Calibration of photon and electron beams with an extrapolation chamber

Abstract

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Extrapolation chamber mounted on perspex for calibration of high energy photon and electron beams from a clinical linear accelerator

“…Earlier reports by Klevenhagen[8] and Zankowski and Podgorsak[910] have brought out the possibility of using such designed prototype versions of extrapolation chambers for the estimation of the absorbed dose from clinical linacs and tele-cobalt beams. These authors have used polystyrene and solid water phantoms for the construction of extrapolation chambers, and measured absorbed doses at selected depths in tissue-equivalent media.…”

“…The charge in nC, corresponding to 1.5mm plate separation, is used for calculation of the absorbed dose (based on the recommendation of Zankowski and Podgorsak,[10] suggesting 1 to 2mm as the true Bragg-Gray region for the EC). Recombination correction is carried out by using the standard two voltage technique +400V and +200V.…”

“…[14]), and solid water to air (Zankowski and Podgorsak[10]) used in Eq. (4) to derive the absorbed dose to water.…”

“…The principle and application of the EC for measurement of x-ray outputs have been reported in literature. [6–8] Zankowski and Podgorsak[910] reported the efficacy of specially built extrapolation chambers as an integral part of polystyrene and solid water phantom, to measure absorbed doses in cobalt-60 gamma beam, 4 to 18 MV x-rays, and 6 to 22 MeV electron beams. Mehenna Arib[11] reported their experience in performing absolute dosimetry with high-energy photon beams using a commercially available Perspex embedded EC and compared it with water measurements.…”

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

“…92,93 It is often implied or suggested that the extrapolated zero volume ionization ratio (PDI) gives the PDD. 87,90,94 In fact the extrapolated zero volume ionization gives the true ionization that would have occurred if there were no perturbation effects. The change in stopping power ratio from the surface to depth of maximum dose still remains a source of uncertainty in converting PDI to PDD even with an extrapolation chamber.…”