Bio

Academic Appointments


Professional Education


  • Board Certification, American Board of Radiology, Therapeutic Radiologic Physics (2011)
  • Residency, Stanford, Radiation Oncology, Physics (2007)
  • PhD, Queen's University, Kingston, Canada, Physics (2000)
  • MSc, Queen's University, Kingston, Canada, Physics (1995)
  • Bsc, University of Toronto, Toronto, Canada, Physics (1993)

Research & Scholarship

Current Research and Scholarly Interests


Current research interests are inter- and intra- fractional motion, focusing on SBRT and realtime imaging. I have also recently been looking at quality assurance for gated VMAT delivery

Publications

Journal Articles


  • Verification of dosimetric accuracy on the TrueBeam STx: Rounded leaf effect of the high definition MLC MEDICAL PHYSICS Kielar, K. N., Mok, E., Hsu, A., Wang, L., Luxton, G. 2012; 39 (10): 6360-6371

    Abstract

    The dosimetric leaf gap (DLG) in the Varian Eclipse treatment planning system is determined during commissioning and is used to model the effect of the rounded leaf-end of the multileaf collimator (MLC). This parameter attempts to model the physical difference between the radiation and light field and account for inherent leakage between leaf tips. With the increased use of single fraction high dose treatments requiring larger monitor units comes an enhanced concern in the accuracy of leakage calculations, as it accounts for much of the patient dose. This study serves to verify the dosimetric accuracy of the algorithm used to model the rounded leaf effect for the TrueBeam STx, and describes a methodology for determining best-practice parameter values, given the novel capabilities of the linear accelerator such as flattening filter free (FFF) treatments and a high definition MLC (HDMLC).During commissioning, the nominal MLC position was verified and the DLG parameter was determined using MLC-defined field sizes and moving gap tests, as is common in clinical testing. Treatment plans were created, and the DLG was optimized to achieve less than 1% difference between measured and calculated dose. The DLG value found was tested on treatment plans for all energies (6 MV, 10 MV, 15 MV, 6 MV FFF, 10 MV FFF) and modalities (3D conventional, IMRT, conformal arc, VMAT) available on the TrueBeam STx.The DLG parameter found during the initial MLC testing did not match the leaf gap modeling parameter that provided the most accurate dose delivery in clinical treatment plans. Using the physical leaf gap size as the DLG for the HDMLC can lead to 5% differences in measured and calculated doses.Separate optimization of the DLG parameter using end-to-end tests must be performed to ensure dosimetric accuracy in the modeling of the rounded leaf ends for the Eclipse treatment planning system. The difference in leaf gap modeling versus physical leaf gap dimensions is more pronounced in the more recent versions of Eclipse for both the HDMLC and the Millennium MLC. Once properly commissioned and tested using a methodology based on treatment plan verification, Eclipse is able to accurately model radiation dose delivered for SBRT treatments using the TrueBeam STx.

    View details for DOI 10.1118/1.4752444

    View details for Web of Science ID 000310101900055

    View details for PubMedID 23039672

  • Prognostic Value of Metabolic Tumor Volume and Velocity in Predicting Head-and-Neck Cancer Outcomes INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Chu, K. P., Murphy, J. D., La, T. H., Krakow, T. E., Iagaru, A., Graves, E. E., Hsu, A., Maxim, P. G., Loo, B., Chang, D. T., Quynh-Thu Le, Q. T. 2012; 83 (5): 1521-1527

    Abstract

    We previously showed that metabolic tumor volume (MTV) on positron emission tomography-computed tomography (PET-CT) predicts for disease recurrence and death in head-and-neck cancer (HNC). We hypothesized that increases in MTV over time would correlate with tumor growth and biology, and would predict outcome. We sought to examine tumor growth over time in serial pretreatment PET-CT scans.From 2006 to 2009, 51 patients had two PET-CT scans before receiving HNC treatment. MTV was defined as the tumor volume ? 50% of maximum SUV (SUV(max)). MTV was calculated for the primary tumor, nodal disease, and composite (primary tumor + nodes). MTV and SUV velocity were defined as the change in MTV or SUV(max) over time, respectively. Cox regression analyses were used to examine correlations between SUV, MTV velocity, and outcome (disease progression and overall survival).The median follow-up time was 17.5 months. The median time between PET-CT scans was 3 weeks. Unexpectedly, 51% of cases demonstrated a decrease in SUV(max) (average, -0.1 cc/week) and MTV (average, -0.3 cc/week) over time. Despite the variability in MTV, primary tumor MTV velocity predicted disease progression (hazard ratio 2.94; p = 0.01) and overall survival (hazard ratio 1.85; p = 0.03).Primary tumor MTV velocity appears to be a better prognostic indicator of disease progression and survival in comparison to nodal MTV velocity. However, substantial variability was found in PET-CT biomarkers between serial scans. Caution should be used when PET-CT biomarkers are integrated into clinical protocols for HNC.

    View details for DOI 10.1016/j.ijrobp.2011.10.022

    View details for Web of Science ID 000306128100047

    View details for PubMedID 22270168

  • Feasibility of low-dose single-view 3D fiducial tracking concurrent with external beam delivery MEDICAL PHYSICS Speidel, M. A., Wilfley, B. P., Hsu, A., Hristov, D. 2012; 39 (4): 2163-2169

    Abstract

    In external-beam radiation therapy, existing on-board x-ray imaging chains orthogonal to the delivery beam cannot recover 3D target trajectories from a single view in real-time. This limits their utility for real-time motion management concurrent with beam delivery. To address this limitation, the authors propose a novel concept for on-board imaging based on the inverse-geometry Scanning-Beam Digital X-ray (SBDX) system and evaluate its feasibility for single-view 3D intradelivery fiducial tracking.A chest phantom comprising a posterior wall, a central lung volume, and an anterior wall was constructed. Two fiducials were placed along the mediastinal ridge between the lung cavities: a 1.5 mm diameter steel sphere superiorly and a gold cylinder (2.6 mm length × 0.9 mm diameter) inferiorly. The phantom was placed on a linear motion stage that moved sinusoidally. Fiducial motion was along the source-detector (z) axis of the SBDX system with ±10 mm amplitude and a programmed period of either 3.5 s or 5 s. The SBDX system was operated at 15 frames per second, 100 kVp, providing good apparent conspicuity of the fiducials. With the stage moving, detector data were acquired and subsequently reconstructed into 15 planes with a 12 mm plane-to-plane spacing using digital tomosynthesis. A tracking algorithm was applied to the image planes for each temporal frame to determine the position of each fiducial in (x,y,z)-space versus time. A 3D time-sinusoidal motion model was fit to the measured 3D coordinates and root mean square (RMS) deviations about the fitted trajectory were calculated.Tracked motion was sinusoidal and primarily along the source-detector (z) axis. The RMS deviation of the tracked z-coordinate ranged from 0.53 to 0.71 mm. The motion amplitude derived from the model fit agreed with the programmed amplitude to within 0.28 mm for the steel sphere and within -0.77 mm for the gold seed. The model fit periods agreed with the programmed periods to within 7%.Three dimensional fiducial tracking with approximately 1 mm or better accuracy and precision appears to be feasible with SBDX, supporting its use to guide radiotherapy.

    View details for DOI 10.1118/1.3697529

    View details for Web of Science ID 000302371900045

    View details for PubMedID 22482637

  • RADIOTHERAPY DOSE PERTURBATION OF ESOPHAGEAL STENTS EXAMINED IN AN EXPERIMENTAL MODEL INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Atwood, T. F., Hsu, A., Ogara, M. M., Luba, D. G., Tamler, B. J., Disario, J. A., Maxim, P. G. 2012; 82 (5): 1659-1664

    Abstract

    To investigate the radiotherapy dose perturbations caused by esophageal stents in patients undergoing external beam treatments for esophageal cancer.Four esophageal stents were examined (three metallic stents: WallFlex, Ultraflex, and Alveolus; one nonmetallic stent with limited radiopaque markers for visualization: Polyflex). All experiments were performed in a liquid water phantom with a custom acrylic stent holder. Radiochromic film was used to measure the dose distributions adjacent to the stents at locations proximal and distal to the radiation source. The stents were placed in an air-filled cavity to simulate the esophagus. Treatment plans were created and delivered for photon energies of 6 and 15 MV, and data analysis was performed on uniform regions of interest, according to the size and geometric placement of the films, to quantify the dose perturbations.The three metallic stents produced the largest dose perturbations with distinct patterns of "hot" spots (increased dose) measured proximal to the radiation source (up to 15.4%) and both "cold" (decreased dose) and hot spots measured distal to the radiation source (range, -6.1%-5.8%). The polymeric Polyflex stent produced similar dose perturbations when the radiopaque markers were examined (range, -7.6%-15.4%). However, when the radiopaque markers were excluded from the analysis, the Polyflex stent produced significantly smaller dose perturbations, with maximum hot spots of 7.3% and cold spots of -3.2%.The dose perturbations caused by esophageal stents during the treatment of esophageal cancer using external beam radiotherapy should be understood. These perturbations will result in hot and cold spots in the esophageal mucosa, with varying magnitudes depending on the stent. The nonmetallic Polyflex stent appears to be the most suitable for patients undergoing radiotherapy, but further studies are necessary to determine the clinical significance of the dose perturbations.

    View details for DOI 10.1016/j.ijrobp.2011.02.020

    View details for Web of Science ID 000301891300031

    View details for PubMedID 21514064

  • An end-to-end examination of geometric accuracy of IGRT using a new digital accelerator equipped with onboard imaging system PHYSICS IN MEDICINE AND BIOLOGY Wang, L., Kielar, K. N., Mok, E., Hsu, A., Dieterich, S., Xing, L. 2012; 57 (3): 757-769

    Abstract

    The Varian's new digital linear accelerator (LINAC), TrueBeam STx, is equipped with a high dose rate flattening filter free (FFF) mode (6 MV and 10 MV), a high definition multileaf collimator (2.5 mm leaf width), as well as onboard imaging capabilities. A series of end-to-end phantom tests were performed, TrueBeam-based image guided radiation therapy (IGRT), to determine the geometric accuracy of the image-guided setup and dose delivery process for all beam modalities delivered using intensity modulated radiation therapy (IMRT) and RapidArc. In these tests, an anthropomorphic phantom with a Ball Cube II insert and the analysis software (FilmQA (3cognition)) were used to evaluate the accuracy of TrueBeam image-guided setup and dose delivery. Laser cut EBT2 films with 0.15 mm accuracy were embedded into the phantom. The phantom with the film inserted was first scanned with a GE Discovery-ST CT scanner, and the images were then imported to the planning system. Plans with steep dose fall off surrounding hypothetical targets of different sizes were created using RapidArc and IMRT with FFF and WFF (with flattening filter) beams. Four RapidArc plans (6 MV and 10 MV FFF) and five IMRT plans (6 MV and 10 MV FFF; 6 MV, 10 MV and 15 MV WFF) were studied. The RapidArc plans with 6 MV FFF were planned with target diameters of 1 cm (0.52 cc), 2 cm (4.2 cc) and 3 cm (14.1 cc), and all other plans with a target diameter of 3 cm. Both onboard planar and volumetric imaging procedures were used for phantom setup and target localization. The IMRT and RapidArc plans were then delivered, and the film measurements were compared with the original treatment plans using a gamma criteria of 3%/1 mm and 3%/2 mm. The shifts required in order to align the film measured dose with the calculated dose distributions was attributed to be the targeting error. Targeting accuracy of image-guided treatment using TrueBeam was found to be within 1 mm. For irradiation of the 3 cm target, the gammas (3%, 1 mm) were found to be above 90% in all plan deliveries. For irradiations of smaller targets (2 cm and 1 cm), similar accuracy was achieved for 6 MV and 10 MV beams. Slightly degraded accuracy was observed for irradiations with higher energy beam (15 MV). In general, gammas (3%, 2 mm) were found to be above 97% for all the plans. Our end-to-end tests showed an excellent relative dosimetric agreement and sub-millimeter targeting accuracy for 6 MV and 10 MV beams, using both FFF and WFF delivery methods. However, increased deviations in spatial and dosimetric accuracy were found when treating lesions smaller than 2 cm or with 15 MV beam.

    View details for DOI 10.1088/0031-9155/57/3/757

    View details for Web of Science ID 000299542000013

    View details for PubMedID 22252134

  • ON-BOARD IMAGING VALIDATION OF OPTICALLY GUIDED STEREOTACTIC RADIOSURGERY POSITIONING SYSTEM FOR CONVENTIONALLY FRACTIONATED RADIOTHERAPY FOR PARANASAL SINUS AND SKULL BASE CANCER INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Maxim, P. G., Loo, B. W., Murphy, J. D., Chu, K. P., Hsu, A., Quynh-Thu Le, Q. T. 2011; 81 (4): 1153-1159

    Abstract

    To evaluate the positioning accuracy of an optical positioning system for stereotactic radiosurgery in a pilot experience of optically guided, conventionally fractionated, radiotherapy for paranasal sinus and skull base tumors.Before each daily radiotherapy session, the positioning of 28 patients was set up using an optical positioning system. After this initial setup, the patients underwent standard on-board imaging that included daily orthogonal kilovoltage images and weekly cone beam computed tomography scans. Daily translational shifts were made after comparing the on-board images with the treatment planning computed tomography scans. These daily translational shifts represented the daily positional error in the optical tracking system and were recorded during the treatment course. For 13 patients treated with smaller fields, a three-degree of freedom (3DOF) head positioner was used for more accurate setup.The mean positional error for the optically guided system in patients with and without the 3DOF head positioner was 1.4 ± 1.1 mm and 3.9 ± 1.6 mm, respectively (p <.0001). The mean positional error drifted 0.11 mm/wk upward during the treatment course for patients using the 3DOF head positioner (p = .057). No positional drift was observed in the patients without the 3DOF head positioner.Our initial clinical experience with optically guided head-and-neck fractionated radiotherapy was promising and demonstrated clinical feasibility. The optically guided setup was especially useful when used in conjunction with the 3DOF head positioner and when it was recalibrated to the shifts using the weekly portal images.

    View details for DOI 10.1016/j.ijrobp.2010.08.049

    View details for Web of Science ID 000296823600035

    View details for PubMedID 21543166

  • SINGLE-FRACTION STEREOTACTIC BODY RADIATION THERAPY AND SEQUENTIAL GEMCITABINE FOR THE TREATMENT OF LOCALLY ADVANCED PANCREATIC CANCER INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Schellenberg, D., Kim, J., Ciristman-Skieller, C., Chun, C. L., Columbo, L. A., Ford, J. M., Fisher, G. A., Kunz, P. L., Van Dam, J., Quon, A., Desser, T. S., Norton, J., Hsu, A., Maxim, P. G., Xing, L., Goodman, K. A., Chang, D. T., Koong, A. C. 2011; 81 (1): 181-188

    Abstract

    This Phase II trial evaluated the toxicity, local control, and overall survival in patients treated with sequential gemcitabine and linear accelerator-based single-fraction stereotactic body radiotherapy (SBRT).Twenty patients with locally advanced, nonmetastatic pancreatic adenocarcinoma were enrolled on this prospective single-institution, institutional review board-approved study. Gemcitabine was administered on Days 1, 8, and 15, and SBRT on Day 29. Gemcitabine was restarted on Day 43 and continued for 3-5 cycles. SBRT of 25 Gy in a single fraction was delivered to the internal target volume with a 2- 3-mm margin using a nine-field intensity-modulated radiotherapy technique. Respiratory gating was used to account for breathing motion. Follow-up evaluations occurred at 4-6 weeks, 10-12 weeks, and every 3 months after SBRT.All patients completed SBRT and a median of five cycles of chemotherapy. Follow-up for the 2 remaining alive patients was 25.1 and 36.4 months. No acute Grade 3 or greater nonhematologic toxicity was observed. Late Grade 3 or greater toxicities occurred in 1 patient (5%) and consisted of a duodenal perforation (G4). Three patients (15%) developed ulcers (G2) that were medically managed. Overall, median survival was 11.8 months, with 1-year survival of 50% and 2-year survival of 20%. Using serial computed tomography, the freedom from local progression was 94% at 1 year.Linear accelerator-delivered SBRT with sequential gemcitabine resulted in excellent local control of locally advanced pancreatic cancer. Future studies will address strategies for reducing long-term duodenal toxicity associated with SBRT.

    View details for DOI 10.1016/j.ijrobp.2010.05.006

    View details for Web of Science ID 000294093300025

    View details for PubMedID 21549517

  • INTENSITY-MODULATED RADIOTHERAPY FOR ORAL CAVITY SQUAMOUS CELL CARCINOMA: PATTERNS OF FAILURE AND PREDICTORS OF LOCAL CONTROL INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Daly, M. E., Quynh-Thu Le, Q. T., Kozak, M. M., Maxim, P. G., Murphy, J. D., Hsu, A., Loo, B. W., Kaplan, M. J., Fischbein, N. J., Chang, D. T. 2011; 80 (5): 1412-1422

    Abstract

    Few studies have evaluated the use of intensity-modulated radiotherapy (IMRT) for squamous cell carcinoma (SCC) of the oral cavity (OC). We report clinical outcomes and failure patterns for these patients.Between October 2002 and June 2009, 37 patients with newly diagnosed SCC of the OC underwent postoperative (30) or definitive (7) IMRT. Twenty-five patients (66%) received systemic therapy. The median follow-up was 38 months (range, 10-87 months). The median interval from surgery to RT was 5.9 weeks (range, 2.1-10.7 weeks).Thirteen patients experienced local-regional failure at a median of 8.1 months (range, 2.4-31.9 months), and 2 additional patients experienced local recurrence between surgery and RT. Seven local failures occurred in-field (one with simultaneous nodal and distant disease) and two at the margin. Four regional failures occurred, two in-field and two out-of-field, one with synchronous metastases. Six patients experienced distant failure. The 3-year actuarial estimates of local control, local-regional control, freedom from distant metastasis, and overall survival were 67%, 53%, 81%, and 60% among postoperative patients, respectively, and 60%, 60%, 71%, and 57% among definitive patients. Four patients developed Grade ? 2 chronic toxicity. Increased surgery to RT interval predicted for decreased LRC (p = 0.04).Local-regional control for SCC of the OC treated with IMRT with or without surgery remains unsatisfactory. Definitive and postoperative IMRT have favorable toxicity profiles. A surgery-to-RT interval of < 6 weeks improves local-regional control. The predominant failure pattern was local, suggesting that both improvements in target delineation and radiosensitization and/or dose escalation are needed.

    View details for DOI 10.1016/j.ijrobp.2010.04.031

    View details for Web of Science ID 000293207600020

    View details for PubMedID 20675073

  • Intensity-Modulated Radiation Therapy Versus Conventional Radiation Therapy for Squamous Cell Carcinoma of the Anal Canal CANCER Bazan, J. G., Hara, W., Hsu, A., Kunz, P. A., Ford, J., Fisher, G. A., Welton, M. L., Shelton, A., Kapp, D. S., Koong, A. C., Goodman, K. A., Chang, D. T. 2011; 117 (15): 3342-3351

    Abstract

    The purpose of this study was to compare outcomes in patients with anal canal squamous cell carcinoma (SCCA) who were treated with definitive chemoradiotherapy by either intensity-modulated radiation therapy (IMRT) or conventional radiotherapy (CRT).Forty-six patients who received definitive chemoradiotherapy from January 1993 to August 2009 were included. Forty-five patients received 5-fluorouracil with mitomycin C (n = 39) or cisplatin (n = 6). Seventeen (37%) were treated with CRT and 29 (63%) with IMRT. The median dose was 54 Gy in both groups. Median follow-up was 26 months (CRT) and 32 months (IMRT). T3-T4 stage (P = .18) and lymph node-positive disease (P = .6) were similar between groups.The CRT group required longer treatment duration (57 days vs 40 days, P < .0001), more treatment breaks (88% vs 34.5%, P = .001), and longer breaks (12 days vs 1.5 days, P < .0001) than patients treated with IMRT. Eleven (65%) patients in the CRT group experienced grade >2 nonhematologic toxicity compared with 6 (21%) patients in the IMRT group (P = .003). The 3-year overall survival (OS), locoregional control (LRC), and progression-free survival were 87.8%, 91.9%, and 84.2%, respectively, for the IMRT groups and 51.8%, 56.7%, and 56.7%, respectively, for the CRT group (all P < .01). On multivariate analysis, T stage, use of IMRT, and treatment duration were associated with OS, and T stage and use of IMRT were associated with LRC.The use of IMRT was associated with less toxicity, reduced need for treatment breaks, and excellent LRC and OS compared with CRT in patients with SCCA of the anal canal.

    View details for DOI 10.1002/cncr.25901

    View details for Web of Science ID 000293103800008

    View details for PubMedID 21287530

  • INTENSITY-MODULATED RADIOTHERAPY FOR LOCALLY ADVANCED CANCERS OF THE LARYNX AND HYPOPHARYNX HEAD AND NECK-JOURNAL FOR THE SCIENCES AND SPECIALTIES OF THE HEAD AND NECK Daly, M. E., Le, Q., Jain, A. K., Maxim, P. G., Hsu, A., Loo, B. W., Kaplan, M. J., Fischbein, N. J., Colevas, A. D., Pinto, H., Chang, D. T. 2011; 33 (1): 103-111

    Abstract

    Limited data evaluate intensity-modulated radiotherapy (IMRT) for cancers of the hypopharynx and larynx. We report clinical outcomes and failure patterns for these patients.Between September 2001 and December 2007, 42 patients with squamous cell carcinoma (SCC) of the hypopharynx (n = 23) and larynx (n = 19) underwent IMRT, 11 postoperatively and 31 definitively. Thirty-six received systemic therapy. Median follow-up was 30 months among surviving patients.Three local failures occurred within the high-dose region and 3 occurred in regional nodes. Seven patients developed distant metastasis as the initial failure. Three-year actuarial estimates of locoregional control, freedom from distant metastasis, and overall survival rates were, respectively, 80%, 72%, and 46%.IMRT provides good locoregional control for SCC of the hypopharynx and larynx compared with historical controls. Locoregional relapses occurred in the high-dose volumes, suggesting adequate target volume delineation. Hypopharyngeal tumors, which fare worse than laryngeal tumors, warrant investigation of more aggressive treatment.

    View details for DOI 10.1002/hed.21406

    View details for Web of Science ID 000286290400017

    View details for PubMedID 20848427

  • INCIDENTAL TESTICULAR IRRADIATION FROM PROSTATE IMRT: IT ALL ADDS UP INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS King, C. R., Maxim, P. G., Hsu, A., Kapp, D. S. 2010; 77 (2): 484-489

    Abstract

    To identify the technical aspects of image-guided intensity-modulated radiation therapy (IMRT) for localized prostate cancer that could result in a clinically meaningful incidental dose to the testes.We examined three sources that contribute incidental dose to the testes, namely, from internal photon scattering from IMRT small field and large pelvic nodal fields with 6 or 15 MV, from neutrons when >10-MV photons are used, and from daily image-guided fiducial-based portal imaging. Using clinical data from 10 patients who received IMRT for prostate cancer, and thermo-luminescent dosimeter measurements in phantom, we estimated the dose to the testes from each of these sources.A mean testicular dose of 172 and 220 cGy results from internal photon scatter for pelvic nodal fields and 68 and 93 cGy for prostate-only fields, for 6- and 15-MV energies, respectively. For 15-MV photon energies, the mean testicular dose from neutrons is 60 cGy for pelvic fields and 31 cGy for prostate-only fields. From daily portal MV image guidance, the testes-in-field mean dose is 350 cGy, whereas the testes-out-of-field scatter dose is 16 cGy. Dosimetric comparisons between IMRT using 6-MV and 15-MV photon energies are not significantly different. Worst-case scenarios can potentially deliver cumulative incidental mean testicular doses of 630 cGy, whereas best-case scenarios can deliver only 84 cGy.Incidental dose to the testes from prostate IMRT can be minimized by opting to restrict the use of elective pelvic nodal fields, by choosing photon energies <10 MV, and by using the smallest port sizes necessary for daily image guidance.

    View details for DOI 10.1016/j.ijrobp.2009.04.083

    View details for Web of Science ID 000278167500023

    View details for PubMedID 19733013

  • IMAGE-GUIDED RADIOTHERAPY IN NEAR REAL TIME WITH INTENSITY-MODULATED RADIOTHERAPY MEGAVOLTAGE TREATMENT BEAM IMAGING INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Mao, W., Hsu, A., Riaz, N., Lee, L., Wiersma, R., Luxton, G., King, C., Xing, L., Solberg, T. 2009; 75 (2): 603-610

    Abstract

    To utilize image-guided radiotherapy (IGRT) in near real time by obtaining and evaluating the online positions of implanted fiducials from continuous electronic portal imaging device (EPID) imaging of prostate intensity-modulated radiotherapy (IMRT) delivery.Upon initial setup using two orthogonal images, the three-dimensional (3D) positions of all implanted fiducial markers are obtained, and their expected two-dimensional (2D) locations in the beam's-eye-view (BEV) projection are calculated for each treatment field. During IMRT beam delivery, EPID images of the megavoltage treatment beam are acquired in cine mode and subsequently analyzed to locate 2D locations of fiducials in the BEV. Simultaneously, 3D positions are estimated according to the current EPID image, information from the setup portal images, and images acquired at other gantry angles (the completed treatment fields). The measured 2D and 3D positions of each fiducial are compared with their expected 2D and 3D setup positions, respectively. Any displacements larger than a predefined tolerance may cause the treatment system to suspend the beam delivery and direct the therapists to reposition the patient.Phantom studies indicate that the accuracy of 2D BEV and 3D tracking are better than 1 mm and 1.4 mm, respectively. A total of 7330 images from prostate treatments were acquired and analyzed, showing a maximum 2D displacement of 6.7 mm and a maximum 3D displacement of 6.9 mm over 34 fractions.This EPID-based, real-time IGRT method can be implemented on any external beam machine with portal imaging capabilities without purchasing any additional equipment, and there is no extra dose delivered to the patient.

    View details for DOI 10.1016/j.ijrobp.2009.04.068

    View details for Web of Science ID 000269941600040

    View details for PubMedID 19735886

  • A study of image-guided intensity-modulated radiotherapy with fiducials for localized prostate cancer including pelvic lymph nodes INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS Hsu, A., Pawlicki, T., Luxton, G., Hara, W., King, C. R. 2007; 68 (3): 898-902

    Abstract

    To study the impact on nodal coverage and dose to fixed organs at risk when using daily fiducial localization of the prostate to deliver intensity-modulated radiotherapy (IMRT).Five patients with prostate cancer in whom prostate and pelvic nodes were irradiated with IMRT were studied. Dose was prescribed such that 95% of the prostate planning target volume (PTV) and 90% of the nodal PTV were covered. Random and systematic prostate displacements in the anterior-posterior, superior-inferior, and left-right directions were simulated to shift the original isocenter of the IMRT plan. The composite dose during the course of treatment was calculated.Compared with a static setup, simulating random shifts reduced dose by less than 1.5% for nodal hotspot (i.e., dose to 1 cm(3)), by less than 1% for the 90% nodal PTV coverage, and by less than 0.5% for the nodal mean dose. Bowel and femoral head hotspots were reduced by less than 1.5% and 2%, respectively. A 10-mm systematic offset reduced nodal coverage by up to 10%.The use of prostate fiducials for daily localization during IMRT treatment results in negligible changes in dose coverage of pelvic nodes or normal tissue sparing in the absence of a significant systematic offset. This offers a simple and practical solution to the problem of image-guided radiotherapy for prostate cancer when including pelvic nodes.

    View details for DOI 10.1016/j.ijrobp.2007.02.030

    View details for Web of Science ID 000247284600035

    View details for PubMedID 17459610

  • Investigation of linac-based image-guided hypofractionated prostate radiotherapy MEDICAL DOSIMETRY Pawlicki, T., Kim, G., Hsu, A., Cotrutz, C., Boyer, A. L., Xing, L., King, C. R., Luxton, G. 2007; 32 (2): 71-79

    Abstract

    A hypofractionation treatment protocol for prostate cancer was initiated in our department in December 2003. The treatment regimen consists of a total dose of 36.25 Gy delivered at 7.25 Gy per fraction over 10 days. We discuss the rationale for such a prostate hypofractionation protocol and the need for frequent prostate imaging during treatment. The CyberKnife (Accuray Inc., Sunnyvale, CA), a linear accelerator mounted on a robotic arm, is currently being used as the radiation delivery device for this protocol, due to its incorporation of near real-time kV imaging of the prostate via 3 gold fiducial seeds. Recently introduced conventional linac kV imaging with intensity modulated planning and delivery may add a new option for these hypofractionated treatments. The purpose of this work is to investigate the use of intensity modulated radiotherapy (IMRT) and the Varian Trilogy Accelerator with on-board kV imaging (Varian Medical Systems Inc., Palo Alto, CA) for treatment of our hypofractionated prostate patients. The dose-volume histograms and dose statistics of 2 patients previously treated on the CyberKnife were compared to 7-field IMRT plans. A process of acquiring images to observe intrafraction prostate motion was achieved in an average time of about 1 minute and 40 seconds, and IMRT beam delivery takes about 40 seconds per field. A complete 7-field IMRT plan can therefore be imaged and delivered in 10 to 17 minutes. The Varian Trilogy Accelerator with on-board imaging and IMRT is well suited for image-guided hypofractionated prostate treatments. During this study, we have also uncovered opportunities for improvement of the on-board imaging hardware/software implementation that would further enhance performance in this regard.

    View details for DOI 10.1016/j.meddos.2007.01.004

    View details for Web of Science ID 000246485600002

    View details for PubMedID 17472885

  • Feasibility of using ultrasound for real-time tracking during radiotherapy MEDICAL PHYSICS Hsu, A., Miller, N. R., Evans, P. M., Bamber, J. C., Webb, S. 2005; 32 (6): 1500-1512

    Abstract

    This study was designed to examine the feasibility of utilizing transabdominal ultrasound for real-time monitoring of target motion during a radiotherapy fraction. A clinical Acuson 128/XP ultrasound scanner was used to image various stationary and moving phantoms while an Elekta SL25 linear accelerator radiotherapy treatment machine was operating. The ultrasound transducer was positioned to image from the outer edge of the treatment field at all times. Images were acquired to videotape and analyzed using in-house motion tracking algorithms to determine the effect of the SL25 on the quality of the displacement measurements. To determine the effect on the dosimetry of the presence of the transducer, dose distributions were examined using thermoluminescent dosimeters loaded into an Alderson Rando phantom and exposed to a 10 x 10 cm2 treatment field with and without the ultrasound transducer mounted 2.5 cm outside the field edge. The ultrasound images acquired a periodic noise that was shown to occur at the pulsing frequency of the treatment machine. Images of moving tissue were analyzed and the standard deviation on the displacement estimates within the tissue was identical with the SL25 on and off. This implies that the periodic noise did not significantly degrade the precision of the tracking algorithm (which was better than 0.01 mm). The presence of the transducer at the surface of the phantom presented only a 2.6% change to the dose distribution to the volume of the phantom. The feasibility of ultrasonic motion tracking during radiotherapy treatment is demonstrated. This presents the possibility of developing a noninvasive, real-time and low-cost method of tracking target motion during a treatment fraction.

    View details for DOI 10.1118/1.1915934

    View details for Web of Science ID 000229908600007

    View details for PubMedID 16013706

Conference Proceedings


  • Comparison of Intensity-Modulated Radiotherapy and 3-Dimensional Conformal Radiotherapy as Adjuvant Therapy for Gastric Cancer Minn, A. Y., Hsu, A., La, T., Kunz, P., Fisher, G. A., Ford, J. M., Norton, J. A., Visser, B., Goodman, K. A., Koong, A. C., Chang, D. T. JOHN WILEY & SONS INC. 2010: 3943-3952

    Abstract

    The current study was performed to compare the clinical outcomes and toxicity in patients treated with postoperative chemoradiotherapy for gastric cancer using intensity-modulated radiotherapy (IMRT) versus 3-dimensional conformal radiotherapy (3D CRT).Fifty-seven patients with gastric or gastroesophageal junction cancer were treated postoperatively: 26 with 3D CRT and 31 with IMRT. Concurrent chemotherapy was capecitabine (n=31), 5-fluorouracil (5-FU) (n=25), or none (n=1). The median radiation dose was 45 Gy. Dose volume histogram parameters for kidney and liver were compared between treatment groups.The 2-year overall survival rates for 3D CRT versus IMRT were 51% and 65%, respectively (P=.5). Four locoregional failures occurred each in the 3D CRT (15%) and the IMRT (13%) patients. Grade>or=2 acute gastrointestinal toxicity was found to be similar between the 3D CRT and IMRT patients (61.5% vs 61.2%, respectively) but more treatment breaks were needed (3 vs 0, respectively). The median serum creatinine from before radiotherapy to most recent creatinine was unchanged in the IMRT group (0.80 mg/dL) but increased in the 3D CRT group from 0.80 mg/dL to 1.0 mg/dL (P=.02). The median kidney mean dose was higher in the IMRT versus the 3D CRT group (13.9 Gy vs 11.1 Gy; P=.05). The median kidney V20 was lower for the IMRT versus the 3D CRT group (17.5% vs 22%; P=.17). The median liver mean dose for IMRT and 3D CRT was 13.6 Gy and 18.6 Gy, respectively (P=.19). The median liver V30 was 16.1% and 28%, respectively (P<.001).Adjuvant chemoradiotherapy was well tolerated. IMRT was found to provide sparing to the liver and possibly renal function.

    View details for DOI 10.1002/cncr.25246

    View details for Web of Science ID 000280677100025

    View details for PubMedID 20564136

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