Picture of Vimal Desai

Vimal Desai

Radiation Oncology Physics Resident

Department of Human Oncology


PhD, University of Wisconsin–Madison, Medical Physics (2019)

MS, University of Wisconsin–Madison, Medical Physics (2015)

BS, University of Virginia, Mechanical Engineering (2013)

BA, University of Virginia, Physics (2013)

Selected Honors and Awards

North Central Chapter of the American Association of Physicists in Medicine Young Investigators Competition, 2nd place (2017)

North Central Chapter of the American Association of Physicists in Medicine Young Investigators Competition, 3rd place (2016)

  • On the implementation of the plan-class specific reference field using multidimensional clustering of plan features and alternative strategies for improved dosimetry in modulated clinical linear accelerator treatments. Med Phys
    Desai VK, Labby ZE, DeWerd LA, Culberson WS
    2020 Apr 26; :
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      PURPOSE: The plan-class specific reference field concept could theoretically improve the calibration of radiation detectors in a beam environment much closer to clinical deliveries than existing broad beam dosimetry protocols. Due to a lack of quantitative guidelines and representative data, however, the pcsr field concept has not yet been widely implemented. This work utilizes quantitative plan complexity metrics from modulated clinical treatments in order to investigate the establishment of potential plan classes using two different clustering methodologies. The utility of these potential plan clusters is then further explored by analyzing their relevance to actual dosimetric correction factors.

      METHODS: Two clinical databases containing several hundred modulated plans originally delivered on two Varian linear accelerators were analyzed using 21 plan complexity metrics. In the first approach, each database's plans were further subdivided into groups based on the anatomic site of treatment and then compared to one another using a series of non-parametric statistical tests. In the second approach, objective clustering algorithms were used to seek potential plan clusters in the multidimensional complexity-metric space. Concurrently, beam- and detector-specific dosimetric corrections for a subset of the modulated clinical plans were determined using Monte Carlo for three different ionization chambers. The distributions of the dosimetric correction factors were compared to the derived plan clusters to see which plan clusters, if any, could help predict the correction factor magnitudes. Ultimately, a simplified volume averaging metric (SVAM) is shown to be much more relevant to the total dosimetric correction factor than the established plan clusters.

      RESULTS: Plan groups based on the site of treatment did not show noticeable distinction from one another in the context of the metrics investigated. An objective clustering algorithm was able to discriminate VMAT plans from step-and-shoot IMRT plans with an accuracy of 90.8%, but no clusters were found to exist at any level more specific than delivery modality. Monte Carlo determined correction factors for the modulated plans ranged from 0.970-1.104, 0.983-1.027, and 0.986-1.009 for the A12, A1SL, and A26 chambers, respectively, and were highly variable even within the treatment modality plan clusters. The magnitudes of these correction factors were explained almost entirely by volume averaging with SVAM demonstrating positive correlation with all Monte Carlo established total correction factors.

      CONCLUSIONS: Plan complexity metrics do provide some quantitative basis for the investigation of plan clusters, but an objective clustering algorithm demonstrated that quantifiable differences could only be found between VMAT and step-and-shoot beams delivered on the same treatment machine. The inherent variability of the Monte Carlo determined correction factors could not be explained solely by the modality of the treatment but were instead almost entirely dependent upon the volume averaging correction, which itself depends on the detector position within the dose distribution, dose gradients, and other factors. Considering the continued difficulty of determining a relevant plan metric to base plan clusters on, case-by-case corrections may instead obviate the need for the pcsr field concept in the future.

      View details for PubMedID 32337734
  • VMAT and IMRT plan-specific correction factors for linac-based ionization chamber dosimetry. Med Phys
    Desai VK, Labby ZE, Hyun MA, DeWerd LA, Culberson WS
    2019 Feb; 46 (2): 913-924
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      PURPOSE: The determination of absorbed dose to water from external beam radiotherapy using radiation detectors is currently rooted in calibration protocols that do not account for modulations encountered in patient-specific deliveries. Detector response in composite clinical fields has not been extensively studied due to the time and effort required to determine these corrections on a case-by-case basis. To help bridge this gap in knowledge, corrections for the Exradin A1SL scanning chamber were determined in a large number of composite clinical fields using Monte Carlo methods. The chamber-specific perturbations that contribute the most to the overall correction factor were also determined.

      METHODS: A total of 131 patient deliveries comprised of 834 beams from a Varian C-arm linear accelerator were converted to EGSnrc Monte Carlo inputs. A validated BEAMnrc 21EX linear accelerator model was used as a particle source throughout the EGSnrc simulations. Composite field dose distributions were compared against a commercial treatment planning system for validation. The simulation geometry consisted of a cylindrically symmetric water-equivalent phantom with the Exradin A1SL scanning chamber embedded inside. Various chamber perturbation factors were investigated in the egs_chamber user code of EGSnrc and were compared to reference field conditions to determine the plan-specific correction factor.

      RESULTS: The simulation results indicated that the Exradin A1SL scanning chamber is suitable to use as an absolute dosimeter within a high-dose and low-gradient target region in most nonstandard composite fields; however, there are still individual cases that require larger delivery-specific corrections. The volume averaging and replacement perturbations showed the largest impact on the overall plan-specific correction factor for the Exradin A1SL scanning chamber, and both volumetric modulated arc therapy (VMAT) and step-and-shoot beams demonstrated similar correction factor magnitudes among the data investigated. Total correction magnitudes greater than 2% were required by 9.1% of step-and-shoot beams and 14.5% of VMAT beams. When examining full composite plan deliveries as opposed to individual beams, 0.0% of composite step-and-shoot plans and 2.6% of composite VMAT plans required correction magnitudes greater than 2%.

      CONCLUSIONS: The A1SL scanning chamber was found to be suitable to use for absolute dosimetry in high-dose and low-gradient dose regions of composite IMRT plans but even if a composite dose distribution is large compared to the detector used, a correction-free absorbed dose-to-water measurement is not guaranteed.

      View details for PubMedID 30449040

Contact Information

Vimal Desai, PhD

600 Highland Avenue, K4/B100,
Madison, WI 53792