Premium
Ion recombination correction for very high dose‐per‐pulse high‐energy electron beams
Author(s) -
Di Martino F.,
Giannelli M.,
Traino A. C.,
Lazzeri M.
Publication year - 2005
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1940167
Subject(s) - ionization chamber , dosimetry , pulse (music) , ionization , absorbed dose , percentage depth dose curve , electron , dosimeter , ion , materials science , atomic physics , physics , nuclear medicine , optics , nuclear physics , medicine , quantum mechanics , detector
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.