AbstractID: 9241 Title: Characterization Of A Large-Area Transmission Chamber For Independent
Verification Of IMRT Dose Delivery Constancy
A requirement for the accurate delivery of Intensity Modulated Radiation Therapy (IMRT) fields using Dynamic Multi-leaf
Collimators (DMLC) is accuracy and precision in leaf positioning as well as in dose delivery by the Intensity Modulated Delivery
System (IMDS). Periodic quality assurance ensures that the IMDS is operating within pre-defined tolerances. The pre-treatment
dosimetry of treatment fields is performed using film and point-dose detectors. However, real-time monitoring of the actual treatment
is desirable to verify accuracy of dose delivery. We have investigated the performance characteristics of a large-area transmission
chamber, mounted in the shielding tray of the accelerator, as a dose-area-product meter. Performance characteristics quantified
include: 1) linearity of response with dose delivered, 2) dose rate dependence, 3) response as a function of irradiated area and dose, 4)
spatial uniformity of response, 5) attenuation of beam intensity, 6) effect on surface dose and 7) reproducibility. The transmission
chamber showed a large change in relative response as the irradiated area was varied. Compared to a 10 cm x 10 cm reference field,
the response of the chamber varied from 17% for a 4 cm x 4 cm field to 300% for a 20 cm x 20 cm field. Long-term reproducibility is
shown to be within 1.0 %. The constancy of IMRT field dose delivery was found to be within 2%. The results of this evaluation show
that a large-area transmission chamber can be used as an independent dose-area-product monitor for constancy of IMRT dose
delivery.