SU-F-T-08: Brachytherapy Film Dosimetry in a Water Phantom for a Ring and Tandem HDR Applicator.

PURPOSE: The feasibility of dose measurement using new generation EBT3 film was explored in a water phantom for a ring and tandem HDR applicator for measurements tracking mucosal dose during cervical brachytherapy.

METHODS: An experimental fixture was assembled to position the applicator in a water phantom. Prior to measurement, calibration curves for EBT3 film in water and in solidwater were verified. EBT3 film was placed at different known locations around the applicator in the water tank. A CT scan of the phantom with applicator was performed using clinical protocol. A typical cervical cancer treatment plan was then generated by Oncentra brachytherapy planning system. A dose of 500 cGy was prescribed to point A (2 cm, 2 cm). Locations measured by film included the outer surface of the ring, measurement point A_m (2.2 cm, 2.2 cm), and profiles extending from point A_m parallel to the tandem. Three independent measurements were conducted. The doses recorded by film were carefully analyzed and compared with values calculated by the treatment planning system.

RESULTS: Assessment of the EBT3 films indicate that the dose at point A matches the values predicted by the planning system. Dose to the point A_m was 411.5 cGy, and the outer circumferential surface dose of the ring was between 500 and 1150 cGy. It was found that from the point A_m, the dose drops 60% within 4.5 cm on the line parallel to the tandem. The measurement doses agree with the treatment planning system.

CONCLUSION: Use of EBT3 film is feasible for in-water measurements for brachytherapy. A carefully machined apparatus will likely improve measurement accuracy. In a typical plan, our study found that the ring surface dose can be 2.5 times larger than the point A prescription dose. EBT3 film can be used to monitor mucosal dose in brachytherapy treatments.