Claire Pagulayan
Nelune Comprehensive Cancer Centre, Prince of Wales Hospital
Medical Physics Registrar

Soo Min Heng
Medical Physics Specialist
Nelune Comprehensive Cancer Centre, Prince of Wales Hospital

Stephanie Corde
Deputy Chief Physicist
Nelune Comprehensive Cancer Centre, Prince of Wales Hospital

Background and Purpose

Since the start of 2017, the Oncura Onco seed I-125 (model 6711) was no longer available in Australia. The Theragenics AgX-100 I-125 seed was chosen as an alternative radioisotope for eye plaque brachytherapy because its physical properties matched the requirements for the ROPES plaque used at the Nelune Comprehensive Cancer Centre, Prince of Wales Hospital. This work aims at validating the dosimetry of the AgX-100 loaded ROPES plaques (11 mm diameter, 15 mm diameter with flange, 15 mm diameter with notch, 18 mm diameter) and assess the differences with the commonly used I-125 6711 model.

Methods

The Bebig Plaque Simulator (PS) had to be upgraded to version 6.4.6 in order to activate the use of the AgX-100 seed model. To independently verify the plaque dosimetry, the brachytherapy module of Radcalc version 6.2.3.6 was commissioned for the new AgX-100 I-125 seed using the published TG43 type data from the literature. Experimental dosimetry verification was done using EBT3 Gafchromic film in a specially designed eye ball solid water phantom. The AgX-100 plaque model was also compared to the 6711 plaque model.

Results

Both Radcalc and film confirmed the dosimetry calculated by PS. The dose calculated by PS agrees with Radcalc to within 2% for depths of 1-15 mm for the 4 available ROPES plaques. The PS v6.4.6 includes a systematic backscatter factor to account for the presence of stainless steel backing of the ROPES eye plaque and this effect was also confirmed using film measurement. The AgX-100 plaque models differ from the 6711 plaque models to within 1 % due to small differences in the energy spectrum.

Conclusion

The AgX-100 I-125 seed was a suitable replacement for the Oncura 6711 I-125 seed. Due to the introduction of the stainless steel backscatter factor in PS v6.4.6, the department has decided to report both the homogenous dose and heterogeneity corrected dose for each eye plaque patient.

References

1. RIVARD et al. Supplement 2 for the 2004 update of the AAPM Task Group No 43 Report: Joint recommendations by the AAPM and the GEC-ESTRO. Medical Physics (2017) 44 (9): e297 – e338

2. CHIU-TSAO et al. Dosimetry of I-125 and Pd-103 COMS eye plaques for intraocular tumours: Report of Task Group 129 by the AAPM and ABS. Medical Physics (2012)39(10): 6161 – 6184.


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