DOSIMAP: A HIGH-RESOLUTION 2-D TISSUE EQUIVALENT DOSEMETER FOR LINACQA AND IMRT VERIFICATION V. Collomb-Patton 1 , P. Boher 1, *, T. Leroux 1 , J.-M. Fontbonne 2 , A. Batalla 3 and A. Vela 3 1 ELDIM, 1185 Rue d’Epron, F14200 He ´rouville Saint Clair, France 2 Laboratoire de Physique Corpusculaire, 6 Bd du Mare ´chal Juin, F14050 Caen, France 3 Centre Franc ¸ois Baclesse, Avenue Ge ´ne ´ral Harris, BP 5026, F14076 Caen Cedex 5, France New generation of radiation therapy accelerators requires highly accurate dose measurements with high spatial resolution pat- terns. IMRT is especially demanding since the positioning accuracy of all the multi-leafs should be verified for each applied field and at any incidence. A new 2-D tissue equivalent dosemeter is presentedwith high spatial resolution that can fulfil these tasks. A plastic scintillator sheet is sandwiched between two polystyrene cubes, and the emitted light is observed by a high- resolution camera. A patented procedure allows efficient discrimination of the scintillation proportional to the dose from the parasitic Cerenkov radiation. This extraction made on the cumulated images taken during an irradiation field at a rate of 10 images s 21 provides high-resolution mapping of the dose rate and cumulated dose in quasi real time. The dosemeter is tissue equivalent (ICRU-44) and works both for electrons and photons without complex parameter adjustment, since phantom and detector materials are identical. The calibration is simple and independent of the irradiation conditions (energy, fluence, qualityand so on). The principle of the dosemeter and its calibration procedure are discussed in this paper. The results and, in particular, the dose depth profiles are compared with standard ionisation chamber measurements in polystyrene for both photons and electrons. Finally, the detector specifications are summarised and one example of complex IMRT field is discussed. INTRODUCTION Highly conformal radiation treatments involving intensity-modulated fields are more and more applied since last years (1,2) . The purpose of these techniques is to achieve dose distributions of increased conformity to the target volumes while reducing the doses to non-target organs. Often this can only be achieved by increasing the dose vari- ations on very small scales. Such dose distributions can be created by segmental techniques (‘step and shoot’), dynamic techniques (DMLC) and by the application of absorbers. Since these techniques provide strongly modulated dose profiles, great care must be taken to ensure their correct application. In this context, the dosimetric verification of treat- ment plans is one of the major tasks for correct QA and IMRT verification. Many efforts have been made recently to develop new efficient 2-D dose- meters (3,4) . Film dosimetry is being used in the field since many years and is still the most important technique employed to verify the 2-D dose distri- butions (5,6) . Its main advantages are a very high spatial resolution and water equivalence, but hand- ling and processing are significant variables for dose accuracy. Moreover, it is not a real-time measure- ment, and systematic meticulous calibration is required. Other methods, such as gel dosimetry (7) , portal imaging (8) , semiconductor detector array (9) and 2-D ionisation chamber arrays (10) , have not gained broad acceptance up to now for different reasons. Gel dosimetry is attractive because it pro- vides real 3-D measurements, but it is difficult to handle and not cost-effective. Portal imaging is easy to use, but very difficult to calibrate and real doses mappings cannot be obtained. Semiconductor or ionisation chamber arrays, even if not tissue equival- ent, can be calibrated to provide the real doses but their spatial resolution is limited to 7 –10 mm. The occurrence of dead zones on the irradiation plane does not allow a real cumulated dose measurement except if different irradiations are performed at different locations. Moreover, their duration is limited by radiation hardness effects, especially for semiconductor diodes. Plastic scintillators show many desirable qualities for dosimetry, including tissue equivalence, dose lin- earity, energy independence, reproducibility, resist- ance to radiation damage and near temperature independence (11) . Therefore, these detectors do not require the usual conversion and correction factors used for other commonly used detectors to convert the dosemeter reading to absorbed dose. They also work for photons and electrons without additional calibration. Their main problem is the occurrence of parasitic Cerenkov radiation that must be discrimi- nated in order to measure the dose accurately. For punctual detectors, different solutions have been investigated. Archambauld et al. (12) proposed the use of green light emitters but Cerenkov contribution is never completely suppressed. A better solution based on spectral filtering was proposed by Frelin *Corresponding author: pdoher@eldim.fr # The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org Radiation Protection Dosimetry (2008), Vol. 131, No. 1, pp. 100–109 doi:10.1093/rpd/ncn228 Advance Access publication 30 August 2008