Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high‐energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low‐dose megavoltage cone‐beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high‐efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone‐beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone‐beam and 6X megavoltage cone‐beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone‐beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone‐beam images is 2.4−3.4 MU compared to 26 MU in 6 MV cone‐beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone‐beam, the 2.5 MV imaging system does not suffer from high‐Z image artifacts. This can be very useful for treatment planning in cases where high‐Z prostheses are present.PACS number(s): 87.57.Q‐

Purpose: To evaluate the feasibility of 2.5MV cone‐beam computed tomography (MVCB) for patient setup and verification. Method and Materials: A new linac (Varian Trilogy Mx) produces an unfiltered 2.5MV imaging beam (2xMVCB) in addition to the higher energy treatment beams. In conjunction with a standard electronic portal imaging device (EPID), 2xMVCB scans were acquired of CT calibration and anthropomorphic phantoms. Approximately 500 projections were collected in each 360‐degree scan using a total of 7.5 – 50 monitor units (MU). Depending on total MU, scanning time was as low as 1.5 minutes. Intrinsic calibration corrections were applied to all raw projections prior to reconstruction. Reconstructed images were further processed to correct for imaging artifacts and compared to kilovoltage cone‐beam (KVCB) and 6MV MVCB scans (6xMVCB). Results: Preliminary results demonstrate that it is possible to obtain 3D images with adequate image quality using relatively low doses (7.5 MU, approximately 4 cGy to isocenter) which are comparable to either a pair of double‐exposed MV portal images or a KVCB. Although image quality is inferior to KVCB, 2xMVCB still permits assessment of bony alignment for patient setup and has better tissue contrast than 6xMVCB. Soft tissue registration may also be possible for 2xMVCB in the thorax and other anatomical regions. These results provide evidence that 2xMVCB may be an efficient image guidance procedure for clinical routine. Conclusion: MVCB using 2.5MV x‐rays can provide volumetric images with adequate quality for patient setup verification. An improved EPID optimized for 2.5MV beam having ∼14 times higher quantum efficiency than conventional EPIDs is being tested. This will enable even lower imaging doses, and also improve image contrast in the abdominal and pelvic regions. Reconstruction algorithms optimized for 2.5MV are also under development. Acknowledgements: Research sponsored by Varian Medical Systems.

Purpose: To evaluate the feasibility of CsI detector for low dose 2.5 MV cone‐beam computed tomography (MVCB). Methods: The new Varian TrueBeamTM at our center is equipped with a prototype non‐clinical 2.5 MV imaging system with a 9 mm thick cesium iodide (CsI) scintillator EPID. Both static portal radiographs and MVCB images can be obtained. The minimum dose per beam pulse or frame is ∼0.0025 cGy. Approximately 450 projections were collected at up to 9 frames/second in a full 360 degrees rotation that may take 60–90 seconds. MVCB scans of a RANDO phantom were acquired using 2.5 MV and 6 MV x‐rays, the latter taken with conventional EPID (aS1000, Varian Medical Systems). KVCB scans were also taken for comparative purposes. An acrylic phantom containing high‐Z inserts was imaged with all beams to assess image artifacts. Results: The new 2.5X prototype system required only ∼2.4–3.4 cGy to achieve clinically acceptable image quality in the head, thorax, and pelvis, while standard 6X MVCB required ∼26 cGy. Image quality approaching kVCB can be obtained with 2.5X MVCB using doses of ∼ 9cGy, although images acquired using 2.4 cGy were sufficient for bony alignment purposes. As expected, MVCB images in the head and thorax are superior to pelvis. In addition, the 2.5 MV imaging beam greatly reduces high‐Z image artifacts that can significantly perturb kV image quality. Conclusions: 2.5X MVCB with high quantum efficiency CsI detector can replace kVCB in most clinical situations at greatly reduced cost and fewer high‐Z artifacts. Unlike kV, treatment planning systems can easily incorporate MV dose into plan optimization. Further, patients having metallic implants (e.g. dental fillings, hip prosthesis, etc.) can benefit from MVCB due to its insensitivity to high‐Z materials and its ability to yield true electron densities for treatment planning. Research partially supported by Varian Medical Systems.