Room: AAPM ePoster Library
Purpose: To test a new method of confirming the activity of Ru-106 plaques used to treat uveal melanoma.
Methods: Three Ru-106 plaques (CCA, CCB, and COB) were obtained from Eckert & Ziegler (BEBIG) along with their NIST-traceable calibration certificates. Our current protocol is to measure the dose rate at a prescribed depth along the central axis of the plaque in a water phantom using Gafchromic film (EBT2 and EBT3). Calibration curves (Dose vs. OD) were obtained by irradiating film with 6 MeV electrons from a TrueBeam LINAC in clinical use. The new method studied here uses a Ludlum Model 44-3 NaI scintillation counter. The measured plaque was placed at the center of the holder at a distance of 4 cm from the probe. A 2 mm sheet of solid water was placed between the end of the probe and the plaque. Orientation of the plaque (rotation and displacement from the center) was varied to determine optimal placement for measurement. The linearity of the readings (counts per minute) was verified for the range of 0.1-0.5 minutes with 10 repeated measurements at each time. A “calibration factor” or efficiency was then calculated.
Results: The average dose difference for all three plaques between film measurements and the supplied data was -0.49 % (-10.87% to 7.96%). The scintillation detector’s linearity had R2 values ranging from 0.9992 to 1. Efficiencies (cpm/activity in Bq) ranged from 0.00141 to 0.00156, yielding an average of 0.00147.
Conclusion: The uncertainties found in the scintillation method are similar in size to what was found using film dosimetry. The scintillation method is simpler and may be useful until calibrated dosimeters are available.