Evaluation of Changes in Dose Estimation on Abdomen CT Scan with Automatic Tube Current Modulation Using In-House Phantom

This study evaluates the effect of the Automatic Tube Current Modulation (ATCM) technique on pitch and effective diameter variation in estimating dose values and noise levels for abdominal examination on Philips Ingenuity CT scan machine using in-house Phantoms. The in-house phantoms are oval in shape with three effective diameter sizes, namely 23.2 cm, 28.3 cm, and 33.3 cm to represent abdominal region. The three size Phantoms were scanned using an Ingenuity 128 Philips CT scan with the abdominal protocol exposure parameters of 120 kVp tube voltage, Dose Right Index (DRI) variations of 10,11,12,13, and 14, and pitch variations of 0.6; 0.8; 1.0; 1.2; and 1.49. The changes in mAs, CTDIvol, and noise to the Philips reference value were then verified (i.e. an addition of one DRI value increases mAs by 12 %). For evaluation, a metric to express the change in DRI is defined as ΔDRI. The study demonstrates that noise level is influenced by object size; size information of the object could be useful to predict the change of tube current and pitch due to ATCM with respect to selected DRI. The DRI value is proportional to the tube current, thus selecting the DRI at a certain pitch will directly determine tube current. The ΔDRI in general, according to Philips specifications, is verified to be approximately 10 % to 13 %, except for DRI 10 to 11 which is relatively high on average 15 % to 17 %. Increasing DRI increases the CTDIvol. The CTDI/mAs constantly ranges of 0.06 to 0.07. The value could serve as a characteristic parameter for quality assurance. The ATCM specifications of the Ingenuity 128 CT Scanner is according to Philips regulations

Stereotactic ultrasound for target volume definition in a patient with prostate cancer and bilateral total hip replacement

Purpose: Target-volume definition for prostate cancer in patients with bilateral metal total hip replacements (THRs) is a challenge because of metal artifacts in the planning computed tomography (CT) scans. Magnetic resonance imaging (MRI) can be used for matching and prostate delineation; however, at a spatial and temporal distance from the planning CT, identical rectal and vesical filling is difficult to achieve. In addition, MRI may also be impaired by metal artifacts, even resulting in spatial image distortion. Here, we present a method to define prostate target volumes based on ultrasound images acquired during CT simulation and online-matched to the CT data set directly at the planning CT. Methods and materials: A 78-year-old patient with cT2cNxM0 prostate cancer with bilateral metal THRs was referred to external beam radiation therapy. T2-weighted MRI was performed on the day of the planning CT with preparation according to a protocol for reproducible bladder and rectal filling. The planning CT was obtained with the immediate acquisition of a 3-dimensional ultrasound data set with a dedicated stereotactic ultrasound system for online intermodality image matching referenced to the isocenter by ceiling-mounted infrared cameras. MRI (offline) and ultrasound images (online) were thus both matched to the CT images for planning. Daily image guided radiation therapy (IGRT) was performed with transabdominal ultrasound and compared with cone beam CT. Results: Because of variations in bladder and rectal filling and metal-induced image distortion in MRI, soft-tissue-based matching of the MRI to CT was not sufficient for unequivocal prostate target definition. Ultrasound-based images could be matched, and prostate, seminal vesicles, and target volumes were reliably defined. Daily IGRT could be successfully completed with transabdominal ultrasound with good accordance between cone beam CT and ultrasound. Conclusions: For prostate cancer patients with bilateral THRs causing artifacts in planning CTs, ultrasound referenced to the isocenter of the CT simulator and acquired with intermodal online coregistration directly at the planning CT is a fast and easy method to reliably delineate the prostate and target volumes and for daily IGRT

Ultrasound-based repositioning and real-time monitoring for abdominal SBRT in DIBH

Aim: Ultrasound-based repositioning and real-time-monitoring aim at the improvement of the precision of SBRT in deep inspiration breath-hold (DIBH). Accuracy of ultrasound-based daily repositioning was estimated by comparison with DIBH-cone-beam-CT. Intrafraction motion during beam-delivery was assessed by ultrasound-real-time-monitoring. Patients/methods: Residual error after ultrasound-based interfractional repositioning (85 fractions, 16 SBRT-series; 14 patients) was assessed by marker-based (7 series) or liver-contour-based (9 series) matching in DIBH-CBCT. During beam-delivery, the percentage of 3D misalignment vector below 2 mm, between 2 and 5 mm, 5–7 mm and over 7 mm was estimated. Percentage of relevant target-displacements was analyzed as a function of DIBH-duration. Results: Residual error after ultrasound-based positioning was 0.4 ± 3.3 mm in LR (left-right), 0.2 ± 4.3 mm in CC (cranio-caudal) and 1.0 ± 3.0 mm in AP (anterior-posterior) directions (vector magnitude 5.4 ± 3.3 mm, MV ± SD). Over 544 DIBHs, target displacement was 1.3 ± 0.5 mm, 0.7 ± 0.3 mm, 1.6 ± 0.6 mm for CC, LR and AP directions, respectively (3D-vector 2.5 ± 0.7 mm). 3D misalignment vector length was below 2 mm in 49.8%, between 2 and 7 mm in 46.3%, and over 7 mm in 3.9% of the beam-delivery-time. During the first 5 s of the DIBH, 3D-misalignment vector length was always below 10 mm. Percentage of target displacements over 10 mm was 0.2%, 0.5% and 0.8% for 10 s, 15 s and 20 s DIBH-duration. Conclusions: Ultrasound-based interfractional repositioning is an accurate method for daily localization of abdominal DIBH-SBRT targets. Residual motion is <7 mm in 96% of the beam-delivery-time. Deviations >10 mm occur rarely and can be avoided by gating the beam at a predefined threshold. Ideal DIBH-duration should not exceed 15 s