Room: Karl Dean Ballroom A1
Purpose: To demonstrate proof of principle for end-to-end evaluation of a single isocenter VMAT radiosurgery of multifocal metastases using a NIPAM 3D dosimeter for which dose is observed as increased electron density on x-ray CT.
Methods: Two dosimeters were prepared at one facility and shipped cold packed to a second (remote) facility for irradiation. A simulation CT was acquired and radiotherapy plans prepared including a dose calibration plan and a 4 arc VMAT SRS plan to 6 targets with 1cm diameter. Dosimeters were aligned via CBCT and irradiated, followed by 3 CBCTs acquired immediately, and 5 diagnostic CTs acquired after >24 hours, which were averaged for dose analysis. Various methods were evaluated for absolute dose calibration, including eliminating need for a background subtraction measured with a blank dosimeter. We also developed a method to analyze dose and spatial accuracy using a quasi-dose calibration based solely on the multifocal SRS irradiation (not requiring a second dose calibration irradiation), and using image thresholding in the clinical TPS for analysis.
Results: Dose was proportional to electron density above 10-12Gy. 3D gamma pass rates were 94.0% and 99.5% using a 5% / 1mm and 3% / 2mm criteria, respectively (threshold = 15Gy, using a global dose criteria). The centroid of each target was compared with the treatment plan using image thresholding on the average of the post-irradiation diagnostic CTs; which indicated submillimeter agreement. The SRS targets were also visible on the post-irradiation CBCTs.
Conclusion: This work demonstrates the potential for a powerful and convenient end-to-end QA technique which incorporates: (1) capability of remote dosimetry, (2) minimal measurements and analysis, (3) ease of analysis within the clinical TPS and (4) a shared frame of reference (from CBCT) eliminating the need to register the plan and measured distributions.
Funding Support, Disclosures, and Conflict of Interest: Dr. Adamson reports ownership in Clearsight RT LLC, which is not related to this study. Authors Carroll, Trager, Yoon, Kodra, Maynard, Hilts, Oldham, and Jirasek have nothing to disclose.
3D, Radiosurgery, Quality Assurance
TH- Radiation dose measurement devices: 3D solid gel/plastic