Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_17_3281 - MRI based Treatment Planning of Spinal Stereotactic Radiation Therapy

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

MRI based Treatment Planning of Spinal Stereotactic Radiation Therapy
J. R. Teruel1, H. Wang1, A. McCarthy1, K. S. Osterman1, P. B. Schiff1, H. Chandarana2, and I. J. Das1; 1Department of Radiation Oncology, NYU Langone Health, New York, NY, 2Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, NYU Langone Health, New York, NY

Purpose/Objective(s): Patients treated with stereotactic body radiation therapy (SBRT) benefit from magnetic resonance imaging (MRI) exams to accurately delineate the lesion to be treated. MRI based synthetic CT (sCT) has shown promising results for MRI-only planning for various treatment sites. The purpose of this study is to evaluate the dependence of spinal/sacral SBRT dosimetry on bone Hounsfield units (HU), and its implications for MRI based sCT planning.

Materials/Methods: Ten treated spinal/sacral SBRT plans were retrospectively evaluated. All treatments were planned in a commercially available treatment planning system using the AAA dose calculation algorithm with grid size of 0.1 or 0.15 cm (VMAT technique, 6 MV photons). The HU value of bone for each plan was manually overwritten, and the plan recalculated with the same MUs without optimization for the following HU values [0, 300, 600, 900, 1200, 1500 and 2000]. Dosimetric parameters for the PTV and spinal cord were obtained for each HU value and the percent dose deviation from the original plan calculated. Additionally, an SBRT treatment plan was simulated for one subject that received diagnostic CT and MRI. The plan was created by placing a virtual PTV in the spinal region and optimized on MRI based sCT. MRI based sCT was generated using a Dixon plus atlas based method. The plan was copied onto the diagnostic CT and calculated for comparison.

Results: The mean bone HU in 10 treatment plans ranged between 208 and 385 with an average of 291 prior to modification. Percent deviation of dose parameters for PTV (n=10) and spinal cord, if in the field, (n=5) are shown in table. PTV dose deviations were below 1% for 0, 300 and 600 bone HU, and increased up to 4% for 2000 bone HU. For the spinal cord, percent deviations of Dmax and D0.35cc were below 1% up to 900 bone HU and reached 2% for 2000 bone HU. Percent deviations in PTV dose introduced by optimizing on a Dixon+atlas sCT instead of the diagnostic CT were: 0.29% for Dmax, 0.20% for Dmin, -0.06% for Dmedian, and -0.09% for D95%. And for the spinal cord were: -0.12% for Dmax, and -0.15% for D0.35cc.

Conclusion: MRI based sCT can provide accurate treatment plans for spinal/sacral SBRT with a minimum dose deviation from conventional CT plans; thus, MRI only planning could be used for spinal SBRT.
Bone HU

0

300

600

900

1200

1500

2000

PTV Dmax 0.60 [0.25] 0.15 [0.09] -0.14 [0.26] -0.36 [0.50] -0.64 [0.79] -0.90 [1.03] -1.25[1.31]
PTV Dmin 0.64 [0.47] 0.12 [0.14] -0.56 [0.52] -1.33[0.95] -2.25 [1.39] -3.13 [1.75] -4.35 [2.21]
PTV Dmedian 0.55 [0.25] 0.09 [0.14] -0.50 [0.23] -1.18 [0.39] -1.98 [0.57] -2.74 [0.73] -3.85 [0.95]
PTV D95% 0.41 [0.24] 0.10 [0.13] -0.44 [0.19] -1.12 [0.33] -1.99 [0.52] -2.82 [0.70] -4.02 [0.97]
Spinal Cord Dmax 0.73 [0.12] 0.15 [0.13] -0.29 [0.13] -0.67 [0.16] -1.08 [0.18] -1.46 [0.23] -2.04 [0.34]
Spinal Cord D0.35cc 0.55 [0.11] 0.12 [0.13] -0.28 [0.13] -0.69 [0.15] -1.17 [0.18] -1.62 [0.23] -2.29 [0.35]
Percent dose deviation from original plan. Values reported as mean and [95% confidence level].

Author Disclosure: J.R. Teruel: Employee; Mount Sinai. H. Wang: None. A. McCarthy: None. K. Osterman: Committee Member; AAPM TG-109 Task Group. P.B. Schiff: Independent Contractor; Guidepoint Global, Schlesinger Associates. H. Chandarana: None. I.J. Das: Honoraria; JASTRO, Japanese Society of Therapeutic Radiation. Speaker's Bureau; JASTRO, Japanese Society of Therapeutic Radiation. Travel Expenses; JASTRO, Japanese Society of Therapeutic Radiation. Committee Member and surveyor; ACR. Associate Editor; Br J Radiology, Medical Physics. examiner; ABR. Committe member and Chair; AAPM.

Jose Teruel, PhD

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