Australasian Physical & Engineering Sciences in Medicine Volume 30 Number 1, 2007 25 Intensity modulated radiotherapy dosimetry with ion chambers, TLD, MOSFET and EDR2 film R. A. Kinhikar, R. Upreti, S. Sharma, C. M. Tambe and D. D. Deshpande Department of Medical Physics, Tata Memorial Hospital, Dr. Ernest Borges Marg, Parel, Mumbai, India Abstract Purpose of this study was to report in a together our experience of using ion chambers, TLD, MOSFET and EDR2 film for dosimetric verification of IMRT plans delivered with dynamic multileaf collimator (DMLC). Two ion chambers (0.6 and 0.13 CC) were used. All measurements were performed with a 6MV photon beam on a Varian Clinac 6EX LINAC equipped with a Millennium MLC. All measurements were additionally carried out with (LiF:Mg,TI) TLD chips. Five MOSFET detectors were also irradiated. EDR2 films were used to measure coronal planar dose for 10 patients. Measurements were carried out simultaneously for cumulative fields at central axis and at off-axis at isocenter plane (+ 1, and + 2 cm). The mean percentage variation between measured cumulative central axis dose with 0.6 cc ion chamber and calculated dose with TPS was -1.4% (SD 3.2). The mean percentage variation between measured cumulative absolute central axis dose with 0.13 cc ion chamber and calculated dose with TPS was -0.6% (SD 1.9). The mean percentage variation between measured central axis dose with TLD and calculated dose with TPS was -1.8% (SD 2.9). A variation of less than 5% was found between measured off-axis doses with TLD and calculated dose with TPS. For all the cases, MOSFET agreed within + 5%. A good agreement was found between measured and calculated isodoses. Both ion chambers (0.6 CC and 0.13 CC) were found in good agreement with calculated dose with TPS. Key words IMRT, Ion chambers, TLD, MOSFET, EDR2 film Introduction Intensity-modulated radiotherapy (IMRT) is a highly conformal treatment modality that requires precise verification. In our facility, IMRT is delivered with a dynamic multileaf collimator (dMLC) to treat cervix, prostate and head and neck cancers. Due to the complexity of technique and high dose gradient region, significant differences might be found between measured and treatment planning systems (TPS) calculated dose, especially in the vicinity of critical structures. Hence this technique demands a high level of dosimetric accuracy. The verification of these dose distributions is a prerequisite for a safe and efficient application. Patient-specific IMRT quality assurance is routinely performed for each patient at our centre. Approaches used to verify patient IMRT treatment fields prior to delivery are discussed elsewhere. 1-4 The choice of dosimeter for a given measurement depends on the geometry, the accuracy needed, and how extensive the measurements will be. 5 IMRT plans are Corresponding author: Rajesh A. Kinhikar, Department of Medical Physics, Tata Memorial Hospital, Dr. Ernest Borges Marg, Parel, Mumbai, India 400012 Tel: (911) 022 24177000, Fax: (911) 022 – 24146937 Email: rkinhikar@rediffmail.com Received: 21 July 2006; Accepted: 22 February 2007 Copyright © 2007 ACPSEM usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured doses are compared against those calculated. Ion chambers are ideal for measuring absolute dose at a single point in a low gradient region. Use of ion chambers for IMRT dose verification has been reported. 6-7 Use of miniature volume ion chambers can significantly reduce errors in IMRT dosimetry due to volume averaging. 8 However, there are difficulties with using ionization chambers for IMRT dose verification. 9 Thermoluminescent dosimeters (TLD) are often more suited to point dose measurements in high gradient region due to their small size relative to ion chambers. TLDs also give the option of measuring multiple points simultaneously. With careful handling and attention to standardized annealing and readout protocols, TLD dosimetry can routinely achieve an accuracy of at least 3%. Although TLD can measure multiple point doses and is suitable for in-vivo measurement, the accuracy of the measurement depends heavily on the quality control of the TLD reading and calibration procedures. Because of the requirement of intensive post-processing and calibration procedures in both film and TLD measurements, the results of these two methods cannot be obtained immediately. Since the variation of fluence always exists in 2D and in 3D of IMRT plans, film plays an important role in any IMRT QA program. Radiographic films due to their high spatial accuracy are preferred as a detector material. Kodak extended dose range (EDR2) film has been shown to be highly useful for commissioning and quality assurance