Assessment of volumetric absorbed dose for mobile fluoroscopic 3D image acquisition

Mobile fluoroscopy (c-arm) units offering 3D image reconstruction are becoming more common in surgical settings. Although these images are "CT-like" and sometimes replace the postoperative CT, the acquisition is technically very different from a traditional CT acquisition. Dose assessment is complicated by a large beam width, automatic exposure rate control, and a rotation of less than 360°. The purpose of this work was to explore the impact of these factors on the volumetric dose calculation and to provide practical recommendations for clinical physicists assessing dose from these units using commonly available equipment. CTDI_W was calculated using the IAEA method for dosimetry of wide beams and compared to scans of the 32-cm CTDI phantom using the full beam width and a 20-mm collimated beam width. The impact of the partial rotation on the CTDI_W calculation was assessed by acquiring measurements at four and twelve positions on the phantom periphery. For the system tested, the CTDI_W was calculated to be 16.1 mGy using the IAEA method with default clinical protocol. Results showed that measuring CTDI_W with the full beam width or a collimated beam width alone resulted in CTDI values of 19.0 mGy and 19.5 mGy, respectively. Using four peripheral measurements instead of 12 resulted in a difference of 4% for a collimated beam and 6% for an open beam. Variations in positioning on the order of a few centimeters resulted in a variation of only 4% with an open beam. The excellent reproducibility of the measurements using the full beam width suggests that this simple method is adequate for year-to-year comparisons. In contrast, the IAEA method is difficult to employ, particularly with 180° acquisitions. Use of peripheral measurements in excess of the usual four is time-consuming and not necessary for most applications obtained with the geometry specific to this system.