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.