Mahsheed Sabet
Sir Charles Gairdner Hospital
Radiation Oncology Medical Physics Registrar

David Waterhouse
Acting Principal Medical Physicist
Sir Charles Gairdner Hospital

Martin Ebert
Director of Physics Research
Sir Charles Gairdner Hospital

Background and Purpose

LDR brachytherapy of the prostate using permanent seed implants is an effective treatment for localised low risk tumours with high survival rates, fewer metastases, less toxicity and better local control. At our centre, LDR treatments are delivered using I-125 seeds with the intended PTV dose of at least 144 Gy. Post-implant dosimetry is performed using post-implant CT images. We use the VariSeed planning system for LDR planning based on the AAPM TG43 method. The TPS was upgraded from version 8.0.2 to 9.0.1 and this work presents the post-upgrade test procedures and outcomes. The changes that affect calculations include:

• The use of 1.0/ln(2) value instead of 1.44 for calculations.

• more calculation points for manually specified dose volume resolution

• The prostate volume excludes urethra by default

Methods

After general post-upgrade tests (functionality of hardware, accessories, orientation, distances, documentation,...) three types of tests were performed:

A) User Test Procedures: The system was tested for the Advantage I-125 seeds (IsoAid, model: IAI-125A) used at our hospital for LDR implants. Test procedures specified by Varian were followed to verify source specifications, dose point calculation, isodose levels, dose volumes, and anisotropy function/line source calculations.

B) Independent volume calculation checks: A cube was defined in both versions of Variseed (4×3×3cm3) and the reported volumes were compared.

Then another volume (1×1×0.5cm3) was added to two sides of the cube. The volumes calculated in the two versions were compared.

C) Clinical Tests: Ten LDR patient plans which were already calculated in the old version were exported to version 9.0.1 and recalculated for comparison of pre- and post-implant doses and volumes. Comparisons include:

• Pre-implant: prostate volume, D90, D100, V100, V150, V200 for prostate, and D30 and D5 for urethra

• Post-implant:

– Prostate: volume, D90, D100, V100, V150, V200

– Base, Mid-gland, Apex: D90, D100, V100

– Urethra: D30, D5

Results

Following the user test procedures, all of the test results were within tolerance levels. Independent volume calculations showed 0.03 cc difference in both cases. The test on patient plans showed <0.1% difference in all volumes between the two versions. A systematic difference of ~0.35 Gy in all D90’s and in dose to urethra, and ~0.25 Gy in all D100’s was observed for all patients.

Conclusion

Differences in the calculated volumes and doses between version 9.0.1 and 8.0.2 were small and clinically insignificant, and were attributed to the changes in calculation method.


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