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The online image-guided localization data from 696 ultrasound (US), 598 mega-voltage cone-beam computed tomography (MV-CBCT), and 393 seed markers (SMs) couch alignments for patients undergoing intensity modulation radiotherapy of the prostate were analyzed. Daily US, MV-CBCT and SM images were acquired for 19, 17 and 12 patients, respectively, after each patient was immobilized in a vacuum cradle and setup to skin markers as the center of mass. The couch shifts applied in the lateral (left-right/LR), vertical (anterior-posterior/AP), and longitudinal (superior-inferior/SI) directions, along with the magnitude of the three-dimensional (3D) shift vector, were analyzed and compared for all three methods. The percentage of shifts larger than 5 mm in all directions was also compared. Clinical target volume-planning target volume (CTV-to-PTV) expansion margins were estimated based on the localization data with US, CB, and SM image guidance. Results show the US data have greater variability. Systematic and random shifts were -1.2±6.8 mm (LR), -2.8±5.1 mm (SI) and -1.0±5.9 mm (AP) for US, 1.0±3.9 mm (LR), -1.3±2.5 mm (SI) and -0.3±3.9 mm (AP) for CB, and -1.0±3.4 mm (LR), 0.0±3.4 mm (SI) and 0.5±4.1 mm (AP) for SM. The mean 3D shift distance was larger using US (8.8±6.2 mm) compared to CB and SM (5.3±3.4 mm and 5.2±3.7 mm, respectively). The percentage of US shifts larger than 5 mm were 34%, 31%, and 38% in the LR, SI, and AP directions, respectively, compared to 18%, 6%, and 16% for CB and 14%, 10%, and 20% for SM. MV-CBCT and SM localization data suggest a different distribution of prostate center-of-mass shifts with smaller variability, compared to US. The online MV-CBCT and SM image-guidance data show that for treatments that do not include daily prostate localization, one can use a CTV-to-PTV margin that is 4 mm smaller than the one suggested by US data, hence allowing more rectum and bladder sparing and potentially improving the therapeutic ratio. [ABSTRACT FROM AUTHOR]

The technology of online mega-voltage cone-beam (CB) computed tomography (MV-CBCT) imaging is currently used in many institutions to generate a 3D anatomical dataset of a patient in treatment position. It utilizes an accelerator therapy beam, delivered with 200° gantry rotation, and captured by an electronic portal imager to account for organ motion and setup variations. Although the patient dose exposure from a single volumetric MV-CBCT imaging procedure is comparable to that from standard double-exposure orthogonal portal images, daily image localization procedures can result in a significant dose increase to healthy tissue. A technique to incorporate the daily dose, from a MV-CBCT imaging procedure, in the IMRT treatment planning optimization process was developed. A composite IMRT plan incorporating the total dose from the CB was optimized with the objective of ensuring uniform target coverage while sparing the surrounding normal tissue. One head and neck cancer patient and four prostate cancer patients were planned and treated using this technique. Dosimetric results from the prostate IMRT plans optimized with or without CB showed similar target coverage and comparable sparing of bladder and rectum volumes. Average mean doses were higher by 1.6±1.0 Gy for the bladder and comparable for the rectum (-0.3±1.4 Gy). In addition, an average mean dose increase of 1.9±0.8 Gy in the femoral heads and 1.7±0.6 Gy in irradiated tissue was observed. However, the V65 and V70 values for bladder and rectum were lower by 2.3±1.5% and 2.4±2.1% indicating better volume sparing at high doses with the optimized plans incorporating CB. For the head and neck case, identical target coverage was achieved, while a comparable sparing of the brain stem, optic chiasm, and optic nerves was observed. The technique of optimized planning incorporating doses from daily online MV-CBCT procedures provides an alternative method for imaging IMRT patients. It allows for daily treatment modifications where other volumetric tomographic imaging techniques may not be feasible and/or available and where accurate patient localization with a high degree of precision is required. [ABSTRACT FROM AUTHOR]

The improvement in conformal radiotherapy techniques with steep dose gradients has allowed for the delivery of higher doses to a tumor volume while maintaining the sparing of surrounding normal tissue. In this situation, verification of patient setup and evaluation of internal organ motion just prior to radiation delivery is a crucial step. To this end, several volumetric image-guided techniques have been developed for patient localization, such as the Siemens MVision mega-voltage cone beam CT (MV-CBCT) system. In this work, the commissioning and clinical implementation of the MVision system is presented. The geometry and gain calibration procedures for the system are described, and guidelines for quality assurance procedures are provided. Different MV-CBCT clinical protocols, ranging from daily to weekly image-guidance, which includes image acquisition, reconstruction, registration with planning CT, and treatment couch offsets corrections, were commissioned. The image quality characteristics of the MVision system were measured and assessed qualitatively and quantitatively, including the image noise and uniformity, low-contrast resolution, and spatial resolution. Furthermore, the image reconstruction and registration software was evaluated. Data show that a 2 cm large object with 1% electron density contrast can be detected with the MVision system with 10 cGy at isocenter and that the registration software is accurate within 2 mm in the anterior-posterior, left-right, and superior-inferior directions. [ABSTRACT FROM AUTHOR]