TITLE:
Absorbed Dose to Water Rate in a Cyberknife VSI System Reference Field Using Ionization Chambers and Gafchromic Films
AUTHORS:
Guerda Massillon-JL, Nestor Aragón-Martínez, Arnulfo Gómez-Muñoz
KEYWORDS:
Small Field Dosimetry, Reference Field, Cyberknife, Gafchromic Films, Ionization Chambers, Dose Rate, EBT3, MD-V3, 6 MV X-Rays
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.6 No.1,
February
24,
2017
ABSTRACT: This work investigated the absorbed dose to
water rate under reference conditions in a Cyberknife VSI system using
radiochromic films EBT3 and MD-V3 and three ionization chambers: an Exradin A12
and two FC65P Welhöfer Scanditronix
with different serial numbers. The correction factor,, was studied using a
Varian iX linac and the Cyberknife system. The measurements in the Varian iX
were performed in a 10 × 10 cm2 field, 10 cm depth in
liquid water at 90 cm and 70 cm SSD and in a 5.4 × 5.4 cm2
field, 10 cm depth at 70 cm SSD to simulate the Cyberknife conditions. In the
Cyberknife system, measurements were performed using ionization chambers and
both film types at 70 cm SSD and 10 cm depth in its 6 cm diameter reference
field. The results indicate that is independent of the dosimeters and the
evaluation methods. Maximum differences of 0.22% - 0.55% (combined
uncertainties of 1.22% - 1.98%, k = 1) are obtained on using Varian iX,
whereas discrepancies of 2.08% - 2.09% (combined uncertainties of 1.87% - 2.13%, k = 1) are observed using the Cyberknife system. Given the agreement
between detectors and the combined standard uncertainties, the data from Varian
iX could be considered the most accurate and consequently a weighted average
factor of 0.902 ± 0.006 could be used for the Cyberknife VSI system reference
field. Within measurement uncertainties, the absorbed dose rate measured in the
Cyberknife VSI system reference field was found to be independent of the
dosimeters used. These results suggest that the absorbed dose measured at a
point within a given field size should be the same, regardless the dosimeter
used, if their dosimetric characteristics are well known. This highlighted the
importance of performing dosimetry by controlling all parameters that could
affect the dosimeter response. One can conclude that radiochromic film
dosimetry can be considered as an appropriate alternative for measuring
absorbed dose to water rate.