Room: AAPM ePoster Library
Purpose: To quantify the patient specific imaging dose from ExacTrac system during marker-less real-time tumour monitoring (RTTM) for lung stereotactic body radiotherapy (SBRT) patients.
Methods: CT images of an adult female Rando ATOM phantom were acquired with a cylindrical solid water tumour placed in her left lung. Contours of the lungs, tumour, all bones, spinal cord, and skin were created. Three-dimensional dose distribution from the ExacTrac system was computed using a validated DoseXYZ Monte Carlo (EGSnrc) simulation of the ExacTrac system with standard thorax clinical protocol (120 kVp, 25 mAs). The Monte Carlo dose was calibrated against experimental surface dose measurements using TG-61 methodology using solid water slabs and a calibrated dosimeter. The dose for 50% volume (D50) and max dose deposited in each contoured organ were calculated.
To simulate RTTM for a typical SBRT patient, four fractions of two minutes each, with an imaging rate of 15 Hz was used (a total of 7200 exposures).
Results: The max dose/D50 (mGy) for each organ from single ExacTrac image pair was 0.0093/0.0078, 0.0147/0.001, 0.0633/0.0025, 0.0206/0.0034, 0.1164/0.0004, and 0.1081/0.0078 for the PTV, right lung, left lung, spinal cord, skin, and bone respectively.
For the simulated RTTM condition, the D50 (mGy) of the organs become 56.22, 7.21, 18.02, 24.50, 2.88, 33.87 for the PTV, right lung, left lung, spinal cord, skin, and bone respectively. The skin and bone received the highest maximum dose of 838.1 mGy and 779.11 mGy respectively.
Conclusion: Preliminary data shows that single stereoscopic ExacTrac imaging is not expected to significantly impact SBRT patient organ dose. However, real time imaging dose during RTTM may be clinically significant for planning purposes. The organs with the greatest risk of radiation toxicity during RTTM are the skin and bone. Further research utilizing patient data is ongoing.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by funding from the Atlantic Canada Opportunities Agency, Grant # 208515.