Browsing by Author "Malcolm, Javian"

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    Design, Evaluation and First Applications of a State-of-the-art 3D Dosimetry System
    (2015) Malcolm, Javian

    3D dose verification has important advantages for comprehensive verification of advanced radiation treatments. We have designed, constructed, installed, and commissioned an in-house prototype three dimensional (3D) dose verification system for a joint international collaboration (Duke and Princess Margaret Cancer Centre (PMCC), Toronto) to investigate the comprehensive accuracy of stereotactic body radiotherapy (SBRT) treatments. The potential of this system to achieve sufficient performance (1 mm resolution, 3 % noise, within 3 % bias) for SBRT verification was investigated.

    The system, termed PMOS, was designed utilizing a parallel ray geometry instigated by precision telecentric lenses and an 630 nm LED light source. Using PRESAGE® radiochromic dosimeters, a 3D dosimetric comparison with our gold-standard system and treatment planning software (Eclipse®) was performed for a four-field box treatment (8 Gy per fraction), under gamma passing criteria of 3 %/3 mm/10 % dose threshold. Post off-site installation, deviations in the system's dose readout performance was assessed by rescanning the four-field box irradiated dosimeter and using line-profiles to compare on-site and off-site mean and noise levels in four distinct dose regions. As a final step, an end-to-end test of the system was completed at the off-site location, including CT-simulation, irradiation of the dosimeter and a 3D dosimetric comparison of the planned dose (Pinnacle3) to delivered dose for a spinal SBRT treatment(12 Gy per fraction).

    The gamma pass rate for the 3D gamma comparison between the PMOS and our gold-standard Duke Large Optical Scanner (DLOS) dose distribution was 99.1% under (3 %, 2mm, 5 % threshold) criteria. Under 3 %, 1mm, 5 % threshold, the gamma comparison between the systems was 95.6 %. Using line profiles, the dose difference between the readout of the PMOS at Duke and PMCC for the same irradiated dosimeter was between 0-1 Gy. At 1mm reconstructed dose voxels, the gamma pass rate was 95. 27% (3 %, 2 mm, 5 % threshold) for the gamma comparison between the measured PMOS and calculate Pinnacle3 dose distribution. The majority of the failing voxels of the gamma map were in the high dose regions of the dose distribution. By rescanning the dosimeter in the DLOS, the PMOS' dose readout was ruled out as the source of the high dose (>14Gy) disagreement. The sensitivity of the PRESAGE® dosimeter and the parameters for the Pinnacle3 dose calculation will be further investigated as possible sources for the error.

    This work will describe the end-to-end process and results of designing, installing, and commissioning a state-of-the-art 3D dosimetry system created for verification of advanced radiation treatments including spinal radiosurgery.

    As an initial step towards this goal, I worked on a related project to evaluate a prototype of our in-house 3D dosimetry system using Fresnel lens and a `solid tank' made of PRESAGE® like material.

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    ItemOpen Access
    Optical-CT 3D Dosimetry Using Fresnel Lenses with Minimal Refractive-Index Matching Fluid.
    (PLoS One, 2016) Bache, Steven; Malcolm, Javian; Adamovics, John; Oldham, Mark
    Telecentric optical computed tomography (optical-CT) is a state-of-the-art method for visualizing and quantifying 3-dimensional dose distributions in radiochromic dosimeters. In this work a prototype telecentric system (DFOS-Duke Fresnel Optical-CT Scanner) is evaluated which incorporates two substantial design changes: the use of Fresnel lenses (reducing lens costs from $10-30K t0 $1-3K) and the use of a 'solid tank' (which reduces noise, and the volume of refractively matched fluid from 1 ltr to 10 cc). The efficacy of DFOS was evaluated by direct comparison against commissioned scanners in our lab. Measured dose distributions from all systems were compared against the predicted dose distributions from a commissioned treatment planning system (TPS). Three treatment plans were investigated including a simple four-field box treatment, a multiple small field delivery, and a complex IMRT treatment. Dosimeters were imaged within 2 h post irradiation, using consistent scanning techniques (360 projections acquired at 1 degree intervals, reconstruction at 2mm). DFOS efficacy was evaluated through inspection of dose line-profiles, and 2D and 3D dose and gamma maps. DFOS/TPS gamma pass rates with 3%/3mm dose difference/distance-to-agreement criteria ranged from 89.3% to 92.2%, compared to from 95.6% to 99.0% obtained with the commissioned system. The 3D gamma pass rate between the commissioned system and DFOS was 98.2%. The typical noise rates in DFOS reconstructions were up to 3%, compared to under 2% for the commissioned system. In conclusion, while the introduction of a solid tank proved advantageous with regards to cost and convenience, further work is required to improve the image quality and dose reconstruction accuracy of the new DFOS optical-CT system.