Clinical Instructor, Radiation Oncology - Radiation Physics
Current Research and Scholarly Interests
Image guided robotic radiosurgery
Implementation of the Electronic Portal Imaging Device for Daily CyberKnife Quality Assurance
WILEY. 2018: E327
View details for Web of Science ID 000434978002053
Dose-Response Modeling of the Visual Pathway Tolerance to Single-Fraction and Hypofractionated Stereotactic Radiosurgery
SEMINARS IN RADIATION ONCOLOGY
2016; 26 (2): 97-104
Patients with tumors adjacent to the optic nerves and chiasm are frequently not candidates for single-fraction stereotactic radiosurgery (SRS) due to concern for radiation-induced optic neuropathy. However, these patients have been successfully treated with hypofractionated SRS over 2-5 days, though dose constraints have not yet been well defined. We reviewed the literature on optic tolerance to radiation and constructed a dose-response model for visual pathway tolerance to SRS delivered in 1-5 fractions. We analyzed optic nerve and chiasm dose-volume histogram (DVH) data from perioptic tumors, defined as those within 3mm of the optic nerves or chiasm, treated with SRS from 2000-2013 at our institution. Tumors with subsequent local progression were excluded from the primary analysis of vision outcome. A total of 262 evaluable cases (26 with malignant and 236 with benign tumors) with visual field and clinical outcomes were analyzed. Median patient follow-up was 37 months (range: 2-142 months). The median number of fractions was 3 (1 fraction n = 47, 2 fraction n = 28, 3 fraction n = 111, 4 fraction n = 10, and 5 fraction n = 66); doses were converted to 3-fraction equivalent doses with the linear quadratic model using α/β = 2Gy prior to modeling. Optic structure dose parameters analyzed included Dmin, Dmedian, Dmean, Dmax, V30Gy, V25Gy, V20Gy, V15Gy, V10Gy, V5Gy, D50%, D10%, D5%, D1%, D1cc, D0.50cc, D0.25cc, D0.20cc, D0.10cc, D0.05cc, D0.03cc. From the plan DVHs, a maximum-likelihood parameter fitting of the probit dose-response model was performed using DVH Evaluator software. The 68% CIs, corresponding to one standard deviation, were calculated using the profile likelihood method. Of the 262 analyzed, 2 (0.8%) patients experienced common terminology criteria for adverse events grade 4 vision loss in one eye, defined as vision of 20/200 or worse in the affected eye. One of these patients had received 2 previous courses of radiotherapy to the optic structures. Both cases were meningiomas treated with 25Gy in 5 fractions, with a 3-fraction equivalent optic nerve Dmax of 19.2 and 22.2Gy. Fitting these data to a probit dose-response model enabled risk estimates to be made for these previously unvalidated optic pathway constraints: the Dmax limits of 12Gy in 1 fraction from QUANTEC, 19.5Gy in 3 fractions from Timmerman 2008, and 25Gy in 5 fractions from AAPM Task Group 101 all had less than 1% risk. In 262 patients with perioptic tumors treated with SRS, we found a risk of optic complications of less than 1%. These data support previously unvalidated estimates as safe guidelines, which may in fact underestimate the tolerance of the optic structures, particularly in patients without prior radiation. Further investigation would refine the estimated normal tissue complication probability for SRS near the optic apparatus.
View details for DOI 10.1016/j.semradonc.2015.11.008
View details for Web of Science ID 000373242700003
Stereotactic Arrhythmia Radioablation (STAR) of Ventricular Tachycardia: A Treatment Planning Study.
2016; 8 (7)
The first stereotactic arrhythmia radioablation (STAR) of ventricular tachycardia (VT) was delivered at Stanford on a robotic radiosurgery system (CyberKnife® G4) in 2012. The results warranted further investigation of this treatment. Here we compare dosimetrically three possible treatment delivery platforms for STAR.The anatomy and target volume of the first treated patient were used for this study. A dose of 25 Gy in one fraction was prescribed to the planning target volume (PTV). Treatment plans were created on three treatment platforms: CyberKnife® G4 system with Iris collimator (Multiplan, V. 4.6)(Plan #1), CyberKnife® M6 system with InCise 2(TM) multileaf collimator (Multiplan V. 5.3)(Plan #2) and Varian TrueBeam(TM) STx with HD 120(TM) MLC and 10MV flattening filter free (FFF) beam (Eclipse planning system, V.11) (Plan #3 coplanar and #4 noncoplanar VMAT plans). The four plans were compared by prescription isodose line, plan conformity index, dose gradient, as well as dose to the nearby critical structures. To assess the delivery efficiency, planned monitor units (MU) and estimated treatment time were evaluated.Plans #1-4 delivered 25 Gy to the PTV to the 75.0%, 83.0%, 84.3%, and 84.9% isodose lines and with conformity indices of 1.19, 1.16, 1.05, and 1.05, respectively. The dose gradients for plans #1-4 were 3.62, 3.42, 3.93, and 3.73 with the CyberKnife® MLC plan (Plan #2) the best, and the TrueBeam(TM) STx co-planar plan (Plan #3) the worst. The dose to nearby critical structures (lung, stomach, bowel, and esophagus) were all well within tolerance. The MUs for plans #1-4 were 27671, 16522, 6275, and 6004 for an estimated total-treatment-time/beam-delivery-time of 99/69, 65/35, 37/7, and 56/6 minutes, respectively, under the assumption of 30 minutes pretreatment setup time. For VMAT gated delivery, a 40% duty cycle, 2400MU/minute dose rate, and an extra 10 minutes per extra arc were assumed.Clinically acceptable plans were created with all three platforms. Plans with MLC were considerably more efficient in MU. CyberKnife® M6 with InCise 2(TM) collimator provided the most conformal plan (steepest dose drop-off) with significantly reduced MU and treatment time. VMAT plans were most efficient in MU and delivery time. Fluoroscopic image guidance removes the need for additional fiducial marker placement; however, benefits may be moderated by worse dose gradient and more operator-dependent motion management by gated delivery.
View details for DOI 10.7759/cureus.694
View details for PubMedID 27570715
- Stereotactic ablative radiotherapy for the treatment of refractory cardiac ventricular arrhythmia. Circulation. Arrhythmia and electrophysiology 2015; 8 (3): 748-750
Trigeminal neuralgia treatment dosimetry of the Cyberknife
2012; 37 (1): 42-46
There are 2 Cyberknife units at Stanford University. The robot of 1 Cyberknife is positioned on the patient's right, whereas the second is on the patient's left. The present study examines whether there is any difference in dosimetry when we are treating patients with trigeminal neuralgia when the target is on the right side or the left side of the patient. In addition, we also study whether Monte Carlo dose calculation has any effect on the dosimetry. We concluded that the clinical and dosimetric outcomes of CyberKnife treatment for trigeminal neuralgia are independent of the robot position. Monte Carlo calculation algorithm may be useful in deriving the dose necessary for trigeminal neuralgia treatments.
View details for DOI 10.1016/j.meddos.2010.12.012
View details for Web of Science ID 000301035000009
View details for PubMedID 21723113
- TESTICULAR DOSE FROM PROSTATE CYBERKNIFE: A CAUTIONARY NOTE INTERNATIONAL JOURNAL OF RADIATION ONCOLOGY BIOLOGY PHYSICS 2009; 73 (2): 636-637
Stereotactic radiosurgery for a cardiac sarcoma: A case report
TECHNOLOGY IN CANCER RESEARCH & TREATMENT
2008; 7 (5): 363-367
Pulmonary artery intimal sarcoma is an uncommon tumor with a poor prognosis. We report a case of a 75-year-old man with a pulmonary artery sarcoma, recurrent following surgical resection. To palliate symptoms of this recurrence, he underwent CyberKnife stereotactic radiosurgery with a clinical and radiographic response of his treated disease. No acute or sub-acute toxicity was seen until the patient's death due to metastatic disease 10 weeks following treatment. The feasibility and short-term safety of this technique are reviewed, with emphasis on the stereotactic planning considerations, such as mediastinal organ movement and radiation tolerance.
View details for Web of Science ID 000259799000003
View details for PubMedID 18783285
Mapping of the prostate in endorectal coil-based MRI/MRSI and CT: A deformable registration and validation study
45th Annual Meeting of the American-Society-for-Therapeutic-Radiology-and-Oncology (ASTRO)
AMER ASSOC PHYSICISTS MEDICINE AMER INST PHYSICS. 2004: 3087–94
The endorectal coil is being increasingly used in magnetic resonance imaging (MRI) and MR spectroscopic imaging (MRSI) to obtain anatomic and metabolic images of the prostate with high signal-to-noise ratio (SNR). In practice, however, the use of endorectal probe inevitably distorts the prostate and other soft tissue organs, making the analysis and the use of the acquired image data in treatment planning difficult. The purpose of this work is to develop a deformable image registration algorithm to map the MRI/MRSI information obtained using an endorectal probe onto CT images and to verify the accuracy of the registration by phantom and patient studies. A mapping procedure involved using a thin plate spline (TPS) transformation was implemented to establish voxel-to-voxel correspondence between a reference image and a floating image with deformation. An elastic phantom with a number of implanted fiducial markers was designed for the validation of the quality of the registration. Radiographic images of the phantom were obtained before and after a series of intentionally introduced distortions. After mapping the distorted phantom to the original one, the displacements of the implanted markers were measured with respect to their ideal positions and the mean error was calculated. In patient studies, CT images of three prostate patients were acquired, followed by 3 Tesla (3 T) MR images with a rigid endorectal coil. Registration quality was estimated by the centroid position displacement and image coincidence index (CI). Phantom and patient studies show that TPS-based registration has achieved significantly higher accuracy than the previously reported method based on a rigid-body transformation and scaling. The technique should be useful to map the MR spectroscopic dataset acquired with ER probe onto the treatment planning CT dataset to guide radiotherapy planning.
View details for DOI 10.1118/1.106292
View details for Web of Science ID 000225372300019
View details for PubMedID 15587662
Long term results of radioactive gold grain implantation for the treatment of persistent and recurrent nasopharyngeal carcinoma
2001; 91 (6): 1105-1113
Brachytherapy is useful for the reirradiation of nasopharyngeal carcinoma. In the current study, the long term treatment results of permanent radioactive gold(198) grain interstitial implantation in patients with persistent and recurrent nasopharyngeal carcinoma were reviewed.Gold grain implantation was performed under direct vision with a split palate approach to provide 60 grays (Gy) 0.5 cm away from the plane of implantation. Between August 1986 and May 1999, 106 patients were treated with gold grain implantation (45 patients for persistent disease, 53 patients for first recurrence, and 8 patients for second recurrence in the nasopharynx). All patients had histologically proven disease by biopsy before undergoing implantation.Patients with persistent disease and those with first recurrence did well with the gold grain implantation. The 5-year local control rates for patients with persistent disease, first recurrence, and second recurrence in the nasopharynx were 87.2%, 62.7%, and 23.4%, respectively (P = 0.0004). The 5-year metastasis free survival rates were 68.1%, 60.3%, and 40%, respectively, for the 3 groups (P = 0.048). The overall survival rates at 5 years for the 3 groups were 79.1%, 53.6%, and 42.9%, respectively (P = 0.0047). Patients with computed tomography evidence of disease extension outside the nasopharynx had a lower local control rate compared with patients whose disease was confined to the nasopharynx (5-year local control rate of 52% vs. 72.3%; P = 0.031). The size of the lesion was not found to be an independent prognostic factor for local control after implantation. Multivariate analysis showed only an indication for implantation (persistent disease, first recurrence, and second recurrence) to be a significant prognostic factor for local control. Complications attributed to gold grain implantation included headache, palatal fistula, and mucosal radiation necrosis at the site of implantation, and were reported to occur in 28.3%, 18.9%, and 16%, respectively, of patients.For selected patients with disease confined to the nasopharynx, gold grain implantation is an effective salvage treatment for persistent and recurrent nasopharyngeal carcinoma.
View details for Web of Science ID 000167590400006
View details for PubMedID 11267955
The effect of the nasopharyngeal air cavity on x-ray interface doses
PHYSICS IN MEDICINE AND BIOLOGY
1998; 43 (3): 529-537
We investigated the impact of air cavities in head and neck cancer patients treated by photon beams based on clinical set-ups. The phantom for investigation was constructed with a cubic air cavity of 4 x 4 x 4 cm3 located at the centre of a 30 x 30 x 16 cm3 solid water slab. The cavity cube was used to resemble an extreme case for the nasal cavity. Apart from measuring the dose profiles and central axis percentage depth dose distribution, the dose values in 0.25 x 0.25 x 0.25 cm3 voxels at regions around the air cavity were obtained by Monte Carlo simulations. A mean dose value was taken over the voxels of interest at each depth for evaluation. Single-field results were added to study parallel opposed field effects. For 10 x 10 cm2 parallel opposed fields at 4, 6 and 8 MV, the mean dose at regions near the lateral interfaces of the cavity cube were decreased by 1 to 2% due to the lack of lateral scatter, while the mean dose near the proximal and distal interfaces was increased by 2 to 4% due to the greater transmission through air. Secondary build-up effects at points immediately beyond the air cavity cube are negligible using field sizes greater than 4 x 4 cm2. For most head and neck treatment, the field sizes are usually 6 x 6 cm2 or greater, and most cavity volumes are smaller than our chosen dimensions. Therefore, the influence of closed air cavities on photon interface doses is not significant in clinical treatment set-ups.
View details for Web of Science ID 000072502300005
View details for PubMedID 9533132