Ion recombination correction for very high dose-per-pulse high-energy electron beams

The parallel-plate ionization chamber is the recommended tool for the absorbed dose measurement in pulsed high-energy electron beams. Typically, the electron beams used in radiotherapy have a dose-per-pulse value less then 0.1 cGy ∕ pulse . In this range the factor to correct the response of an ionization chamber for the lack of complete charge collection due to ion recombination ( k sat ) can be properly evaluated with the standard “two voltage” method proposed by the international dosimetric reports. Very high dose-per-pulse electron beams are employed in some special Linac dedicated to the Intra-Operatory-Radiation-Therapy (IORT). The high dose-per-pulse values ( 3 – 13 cGy ∕ pulse ) characterizing the IORT electron beams allow to deliver the therapeutic dose ( 10 – 20 Gy ) in less than a minute. This considerably reduces the IORT procedure time, but some dosimetric problems arise because the standard method to evaluate k sat overestimates its value by 20%. Moreover, if the dose-per-pulse value > 1 cGy ∕ pulse , the dependence of k sat on the dose-per-pulse value cannot be neglected for relative dosimetry. In this work the dependence of k sat on the dose-per-pulse value is derived, based on the general equation that describes the ion recombination in the Boag theory. A new equation for k sat , depending on known or measurable quantities, is presented. The new k sat equation is experimentally tested by comparing the absorbed doses to water measured with parallel-plate ionization chambers (Roos and Markus) to that measured using dose-per-pulse independent dosimeters, such as radiochromic films and chemical Fricke dosimeters. These measurements are performed in the high dose-per-pulse ( 3 – 13 cGy ∕ pulse ) electron beams of the IORT dedicated Linac Hitesys Novac7 (Aprilia—Latina, Italy). The dose measurements made using the parallel-plate chambers and those made using the dose-per-pulse independent dosimeters are in good agreement ( < 3 % ) . This demonstrates the possibility of using the parallel-plate ionization chambers also for the very high dose-per-pulse ( > 1 cGy ∕ pulse ) electron-beam dosimetry.