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Towards the Establishment of Dosimetric References in Small Fields Using the New Concept of Dose-Area Product

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S Dufreneix

S Dufreneix*, J Bordy , F Delaunay , J Daures , J Gouriou , M Le Roy , A Ostrowsky , B rapp , L Sommier , CEA, LIST, Laboratoire National Henri Becquerel, Gif-sur-Yvette, France


TU-F-BRE-9 Tuesday 4:30PM - 6:00PM Room: Ballroom E

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.

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