PHSOR10 Presentation Time: 9:45 AM

Abstract

Purpose/Objective

In radiotherapy, independent verification of the treatment fields is standard practice. However, for ¹⁰⁶Ru eye plaque brachytherapy there is no such method available. This has led to many hospitals performing treatments without verification of plaque absorbed dose rate to water [1], as also addressed by the AAPM TG No 221 [2]. To enhance the safety of brachytherapy treatments for intraocular tumors, we devised an independent measurement protocol to determine depth-dose curves from ¹⁰⁶Ru eye-plaques using a new high precision setup equipment and diode detector.

Material/Methods

The absorbed dose rate to water was measured from five plaques (Bebig Eckert and Ziegler), four CCB types and one CCA type, using three PTW microSilicon detectors with a prototype, dedicated setup equipment from the plaque vendor named BetaCheck-106. The diodes were calibrated in ⁶⁰Co at the Swedish secondary standards metrology laboratory. Beam quality correction factors along the lines of the TRS-398 protocol [3] were calculated with the PENELOPE Monte Carlo code and detector blueprints. Absorbed dose rate measurements were performed in the water-filled PMMA phantom. Furthermore, the depth-dose curves were validated against vendor data and data from alanine detectors measurements with the latter performed at the primary standard laboratory of the National Physical Laboratory, UK and traceable to a ⁶⁰Co primary standard of absorbed dose to water.

Result

The absorbed dose rates to water measured with the diodes and alanine detectors fell within the vendor's expanded measurement uncertainty, 11% (k=2), but were lower than the vendor's values. For the CCB applicators at the 2 mm distance reference point, the absorbed dose rates measured with the diode detectors were on average 9% lower, while the absorbed dose rates measured with alanine detectors were 7.4% lower. For the CCA applicator, the absorbed dose rate was 7% and 4.6% lower for diode- and alanine detectors, respectively. Preliminary results are shown in Figure 1.

Conclusion

Our study shows an efficient measurement protocol for verifying ¹⁰⁶Ru eye-plaque absorbed dose-rates. The dose rates measured by diode and alanine detectors both show lower dose rates compared to vendor certificates, but are still within the vendor's expanded measurement uncertainty. The discrepancy will be further investigated. The dedicated setup equipment provided high repeatability, which is crucial for reliably measuring the steep dose gradients from ¹⁰⁶Ru. Diode detectors calibrated in ⁶⁰Co with Monte Carlo calculated detector correction factors provide vendor independent traceability. The methodology offers hospitals a feasible way to verify absorbed dose-rate to water depth dose curves and so increase safety of patient treatments and fulfilment of regulatory requirements.