Small field dosimetry of high-energy electron beams

Abstract

ABSTRACT Purpose: A comparative dosimetric study was conducted with four detectors manufactured by PTW-Freiburg (Germany) (Markus – type 23343, Advanced Markus – type 34045, PinPoint – type 31006 and a diode type – 60012) for small field high-energy electron beams. The dosimetry of Intraoperative Radiation Therapy (IORT) is challenging because of the type of tertiary collimation used. The cones increase low energy electron contamination and detectors with low energy dependence and high spatial resolution should be used. Materials and Methods: Dosimetric measurements were made using all four detectors. Two applicator systems were studied: The small-field system has 9 cylindrical straight end cones of inner diameters 2 to 8 cm increasing in steps of 1 cm, and two bevel end cones of 2 and 3 cm with a bevel angle of 45°. The periscopic electron cone system is provided with three set of cylindrical cones of inner diameter 3.2, 3.8 and 4.5 cm with straight and bevel ends with bevel angle 30°. Results: All dosimetric data was compared to the Markus chamber. The percent error for the absolute dose measurements for the Advanced Markus and the PinPoint were found to be 3.2% and 5.1% respectively. For the small-field cone system the percent difference in output factors (OF) for the Markus chamber and all other detectors was found to be less than 2.6% for the straight cones and less than 4.8% for the bevelled cones. The percent difference for the Markus and all other detectors was found to be less than 1.9% for straight cones and 4.2% for the bevelled cones for the periscopic system. Conclusions: The Markus, Advanced Markus and the diode detector, may be used for the relative dosimetry of small electron fields. The Markus and diode detector should be crosscalibrated for absolute dose measurements. The PinPoint chamber should not be used for the absolute dosimetry of small electron fields, unless the quality correction factor is determined for the beam quality used. A comparative study should be made with the PinPoint oriented in the two possible positions to determine which position is more suitable for electron dosimetry. Spencer-Attix stopping power ratios for water to air ( ) must be calculated using Monte Carlo simulations for the two applicator systems used.