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PTV Margin Reduction by High Dose Volumetric CT Scan for Radiotherapy of Prostate Cancer

Y Cho*, H Alasti , V Kong , C Charles , Princess Margaret Hospital, Toronto, ON


(Monday, 7/30/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 6

Purpose: To determine if the superior soft tissue contrast provided by high dose volumetric CT (HDVCT) can result in reduction of planning target volume (PTV) for inter observer variation (IOV) and to relate the measured IOV with the required PTV margin.

Methods: Ten prostate patients receiving radiotherapy were imaged with conventional CT (CCT, 120kV, 300mAs), and HDVCT (120kV, 2300mAs, 80 slices of volumetric scan) all with 2mm thickness. Five radiation oncologists delineated clinical target volume (CTV) of prostate gland. Degree of CTV confidence was measured in each image by the number of voxel count included by contours of all five observers. Target volume in agreement was determined by the voxels with the confidence of 50% or more for each image modality used (TVa,m). Required PTV margin was investigated using two
methods: (1) geometric modelling, expanding CTVs in 0.5mm step to find if the expanded CTV completely cover TVa,m more than 90% of patient population and (2) treatment plan study, single-arc VMAT plans (RayStation v6.1) with PTV margin of 4mm and 6mm (totaling 200 plans).

Results: Average volume (±standard variation) of TVa,CCT and TVa,HDVCT were 43.1cc ± 3.9cc and 42.5cc ± 3.8cc respectively. Required inter-observer PTV margin of TVa,CCT and TVa,HDVCT were 5.0mm with CCT and 4.0mm with HDVCT from geometric modelling. Planning study confirmed the same PTV margin. Required PTV margin is found 1.5∑ (systematic)+0.7σ (random). This is smaller than the PTV margin recipe for setup error.

Conclusion: IOV in target delineation is a source of systematic uncertainty. HDVCT demonstrated reduction in IOV and PTV margin when compared to CCT. It should be noted that HDVCT requires smaller PTV margin for both TVa,m, although CTVs in HDVCT appear smaller on average. Smaller target volume combined by smaller PTV margin with HDVCT is promising for radiotherapy planning of prostate cancer.


Not Applicable / None Entered.


IM- CT: Development (New technology and techniques)

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