Prabha Jones
Alfred Health - Gippsland Radiation Oncology
Radiation Oncology Medical Physics Registrar

Prabha Jones
Medical Physics Registrar
Gippsland Radiation Oncology, Alfred Health

Sarah Elliot
Senior Medical Physicist
Alfred Health Radiation Oncology, Alfred Health

Ryan L Smith
Senior Medical Physicist
Alfred Health Radiation Oncology, Alfred Health

Background: Low dose rate (LDR) prostate brachytherapy is a highly conformal internal radiation therapy that delivers a high dose of radiation to the prostate by permanently implanting I-125 seeds. Due to the close proximity of the urethra, bladder and rectum, it is important to ensure precise contouring of structures and the careful planning of source positions. Dose-Volume Histogram (DVH) indices are used to monitor dose to the target and organs at risk. Aim: Variseed treatment planning system (TPS) has introduced an updated version (V9) for which the volume calculation algorithm has changed to a dynamic grid approach, compared to a static grid in the previous version (V8). The aim of this work is to characterise the effects of the dynamic volume calculation on clinical LDR pre-plans for Variseed V9. Method: The pre-planned data of 40 consecutive patients treated between 2015 and 2016 were analysed. Each patient plan was re-calculated in V9 and structure volumes and selected DVH indices (D100%, V100%, V150%, V200%) for the target were compared with V8. This method was also applied to a CIRS multimodality pelvic phantom with known anatomical prostate volume. Results: For the 40 patients evaluated, the total calculated volume of the urethra, rectum and target were identical (within rounding) in both versions of Variseed. There was a 2% variation in the volume of the prostate between V8 (mean 36.74± 10.74 cm3) and V9 (mean 35.97± 10.74 cm3). Although the D100% and V100% for the target are similar in V8 and V9, the V150% and V200% were different and these results are clinically significant. In following our departmental pre-planning protocol, for 6 patients the V200% was below the minimum acceptable value of 13% in V9; for 17 patients the V150% was above the maximum acceptable value of 62% in V9. Further investigation on the prostate phantom showed the V150 % was also higher in V9. However, the V200% was higher in V9 for the prostate phantom which does not follow the trend observed in the patient data analysis. Conclusion: There are differences in the DVH statistics of identical prostate seed plans calculated in Variseed V8 and V9 due to the dynamic volume calculation algorithm introduced in V9. The magnitude of the differences observed in the V150% and V200% would lead to changes in source position and/or source numbers in the pre-planning stage of brachytherapy. To further understand the effects of the dynamic volume calculation algorithm on DVH statistics, the relationship between dose and the structure volume, structure shape, total number of sources and image slice spacing is being further investigated.


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