Demystifying the CT Radiation Dose Sheet.

The radiation dose sheet generated by the CT scanner is a form that displays important information about an examination. It functions as a road map for the examination, detailing what CT examinations were performed and what parameters were used to perform them. One essential element of the radiation dose sheet, the volume CT dose index (CTDIvol), is a commonly used radiation dose index that is displayed on most CT scanners. The CTDIvol is used for quality control and is helpful for comparing the radiation output among different protocols and different scanners. The dose-length product (DLP) is a radiation dose index that builds on the CTDIvol by incorporation of the scan length. The DLP is combined with a conversion coefficient and used to determine the effective dose from the CT examination. Determining the effective dose is a way to estimate the whole-body radiation dose, even if the CT examination is confined to a smaller part of the body. In addition to these values, other data about the study from the CT scanner manufacturer, including the tube voltage and tube current-time product, usually are displayed on CT scanners. These values are major determinants of the image quality and radiation dose. The radiation dose sheet is a useful tool for radiologists, technologists, and physicists, allowing them to comprehend the technical details of a CT examination. The authors describe the components of the radiation dose sheet, the relationships of these components with one another, and the contributions of these components to the radiation dose. ©RSNA, 2022.

The use of pencil ionization chamber with temporal readout capabilities to measure CT beam full width half maximum.

BACKGROUND: Measurement of Computed Tomography (CT) beam width is required by accrediting and regulating bodies for routine physics evaluations due to its direct correlation to patient dose. Current methods for performing CT beam width measurement require special hardware, software, and/or consumable films. Today, most 100-mm pencil chambers with a digital interface used to evaluate Computed Tomography Dose Index (CTDIvol) have a sufficiently high sampling rate to reconstruct a high-resolution dose profile for any acquisition mode. PURPOSE: The goal of this study is to measure the CT beam width from the sampled dose profile under a single helical acquisition with the 100-mm pencil chamber used for CTDIvol measurements. METHODS: The dose profiles for different scanners were measured for helical scans with varying collimation settings using a 100-mm pencil chamber placed at the isocenter and co-moving with the patient table. The measured dose profiles from the 100-mm pencil chamber were corrected for table attenuation by extracting a periodic correction function (PCF) to eliminate table interference. The corrected dose profiles were then deconvolved with the response function of the chamber to compute the beam profile. The beam width was defined by the full width half maximum (FWHM) of the resulting beam profile. Reference dose profiles were also measured using Gafchromic film for comparison. RESULTS: The beam widths, estimated using the innovative deconvolution method from the 100-mm pencil chamber, exhibit an average percentage difference of 1.6 ± 1.8 when compared with measurements obtained through Gafchromic film for beam width assessment. CONCLUSION: The proposed approach to deconvolve the pencil chamber response demonstrates the potential of obtaining the CT beam width at high accuracy without the need of special hardware, software, or consumable films. This technique can improve workflow for routine performance evaluation of CT systems.

Pre-clinical dosimetry of a new six-channel applicator for high-dose-rate treatment of esophageal cancer.

PURPOSE: To evaluate the dosimetry of a six-channel high-dose-rate (HDR) applicator for treatment of esophageal cancer with respect to lateral directionality and heterogeneous media. MATERIAL AND METHODS: A computed tomography (CT)- and magnetic resonance imaging (MRI)-compatible esophageal applicator consisting of 2 inflatable portions (anchor and therapeutic balloons) with 6 longitudinal treatment catheters equally spaced circumferentially was constructed. Treatment plans were prepared using Oncentra 4.5 for various catheter loadings and target locations and sizes. Calculated dose distributions were compared to measured distributions obtained using film and a water phantom. Balloon inflations with water and with air were tested. RESULTS: TG-43 dose calculations matched measurements well when inflation balloons were filled with water. When air was used to inflate, model-based dose calculations (TG-186) improved the comparison with measurement. Several cases with simulated ring targets demonstrated better dose conformity to non-uniform targets compared to a single central catheter. Additionally, the use of this applicator compared to a single catheter, gave rise to considerable improvement in sparing non-target tissue. CONCLUSIONS: Lateral dose modulation is achievable with the applicator described in this work. The use of TG-186 dose calculation made a small improvement in heterogeneous media.

Combining prior day contours to improve automated prostate segmentation.

PURPOSE: To improve the accuracy of automatically segmented prostate, rectum, and bladder contours required for online adaptive therapy. The contouring accuracy on the current image guidance [image guided radiation therapy (IGRT)] scan is improved by combining contours from earlier IGRT scans via the simultaneous truth and performance level estimation (STAPLE) algorithm. METHODS: Six IGRT prostate patients treated with daily kilo-voltage (kV) cone-beam CT (CBCT) had their original plan CT and nine CBCTs contoured by the same physician. Three types of automated contours were produced for analysis. (1) Plan: By deformably registering the plan CT to each CBCT and then using the resulting deformation field to morph the plan contours to match the CBCT anatomy. (2) Previous: The contour set drawn by the physician on the previous day CBCT is similarly deformed to match the current CBCT anatomy. (3) STAPLE: The contours drawn by the physician, on each prior CBCT and the plan CT, are deformed to match the CBCT anatomy to produce multiple contour sets. These sets are combined using the STAPLE algorithm into one optimal set. RESULTS: Compared to plan and previous, STAPLE improved the average Dice's coefficient (DC) with the original physician drawn CBCT contours to a DC as follows: Bladder: 0.81 ± 0.13, 0.91 ± 0.06, and 0.92 ± 0.06; Prostate: 0.75 ± 0.08, 0.82 ± 0.05, and 0.84 ± 0.05; and Rectum: 0.79 ± 0.06, 0.81 ± 0.06, and 0.85 ± 0.04, respectively. The STAPLE results are within intraobserver consistency, determined by the physician blindly recontouring a subset of CBCTs. Comparing plans recalculated using the physician and STAPLE contours showed an average disagreement less than 1% for prostate D98 and mean dose, and 5% and 3% for bladder and rectum mean dose, respectively. One scan takes an average of 19 s to contour. Using five scans plus STAPLE takes less than 110 s on a 288 core graphics processor unit. CONCLUSIONS: Combining the plan and all prior days via the STAPLE algorithm to produce treatment day contours is superior to the current standard of deforming only the plan contours to the daily CBCT. STAPLE also improves the precision, with a substantial decrease in standard deviation, a key for adaptive therapy. Geometrically and dosimetrically accurate contours can be automatically generated with STAPLE on prostate region kV CBCT in a time scale suitable for online adaptive therapy.