Absolute and relative dose measurements with Gafchromic™ EBT film for high energy electron beams with different doses per pulse

The authors have evaluated the accuracy, in absolute and relative dose measurements, of the Gafchromic (TM) EBT film in pulsed high-energy electron beams. Typically, the electron beams used in radiotherapy have a dose-per-pulse value of less than 0.1 mGy/pulse. However, very high dose-per-pulse electron beams are employed in certain linear accelerators dedicated to intraoperatory radiation therapy (IORT). In this study, the absorbed dose measurements with Gafchromic (TM) EBT in both low (less than 0.3 mGy per pulse) and high (30 and 70 mGy per pulse) dose-per-pulse electron beams were compared with ferrous sulfate chemical Fricke dosimetry (operated by the Italian Primary Standard Dosimetry Laboratory), a method independent of the dose per pulse. A summary of Gafchromic (TM) EBT in relative and absolute beam output determination is reported. This study demonstrates the independence of Gafchromic (TM) EBT absorption as a function of dose per pulse at different dose levels. A good agreement (within 3%) was found with Fricke dosimeters for plane-base IORT applicators. Comparison with a diode detector is presented for relative dose measurements, showing acceptable agreement both in the steep dose falloff zone and in the homogeneous dose region. This work also provides experimental values for recombination correction factor (K-sat) of a Roos (plane parallel) ionization chamber calculated on the basis of theoretical models for charge recombination.