TITLE:
Patient-Specific QA of Spot-Scanning Proton Beams Using Radiochromic Film
AUTHORS:
Maria F. Chan, Chin-Cheng Chen, Chengyu Shi, Jingdong Li, Xiaoli Tang, Xiang Li, Dennis Mah
KEYWORDS:
Proton Therapy, Patient-Specific QA, Gaf Chromic EBT3, Film Dosimetry
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.6 No.2,
May
16,
2017
ABSTRACT: Radiochromic
film for spot-scanning QA provides high spatial resolution and efficiency gains
from one-shot irradiation for multiple depths. However, calibration can be a
tedious procedure which may limit widespread use. Moreover, since there may be
an energy dependence, which manifests as a depth dependence, this may require additional
measurements for each patient. We present a one-scan protocol to simplify the
procedure. A calibration using an EBT3 film, exposed by a 6-level step-wedge
plan on a Proteus®PLUS proton system (IBA,
Belgium), was performed at depths of 18, 20, 24 cm using Plastic Water®
(CIRS, Norfolk, VA). The calibration doses ranged from 65 - 250 cGy (RBE) (relative
biological effectiveness) for proton energies of 170 - 200 MeV. A clinical
prostate + nodes plan was used for validation. The planar doses at selected
depths were measured with EBT3 films and analyzed using one-scan protocol
(one-scan digitization of QA film and at least one film exposed to a known
dose). The gamma passing rates, dose-difference maps, and profiles of 2D planar
doses measured with EBT3 film and IBA MatriXX-PT, versus the RayStation TPS
calculations were analyzed and compared. The EBT3 film measurement results
matched well with the TPS calculation data with an average passing rate of ~95%
for 2%/2 mm and slightly lower passing rates were obtained from an ion chamber
array detector. We were able to demonstrate that the use of a proton step-wedge
provided clinically acceptable results and minimized variations between
film-scanner orientation, inter-scan, and scanning conditions. Furthermore, for
relative dosimetry (calibration is not done at the time of experiment), it
could be derived from no more than two films exposed to known doses (one could
be zero) for rescaling the master calibration curve at each depth. The
sensitivity of the calibration to depth variations has been explored. One-scan
protocol results appear to be comparable to that of the ion chamber array
detector. The use of a proton step-wedge for calibration of EBT3 film
potentially increases efficiency in patient-specific QA of proton beams.