I am an assistant professor in the Department of Human Oncology with primary roles in the clinic and research. In the clinic, I focus primarily on the clinical applications of the Siemens Somatom Definition Edge Dual Energy CT scanner. This includes preparation of the CT simulator for clinical use, ongoing quality assurance, implementation of motion management techniques, such as respiratory gating and 4DCT acquisitions, and the use of dual-energy scans to produce images with higher contrast.
My research has been primarily focused on expanding the clinical use of our dual-energy CT (DECT) simulation techniques to provide better options for our physicians to visualize certain tissues. In particular, I am investigating the use of DECT scans to better visualize and delineate bone marrow and bone lesions to provide more information to the physicians during contouring.
PhD, University of Wisconsin–Madison, Medical Physics (2016)
MS, University of Wisconsin–Madison, Medical Physics (2012)
BS, Boston College, Physics (2010)
Assistant Professor, Human Oncology (2016)
Selected Honors and Awards
Scholar Athlete Award, Boston College (2010)
Phi Beta Kappa Honor Society (2010)
Thomas McElroy Memorial Scholarship Recipient (2009)
Dean's Scholar Award, Boston College (2009)
Boards, Advisory Committees and Professional Organizations
UW Accredited Dosimetry Calibration Laboratory Advisory Board Member (2017-pres.)
American Association of Physicists in Medicine (2010-pres.)
UW Medical Radiation Research Center Research Oversight Committee Member (2013-2016)
Imaging, Radiation Measurements, Motion Management, Clinical Use of Dual Energy CT (DECT) Simulation Techniques
High-quality imaging lays an important foundation for a radiation oncology clinic and can aid in other aspects of the treatment delivery process.
The imaging systems in a radiation oncology department can be used for much more than a simple treatment planning aid. In my research, I hope to leverage the tools available in our clinic to produce higher quality treatments for our patients.
Dual Energy Computed Tomography for Bone Delineation
Using the tools available to us as a part of the Siemens Somatom Definition Edge Dual Energy CT (DECT) scanner, we have the ability to better visualize tissues that are historically difficult to differentiate. With this technology, we are working toward being able to delineate the red and yellow bone marrow present near a patient’s treatment site. Additionally, we are investigating the radiation response of bone marrow to doses in the therapeutic range, so as to be able to predict any effect on a patient’s hematopoetic system. Through the creation of virtual non-calcium images, we can better visualize abnormalities within bony structures, which will increase the visibility of bone lesions. The research project is founded in optimizing the algorithm parameters for our purposes and leveraging the system to perform optimally for these purposes.
Cone Beam Computed Tomography Dose Investigation
The dose from pre-treatment cone beam computed tomography scans has been investigated extensively, but most assessments focus on the in-field dose. In certain instances, such as the treatment of pregnant patients, the out-of-field dose from imaging procedures could contribute a significant dose to areas of concern (such as the fetus). Measuring this dose can prove difficult, and this research will investigate which radiation detectors can and should be used for these purposes and how to calibrate them. The dose information gathered from this investigation will be used to inform decisions on imaging protocols for clinical patients.
Monte Carlo and 60 Co-based kilovoltage x-ray dosimetry methods. Med Phys
Lawless MJ, Dimaso L, Palmer B, Micka J, Culberson WS, DeWerd LA
2018 Dec; 45 (12): 5564-5576
PURPOSE: This work seeks to investigate new methods to determine the absorbed dose to water from kilovoltage x rays. Current methods are based on measurements in air and rely on correction factors in order to account for differences between the photon spectrum in air and at depth in phantom, between the photon spectra of the calibration beam and the beam of interest, or in the radiation absorption properties of air and water. This work aims to determine the absorbed dose to water in the NIST-matched x-ray beams at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL). This will facilitate the use of detectors in terms of dose to water, which will allow for a simpler determination of dose to water in clinical kilovoltage x-ray beams.
MATERIALS AND METHODS: A model of the moderately filtered x-ray beams at the UWADCL was created using the BEAMnrc user code of the EGSnrc Monte Carlo code system. This model was validated against measurements and the dose to water per unit air kerma was calculated in a custom built water tank. Using this value and the highly precise measurement of the air kerma made by the UWADCL, the dose to water was determined in the water tank for the x-ray beams of interest. The dose to water was also determined using the formalism defined in the report of AAPM Task Group 61 and using a method that makes use of a 60 Co absorbed dose-to-water calibration coefficient and a beam quality correction factor to account for differences in beam quality between the 60 Co calibration and kilovoltage x-ray beam of interest. The dose to water values as determined by these different methods was then compared.
RESULTS: The BEAMnrc models used in this work produced simulations of transverse and depth dose profiles that agreed with measurements with a 2%/2 mm criteria gamma test. The dose to water as determined from the different methods used here agreed within 3.5% at the surface of the water tank and agreed within 1.8% at a depth of 2 cm in phantom. The dose-to-water values all agreed within the associated uncertainties of the methods used in this work. Both the Monte Carlo-based method and the 60 Co-based method had a lower uncertainty than the TG-61 methodology for all of the x-ray beams used in this work.
CONCLUSION: Two new dose determination methods were used to determine the dose to water in the NIST-matched x-ray beams at the UWADCL and they showed good agreement with previously established techniques. Due to the improved Monte Carlo calculation techniques used in this work, both of the methods have lower uncertainties compared to TG-61. The methods presented in this work compare favorably with calorimetry-based standards established at other institutions.View details for PubMedID 30273996
Design of a modulated orthovoltage stereotactic radiosurgery system. Med Phys
Fagerstrom JM, Bender ET, Lawless MJ, Culberson WS
2017 Jul; 44 (7): 3776-3787
PURPOSE: To achieve stereotactic radiosurgery (SRS) dose distributions with sharp gradients using orthovoltage energy fluence modulation with inverse planning optimization techniques.
METHODS: A pencil beam model was used to calculate dose distributions from an orthovoltage unit at 250 kVp. Kernels for the model were derived using Monte Carlo methods. A Genetic Algorithm search heuristic was used to optimize the spatial distribution of added tungsten filtration to achieve dose distributions with sharp dose gradients. Optimizations were performed for depths of 2.5, 5.0, and 7.5 cm, with cone sizes of 5, 6, 8, and 10 mm. In addition to the beam profiles, 4π isocentric irradiation geometries were modeled to examine dose at 0.07 mm depth, a representative skin depth, for the low energy beams. Profiles from 4π irradiations of a constant target volume, assuming maximally conformal coverage, were compared. Finally, dose deposition in bone compared to tissue in this energy range was examined.
RESULTS: Based on the results of the optimization, circularly symmetric tungsten filters were designed to modulate the orthovoltage beam across the apertures of SRS cone collimators. For each depth and cone size combination examined, the beam flatness and 80-20% and 90-10% penumbrae were calculated for both standard, open cone-collimated beams as well as for optimized, filtered beams. For all configurations tested, the modulated beam profiles had decreased penumbra widths and flatness statistics at depth. Profiles for the optimized, filtered orthovoltage beams also offered decreases in these metrics compared to measured linear accelerator cone-based SRS profiles. The dose at 0.07 mm depth in the 4π isocentric irradiation geometries was higher for the modulated beams compared to unmodulated beams; however, the modulated dose at 0.07 mm depth remained <0.025% of the central, maximum dose. The 4π profiles irradiating a constant target volume showed improved statistics for the modulated, filtered distribution compared to the standard, open cone-collimated distribution. Simulations of tissue and bone confirmed previously published results that a higher energy beam (≥ 200 keV) would be preferable, but the 250 kVp beam was chosen for this work because it is available for future measurements.
CONCLUSIONS: A methodology has been described that may be used to optimize the spatial distribution of added filtration material in an orthovoltage SRS beam to result in dose distributions with decreased flatness and penumbra statistics compared to standard open cones. This work provides the mathematical foundation for a novel, orthovoltage energy fluence-modulated SRS system.View details for PubMedID 28498612
Response of TLD-100 in mixed fields of photons and electrons. Med Phys
Lawless MJ, Junell S, Hammer C, DeWerd LA
2013 Jan; 40 (1): 012103
PURPOSE: Thermoluminescent dosimeters (TLDs) are routinely used for dosimetric measurements of high energy photon and electron fields. However, TLD response in combined fields of photon and electron beam qualities has not been characterized. This work investigates the response of TLD-100 (LiF:Mg,Ti) to sequential irradiation by high-energy photon and electron beam qualities.
METHODS: TLDs were irradiated to a known dose by a linear accelerator with a 6 MV photon beam, a 6 MeV electron beam, and a NIST-traceable (60)Co beam. TLDs were also irradiated in a mixed field of the 6 MeV electron beam and the 6 MV photon beam. The average TLD response per unit dose of the TLDs for each linac beam quality was normalized to the average response per unit dose of the TLDs irradiated by the (60)Co beam. Irradiations were performed in water and in a Virtual Water™ phantom. The 6 MV photon beam and 6 MeV electron beam were used to create dose calibration curves relating TLD response to absorbed dose to water, which were applied to the TLDs irradiated in the mixed field.
RESULTS: TLD relative response per unit dose in the mixed field was less sensitive than the relative response in the photon field and more sensitive than the relative response in the electron field. Application of the photon dose calibration curve to the TLDs irradiated in a mixed field resulted in an underestimation of the delivered dose, while application of the electron dose calibration curve resulted in an overestimation of the dose.
CONCLUSIONS: The relative response of TLD-100 in mixed fields fell between the relative response in the photon-only and electron-only fields. TLD-100 dosimetry of mixed fields must account for this intermediate response to minimize the estimation errors associated with calibration factors obtained from a single beam quality.View details for PubMedID 23298105
Michael Lawless, PhD600 Highland Avenue,
Madison, WI 53792-0001