Technical Note
A proposed method for beam measurements in stereotactic radiotherapy
M.C. Lopes 1 , F. Sánchez-Doblado 2 , K. Theodorou 3,* , A. Mazal 4 , L. Nuñez 5 , A. Chaves 1 , C. Kappas 3 , M. Bazioglou 3 , P. Lassalete 1
1. Centro Regional de
Oncologia I.P.O.F.G., Coimbra (Portugal)
2. Dpto. Fisiología M. y Biofísica, F. Medicina, Universidad de
Sevilla, Sevilla (Spain)
3. Medical Physics Dept., University of Patras, Patras (Hellas)
4. Institut Curie, Paris (France)
5. Clínica Puerta de Hierro, Madrid (Spain)
Manuscript received: April 27, 1999; received: November 13, 1999.
Accepted for publication: December 16, 1999.
Abstract
The dosimetry of small photon fields, as the ones used in Stereotactic Radiotherapy (SRT)/Radiosurgery (SRS) is complicated by means of lateral electronic disequilibrium and detector’s size. Thus, the dosimetry of small beams cannot be carried out by direct application of the various existing international dosimetry protocols, which refer to conventional radiotherapy. The Small Field Dosimetry Task Group (SFDTG) of the European Concerted Action DYNARAD was intended to assess the difficulties of the dosimetry of radiosurgery beams. This paper aims to present the results of the SFDTG effort in the implementation of a common methodology for dose measurements at the beam central axis and its application to different participating centres during the beam calibration phase. The methodology proposed consists of the irradiation of an ionisation chamber with its longitudinal axis parallel to the beam central axis and correct subsequently the reading by a factor, which includes the beam dose profile averaging throughout the chamber volume. The applied methodology proved to be compatible with film and TLD dosimetry and that it is an acceptable method for correcting ionisation chamber measurements of small radiosurgery beams.
KEYWORDS: Radiosurgery, small field dosimetry, beam calibration.