond the edge of the radiation was evaluated by comparing the results of simulations with the PENELOPE and EGSnrc codes, as well as via calculations and measurements of the mean absorbed doses in the core of two graphite calorimeters in small 6 MV beams produced by the GE Saturne 43 accelerator. Specifically a graphite calorimeter (GR11) with a core of 30 mm diameter and 3 mm thickness and a graphite calorimeter (GR10) with a core diameter of 6 mm and 6 mm thickness were used [26,47]. The GR11 calorimeter was designed for absolute measurements of DAP in field sizes smaller than 2 cm [26] and the size of the calorimeter core is the result of a compromise between two conflicting options: enlarging it to cover the greatest possible part of the scattered beam and reducing it to increase the measured temperature elevation in the core.…”

“…Specifically a graphite calorimeter (GR11) with a core of 30 mm diameter and 3 mm thickness and a graphite calorimeter (GR10) with a core diameter of 6 mm and 6 mm thickness were used [26,47]. The GR11 calorimeter was designed for absolute measurements of DAP in field sizes smaller than 2 cm [26] and the size of the calorimeter core is the result of a compromise between two conflicting options: enlarging it to cover the greatest possible part of the scattered beam and reducing it to increase the measured temperature elevation in the core. On the other hand, the size of the GR10 calorimeter ensures that the core is widely within the field size for the largest circular beam (2 cm diameter) considered in this work.…”

“…Conversely, even in the case of a composite clinical field, Monte Carlo calculations in [24] showed that differences in response among point-like detectors substantially diminish, if the detector signal is integrated over the whole radiation field. The Laboratoire National Henri Becquerel (LNE-LNHB) recently developed a calorimeter for absolute measurement of DAP in small field sizes, making this quantity available for transfer from the calibration laboratory to the user's beam [25,26]. Then, in the scenario of a small beam dosimetry based on DAP references, defining the beam quality specifier in terms of DAP to link the calibration to the user's beam becomes an attractive possibility.…”

EMRP Project HLT 09 – Metrology for radiotherapy using complex radiation fields

“…The first part of the work in this WP is described in Dufreinex's paper [2] in this proceeding. It shows the design and construction of the specially designed graphite calorimeter able to be used in small radiation fields (smaller than 2 × 2 cm²) like those used in SRS and SRT (Stereotactic Radio Surgery and Radio Therapy) and IMRT (Imaging Modulated Radio Therapy).…”