PV QA 3 - Poster Viewing Q&A 3
TU_10_3215 - Optimal margin Selection and the Resultant Impact on Organs-at-Risk in Stereotactic Body Radiation Therapy of Liver Tumors
Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3
Habeeb Saleh, PhD
University of Kansas Health System: Chief Medical Physicist: Employee
Optimal margin Selection and the Resultant Impact on Organs-at-Risk in Stereotactic Body Radiation Therapy of Liver Tumors
H. Saleh1, M. A. Thompson2, J. Xu2, K. S. Qamar2, and K. Kauweloa3; 1University of Kansas Medical Center, Kansas City, KS, 2Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS, 3University of Kansas Cancer Center, Kansas City, KS
Purpose/Objective(s): The primary objective of this study was to analyze the impact on dose to organs-at-risk (OAR’s) in liver stereotactic body radiation therapy (SBRT) patients. Analysis of optimal PTV margins derived from electromagnetic transponder information implanted in and around liver tumor was performed
Materials/Methods: Five liver SBRT patients were implanted with 3 electromagnetic transponders for patient setup, breathing motion tracking, and treatment gating. The beacons implanting location and depth were determined by an interventional radiologist (IR) and a medical physicist based on MRI images. For each patient, a 10-phase 4DCT scan was acquired. Patients ITV ranged from 10 – 57 cm3. One patient has a lesion adjacent to the stomach, and one has lesion adjacent to the right kidney. The prescribed dose was 800-1000 cGy per fraction to a total of 4000-5000 cGy. RTOG 1112 protocol recommends 2750 cGy – 5000 cGy dose for liver SBRT depending on the tumor proximity to critical structure. Electromegnatic transponders based breathing motion managment system was used to track patient lesions position at the rate of 30 Hz. Gating thresholds was set to 3 -5 mm. Retrospectively, patients were planned with PTV margins of 1, 3, 5, 7 and 10 mm. The mean PTV, normal liver tissue and critical organ doses were evaluated.
Results: For all patient plans, 100% of ITV, and 95% of PTV are encompassed by the prescription dose. The Liver D700cc doses ranged between 205 cGy – 1287 cGy, the liver 2100 cGy volume ranged between 100-372 cm3, and the stomach D5ccdoses ranged between 316 – 2615 cGy with 1 – 10mm PTV margins respectively. Table 1 shows doses to normal liver tissue and OAR for 3 mm PTV margins. These doses were adjusted to 5000 cGy prescription. With 3 mm margins, the highest dose to 5 cc of stomach is 2140 cGy which is less than the 2650 cGy tolerance. The wide range of doses to critical organs is due to tumor size and proximity to critical organs.
Conclusion: This study shows that using implanted beacons during liver SBRT, tight margin can be selected to significantly reduce normal tissue toxicity without decreasing PTV coverage. In addition, dose escalation can be used. With electromegnatic transponders based beam gating system, PTV margins as small as 3mm can be used. Table 1: Dose to liver normal tissue and OAR for 3 mm margins
| Patients || Liver D700cc (cGy) || Liver V1000 (cm3) || Liver V2100 (cm3) || Stomach D5cc (cGy) || Kidney D200cc (cGy) |
| Patient 1 || 215 || 617 || 108 || 342 || 15 |
| Patient 2 || 23 || 176 || 86 || 587 || 164 |
| Patient 3 || 368 || 460 || 273 || 470 || 117 |
| Patient 4 || 835 || 617 || 262 || 2140 || 43 |
| Patient 5 || 240 || 313 || 112 || 509 || 144 |
Author Disclosure: H. Saleh: None. M. Thompson: None. J. Xu: None. K.S. Qamar: None.