AbstractID: 6853 Title: Possibilities and Limitations in the use of Radiographic Film for the
Evaluation of Dose Delivery in Small Fields
Abstract
The use of radiographic film (X-OMAT V) has been investigated to verify dose distributions, created by sophisticated irradiation techniques, such as obtained in IMRT.
Output factors were measured on the central axis of small fields with a PinPoint
chamber and used to calculate the number of monitor units needed to deliver an
equal dose for single small fields and larger IMRT fields. Films were irradiated, developed and analyzed and readings could be compared directly because the same
dose was delivered for small and larger fields.
For three energies tested (6, 8 and 18 MV) the small fields showed an increase up to
8 %, with decreasing field dimensions. Radiographic film behaves more like a solidstate detector for small fields, than a water-like detector. The over-response could not
be attributed to an under-response of the ionization chamber. Beam hardening introduced by the thickest part of the scattering filter might induce the over-response of
the radiographic film.
This study shows that radiographic film in dose verification of IMRT-plans should be
handled cautiously. When the delivered segments are very small, this type of film requires a correction for its increased sensitivity if the sensitometric curve is obtained
for larger field sizes.