Bolch, Wesley E

Abstract Standard of care RPT may result in dose-limiting side effects of the salivary glands include sialadenitis and xerostomia that are inconsistent with outcomes from external beam for equivalent ADs. This work develops a parametrizable ‘Macro-to-Micro’ (M2μ) model that demonstrates the consideration of small-scale dosimetry in comparison to conventional methods that typically assume uniform voxel or organ-level uptake. Anatomical features of the salivary gland are represented by annular structures (a centralized branching network of ducts, including excretory, lobar, interlobular) and small-scale voxels (intralobular ducts and acinar cells) with dimensions set to reproduce ex-vivo murine histopathology measurements and scaled up in size for extrapolation to humans. Simulations were performed by scoring and recording S-value histograms to the target ductal and acinar cells, for two beta emitters (177Lu, 131I) and one alpha emitter (225Ac). Four idealized activity distributions were created to assign activity to surface and volumes of the annuli, and GEANT4 v11 was used for radiation transport calculations. Comparisons against whole gland uniform spherical self-dose S-value calculations were conducted to validate the radiation transport and to highlight differences between M2μ and conventional dosimetry approaches. Analyses for both models showed greater S-value variation in comparison to the homogeneously distributed activity S-value calculation. The most notable result was that the calculated S-values differed between different branches, depending on the geometric size of the annuli. For the surrounding acinar cells, S-values from ductal cells decreased as a function of distance from the branching structures. High variability of S-values, depending on ductal cell dimensions as well as within the acinar cell distribution of the salivary gland highlights the potential clinical utility of small-scale approaches to RPT salivary gland dosimetry, contingent on clinical translation and validation. To this end, future work should further refine the model and incorporate small-scale activity distributions from pre-clinical and translational studies.

Abstract Background Computed tomography (CT) is an essential imaging modality for disease diagnosis, treatment efficacy, and image‐based guidance of various medical procedures. The locally deposited radiation dose in tissues, as estimated by the computed tomography dose index (CTDI), can vary considerably across exposures delivered by CT scanners from different vendors, even if the scans are performed using similar technique factors, such as tube potential and tube current. The volumetric CTDI (CTDI vol ) is a common dose metric that reports an average radiation dose (in mGy) delivered to a specific volume within a test phantom. The CTDI vol is important in dosimetry applications as the organ absorbed dose within the patient has been shown to scale in near‐linear proportion, creating a basis for comparing organ doses across different scan protocols and scanner models. Purpose To develop a database of tube current‐time product (mAs) normalized CTDI vol values for currently utilized CT scanner models for each of the four primary CT vendors for use in the MIRDct organ dosimetry software available at MIRDsoft.org. This data forms the basis of the MIRDct code, which reports organ doses across a range of computational phantoms based upon axial organ dose coefficient libraries generated through Monte Carlo radiation transport for a reference CT scanner. Organ doses delivered by alternate CT scanner vendors and models may then be reported using ratios of normalized CTDI vol values under similar technique factors. Methods Scanners were selected from four major CT manufacturers: Philips Healthcare, GE Healthcare, Canon Medical Systems, and Siemens Healthineers. Technique parameters were also selected for each scanner that closely matched values used in the generation of an equivalent CT source term (small to large bowtie filters; 80–140‐kVp tube voltage; and 10‐mm to 40‐mm beam collimation). For each scanner chosen, the appropriate technique factors and protocols were selected, and the console‐reported CTDI vol values were recorded and normalized to a set value of 100 mAs. The normalized CTDI vol data collected for use within the MIRDct code were analyzed for noticeable patterns, features, and trends, and were compared to similar normalized CTDI vol datasets used within the National Cancer Institute NCICT software and the Virtual Phantoms, Inc. VirtualDose software. Results For all given CT scanner and technique factor combinations, there was strong agreement in normalized CTDI vol values across all three codes: between 0% and 12% difference for the compared scanners. Ratios of CTDI vol values for various CT scanner vendors and models to the corresponding CTDI vol values for the MIRDct reference scanner (Cannon Aquilion One Genesis) were also compared on the basis of either the 16‐cm head PMMA phantom or the 32‐cm body PMMA phantom. The mean quotient of these normalized CTDI vol ratios (head ratios to body ratios) was found to be approximately 1.06, and thus either ratio may be applied in reporting patient organ dose by MIRDct. Conclusions A database of normalized CTDI vol (mGy/100 mAs) was created for 17 models of CT scanners from four manufacturers at varying tube potentials, collimations, x‐ray bowtie filters, and phantom sizes for use in the MIRDct software.