In:
Medical Physics, Wiley, Vol. 44, No. 1 ( 2017-01), p. 321-332
Abstract:
LIAC ® and NOVAC are two mobile linear accelerators dedicated to intraoperative radiation therapy ( IORT ), generating electron beams in the energy range of 3–12 MeV. Due to high dose‐per‐pulse (up to 70 mG y per pulse), in 2003 the Italian National Institute of Health ( ISS ) stated that “for the measure of dose to water in reference conditions, ionization chambers cannot be employed and no published dosimetry protocol can be used”. As a consequence, ferrous sulphate (or, alternatively alanine) dosimetry was recommended. Based on a retrospective multi‐center survey, a comparison with ferrous sulphate dosimetry is now used to validate the parallel‐plate ionization chambers for reference dosimetry of NOVAC and LIAC . Methods The IAEA TRS ‐398 dosimetry protocol was applied except for the reference irradiation setup and the determination of the ion‐recombination correction factor k s . Moreover the depth of maximum dose (R 100 ) instead of z ref as measurement depth was chosen by the majority of centers, thus implying a renormalization of the beam‐quality correction factor k Q,Qo , based on water‐air stopping power ratios. Regarding the k s determination, a previously published method, independent of ferrous sulphate dosimetry, was adopted. All the centers participating in this study had used both ferrous sulphate dosimeters and ionization chambers in water phantoms for dosimetry under reference conditions. Results The mean percentage difference between ionization chambers and ferrous sulphate dosimetry was −0.5% with a dispersion of 3.9% (2 σ ). Moreover, the uncertainty analysis allowed the agreement between ionization chambers and ferrous sulphate dosimetry to be verified. These results did not show any significant dependence on electron energy, thus indirectly confirming k Q,Qo renormalization. The results from the centers using z ref as the measurement depth were similar to the other data, but further focused studies could aim at investigating possible dependences of the dose differences on the chosen reference depth. Conclusion The present study confirms that parallel‐plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external‐beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT , so that users may benefit from specific, authoritative and up‐to‐date recommendations.
Type of Medium:
Online Resource
ISSN:
0094-2405
,
2473-4209
DOI:
10.1002/mp.2017.44.issue-1
Language:
English
Publisher:
Wiley
Publication Date:
2017
detail.hit.zdb_id:
1466421-5
