AbstractID: 9067 Title: Sensitivity and OD growth effects on the response of
radiochromic films using pre- and post- irradiation double exposure techniques in LDR
brachytherapy
The pre-irradiation double-exposure technique (pre-DET), in which a 20 Gy uniform
dose is delivered to Radiochromic films (RCFs) prior to exposure to the unknown
radiation fields, is widely used to account for spatial nonuniformity of RCF. However,
with respect to post-exposure OD growth, the pre-DET complicates the temporal
mismatch between films exposed to low-dose-rate (LDR) brachytherapy over a protracted
time interval and calibration films exposed acutely, since its measured response is the
superposition of the 20 Gy uniform and the LDR brachytherapy or LINAC exposure
growth profiles. To simplify these kinetics, we have investigated a post-irradiation
double-exposure (post-DET) technique, in which the order of irradiations is reversed
(unknown or calibration field followed by 20 Gy uniform dose 48-72 h later). Post- and
pre-DET exposures were performed both for a Cs-137 source and graded LINAC doses
from 1-200 Gy. Our results show, that post-DET reduces temporal mismatch errors, but
that the marginal response to the 20 Gy uniform dose varies dramatically depending upon
the dose delivered by the initial exposure. Although the sensitivity decreases by less than
3% for initial doses less than 10 Gy, for larger doses, sensitivity rapidly falls, diminishing
by 50% for a film given 200 Gy. Clearly, the post-DET method will not yield accurate
uniformity corrections for films used to measure highly non-uniform dose distributions.
On the other hand, the pre-DET OD shows 60% larger post-exposure growth potential at
low doses than corresponding single-exposure films.
This work was supported by grant R01 CA 46640 awarded by NIH.