Charge recombination correction in the dosimetry by means ionization chambers of a high dose-per-pulse electron accelerator for Intraoperative Radiation Therapy (IORT)

Abstract

The electron beams of some accelerators dedicated to Intraoperative Radiation Therapy (IORT) are characterized by a dose-rate of the order of some cGy/pulse, much higher than the typical values less than 1 mGy/pulse of conventional linacs. Thus dose-rates attain values of some tens of Gy/min reducing considerably the treatment time, but introducing a problem in the use of ionization chambers in beam calibration because of the significant lack of complete charge collection due to ion recombination. At very high dose-per-pulse values the standard "two voltage" [1] method for ksat evaluation is no more valid and yields overestimates of ksat up to 20% [2]. To overcome the problem, dose-rate independent dosimeters (such as chemical or radiochromic dosimeters) should be used. Indeed, the most commonly used chemical Fricke dosimeters have some disadvantages such as scarce spatial resolution, low sensitivity, high cost and post-irradiation reading process that limit their use in routine clinical dosimetry. Radiochromic film dosimeters seem more suitable to the purpose, but they are sensitive to temperature, time after irradiation and they need both a calibration procedure and a post-irradiation procedure to give the dose value. The ionization chamber has a very good sensitivity, it is practical to use, and, last but not least, it gives the dosimetric information on-line. The aim of this work is the determination of ksat [1] for NACP (Scanditronix-Wellöhfer) parallel plate ionization chamber on the basis of the method proposed recently by Laitano et al. [3] exposed to high-dose-per pulse electron beams of a mobile accelerator (LIAC, Sordina, Italy). Dose estimates obtained with ionization chamber were compared with Fricke and radiochromic dosimetry. The good agreement among the dosimetric methods demonstrate that the ionization chamber maintains a key role in dosimetry of high-dose-per-pulse electron beams too. © 2009 Springer-Verlag.