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TU‐F‐BRE‐09: Towards the Establishment of Dosimetric References in Small Fields Using the New Concept of Dose‐Area Product
Author(s) -
Dufreneix S,
Bordy J,
Delaunay F,
Delaunay F,
Daures J,
Gouriou J,
Le Roy M,
Ostrowsky A,
Rapp B,
Sommier L
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4889317
Subject(s) - dosimeter , ionization chamber , calorimeter (particle physics) , graphite , dosimetry , materials science , absorbed dose , ionization , field (mathematics) , radiation , nuclear medicine , radiochemistry , physics , optics , chemistry , mathematics , ion , composite material , medicine , quantum mechanics , detector , pure mathematics
Purpose: To establish dosimetric references of absorbed dose in water in radiation fields smaller than 2 cm used in radiotherapy thanks to a new methodology based on the use of dosimeters larger than the field size. Methods: A new graphite calorimeter was constructed with a large sensitive volume (diameter of the core: 30 mm). This primary dosimeter was fully characterized and compared to previous LNE‐LNHB graphite calorimeters in a 60Co large field. A specially designed graphite parallel‐plate ionization chamber with a 30 mm collecting electrode was also assembled and tested. Measurements were then conducted in two 6 MV small circular fields of 2 cm and 1 cm diameter respectively, using the new concept of dose‐area product instead of punctual dose commonly used in radiotherapy. Results: The dose rate established in a large 60Co field with the new calorimeter is in agreement within 0.4% with previous calorimeters. The ionization chamber shows good characteristics except for a 0.06% drift per hour in water. The ratio of calorimetric against ionometric measurements in the 2 cm diameter field is 1.1% higher than the one in the 1 cm diameter field (with respectively 0.30% and 1.03% type A uncertainty for each field). Conclusion: Results presented here highlight the possibility of measuring dose‐area products in small fields with a graphite calorimeter and a parallel‐plate ionization chamber. Measurements in a 0.75 cm diameter field are already underway to confirm the trend observed in the 2 cm and 1 cm diameter fields. The last step to establish precise dosimetric references in small fields is to calculate correction factors thanks to Monte Carlo simulations.