Radiation dose estimates based on Monte Carlo simulation for spiral breast computed tomography imaging in a large cohort of patients

Abstract

Background: Spiral breast computed tomography (BCT) equipped with a photon-counting detector (PCD) is a new radiological modality allowing for the compression-free acquisition of high-resolution 3-D datasets of the breast.Optimized dose exposu04170/re setups according to breast size were previously proposed but could not effectively be applied in a clinical environment due to ambiguity in measuring breast size.Purpose: This study aims to report the standard radiation dose values in a large cohort of patients examined with BCT, and to provide a mathematical model to estimate radiation dose based on morphological features of the breast. Methods: This retrospective study was conducted on 1657 BCT examinations acquired between 2018 and 2021 from 829 participants (57 ± 10 years, all female). Applying a dedicated breast tissue segmentation algorithm and Monte Carlo (MC) simulation, mean absorbed dose (MAD), mean glandular dose (MGD), mean skin dose (MSD), maximum glandular dose (maxGD), and maximum skin dose (maxSD) were calculated and related to morphological features such as breast volume, effective diameter, breast length, skin volume, and glandularity. Effective dose (ED) was calculated by applying the corresponding beam and tissue weighting factors, 1 Sv/Gy and 0.12 per breast. Relevant morphological features predicting dose values were identified based on the Spearman's rank correlation coefficient. Exponential or bi-exponential models predicting the dose values as a function of morphological features were fitted by using a nonlinear least squares (LS) method. The models were validated by assessing R 2 and residual standard error (RSE). Results: The most relevant morphological features for radiation dose estimation were the breast volume (correlation coefficient: −0.8), diameter (−0.7), and length (−0.6). The glandularity presented a weak-positive correlation (0.4) with MGD and maxGD due to the inhomogeneous distribution of the glandularity and absorbed dose in the 3-D breast volume. The standard MGDs were calculated to be 7.3 ± 0.7, 6.5 ± 0.3, and 5.9 ± 0.3 mGy, MADs to 7.6 ± 0.8, 6.8 ± 0.3, and 6.2 ± 0.3 mGy, maxSDs to 19.9 ± 1.6, 19.5 ± 0.5, and 18.9 ± 0.5 mGy, and EDs to 0.88 ± 0.08, 0.78 ± 0.04, and 0.72 ± 0.04 mSv for small, medium, and large breasts with average breast lengths of 5.9 ± 1.6, 8.7 ± 1.3, and 12.2 ± 2.0 cm, respectively. The estimated glandularity -23.1 ± 16.9, 12.5 ± 11.4, and 6.9 ± 7.3% from small to large breasts. The mathematical models were able to estimate the MAD, MGD, MSD, and maxSD as a function of each morphological feature with only upto 0.5 mGy RSE.

Virtual dosimetry study with three cone-beam breast computed tomography scanners using a fast GPU-based Monte Carlo code

“…gCTD is also more computationally efficient (by a factor 10 2 ) with respect to the gMCDRR code of Qin et al (2022) written for cone-beam CT scatter correction, which run on the same hardware GPU platform. Using a single NVIDIA Quadro P5000 GPU card and the ImpactMC commercial MC dose simulation software, Shim et al (2023) report a computation speed equal to 1.1-2.6 × 10 7 primary photons s -1 for their simulations on patient-derived anthropomorphic breast phantoms, which is a performance two orders of magnitude worse than in our study. In their paper, Shim et al (2023) report a maximum statistical uncertainty of 1% when launching 2 × 10 7 photons per projection with 2000 projections per rotation (i.e., 4 × 10 10 photons in each virtual scan), against about 4 × 10 12 photons launched in each virtual scan of this study (table 2).…”

“…Using a single NVIDIA Quadro P5000 GPU card and the ImpactMC commercial MC dose simulation software, Shim et al (2023) report a computation speed equal to 1.1-2.6 × 10 7 primary photons s -1 for their simulations on patient-derived anthropomorphic breast phantoms, which is a performance two orders of magnitude worse than in our study. In their paper, Shim et al (2023) report a maximum statistical uncertainty of 1% when launching 2 × 10 7 photons per projection with 2000 projections per rotation (i.e., 4 × 10 10 photons in each virtual scan), against about 4 × 10 12 photons launched in each virtual scan of this study (table 2). Hence, by reducing by two orders of magnitude the number of photons launched in each simulation, the fast VCT-BREAST tool can produce 3D dose maps within less than 1 min of GPU time.…”

“…This work was instrumental in testing and validating the whole procedure for 3D virtual dosimetry in BCT within the VCT-BREAST platform. A useful extension foreseen for VCT-BREAST is the digital reproduction of the photon-counting spiral BCT scanner developed by Advanced Breast-CT company, in line with recent reports (Germann et al 2021, Shim et al 2023.…”

“…The procedure was applied also to derive the mean skin dose (MSD), the mean dose value at the skin, and its standard deviation, σ s , as well as the mean breast dose (MBD) in the whole breast (skin included), and its standard deviation, σ b . This definition of the MBD differs from that of Shim et al (2023) who exclude the skin layer from the calculation of their mean absorbed dose value.…”

“…In this work, we simulated the characteristics of three cone-beam BCT scanners for which the geometric orbit of the x-ray source is circular and the views are acquired equally sampled over a full rotation, as described in the following. We note that a virtual dosimetric study has been reported recently for the spiral BCT geometry on a large real patient cohort, for the nu:view commercial scanner (AB-CT-Advanced Breast-CT GmbH, Erlangen, Germany) (Shim et al 2023).…”