Electron beam detection in radiotherapy using a capacitor dosimeter equipped with a silicon photodiode.

In this study, a newly developed capacitor dosimeter was evaluated using electron beams commonly utilized in radiotherapy. The capacitor dosimeter comprised a silicon photodiode, 0.47-µF capacitor, and dedicated terminal (dock). Before electron beam irradiation, the dosimeter was charged using the dock. The doses were measured without using a cable by reducing the charging voltages using the currents from the photodiode during irradiation. A commercially available parallel-plane-type ionization chamber and solid-water phantom were used for dose calibration with an electron energy of 6 MeV. In addition, the depth doses were measured using a solid-water phantom at electron energies of 6, 9, and 12 MeV. The doses were proportional to the discharging voltages, and the maximum dose difference in the calibrated doses measured using a two-point calibration was approximately 5% in the range of 0.25-1.98 Gy. The depth dependencies at energies of 6, 9, and 12 MeV corresponded to those measured using the ionization chamber.

Circulating Cell-Free DNA as a Biomarker for Prognosis and Response to Systemic Therapy in Patients with Unresectable Hepatocellular Carcinoma.

INTRODUCTION: Systemic therapy provides clinical benefits to a subset of patients with advanced unresectable hepatocellular carcinoma (HCC). However, few biomarkers are available for predicting prognosis and treatment response in patients with advanced HCC undergoing treatment with systemic therapies. This study aimed to examine whether circulating cell-free DNA (cfDNA) containing circulating tumor DNA can act as a therapeutic response and prognostic biomarker in patients with advanced HCC. METHODS: We analyzed longitudinally collected plasma cfDNA of patients with advanced HCC who were naïve to systemic therapy, and assessed their prognostic and predictive values to determine treatment responses. RESULTS: cfDNA concentration positively correlated with entire tumor volume on computed tomography before (p = 0.0231) and at the end (p < 0.0001) of the first-line systemic therapy. The overall survival rate was higher in patients with cfDNA concentrations lower than the median cfDNA level at baseline compared to patients with higher cfDNA concentrations (hazard ratio, 0.2765; 95% confidence interval, 0.08-0.81; p = 0.0197). The ratio of cfDNA at 4 weeks to that at baseline was predictive of radiographic disease response. In patients with progressive disease, cfDNA concentration at 4 weeks increased significantly (p = 0.0245), whereas the concentration remained unchanged in patients with other disease courses (p = 0.9375). CONCLUSION: The baseline plasma cfDNA concentration can be used as a prognostic biomarker in patients with advanced HCC. cfDNA kinetics may also predict the tumor response to therapy and disease progression.

Assessment of a computed tomography-based radiomics approach for assessing lung function in lung cancer patients.

PURPOSE: We aimed to assess radiomics approaches for estimating three pulmonary function test (PFT) results (forced expiratory volume in one second [FEV1], forced vital capacity [FVC], and the ratio of FEV1 to FVC [FEV1/FVC]) using data extracted from chest computed tomography (CT) images. METHODS: This retrospective study included 85 lung cancer patients (mean age, 75 years ±8; 69 men) who underwent stereotactic body radiotherapy between 2012 and 2020. Their pretreatment chest breath-hold CT and PFT data before radiotherapy were obtained. A total of 107 radiomics features (Shape: 14, Intensity: 18, Texture: 75) were extracted using two methods: extraction of the lung tissue (<-250 HU) (APPROACH 1), and extraction of small blood vessels and lung tissue (APPROACH 2). The PFT results were estimated using the least absolute shrinkage and selection operator regression. Pearson's correlation coefficients (r) were determined for all PFT results, and the area under the curve (AUC) was calculated for FEV1/FVC (<70 %). Finally, we compared our approaches with the conventional formula (Conventional). RESULTS: For the estimated FEV1/FVC, the Pearson's r were 0.21 (P =.06), 0.69 (P <.01), and 0.73 (P <.01) for Conventional, APPROACH 1, and APPROACH 2, respectively; the AUCs for FEV1/FVC (<70 %) were 0.67 (95 % confidence interval [CI]: 0.55, 0.79), 0.82 (CI: 0.72, 0.91; P =.047) and 0.86 (CI: 0.78, 0.94; P =.01), respectively. CONCLUSIONS: The radiomics approach performed better than the conventional equation and may be useful for assessing lung function based on CT images.

Development of a dose-rate dosimeter for x-ray CT scanner using silicon x-ray diode.

In an x-ray diagnosis, it is important to evaluate the entrance dose rate, as the dose rate of exposure becomes highest in that position. To investigate the effect of the entrance dose rate of x-ray CT scanners, a dose-rate dosimeter comprising a silicon x-ray diode (Si-XD), a CMOS dual operational amplifier, resistors, capacitors, and a mini-substrate measuring 20 × 17 mm2 were developed. The Si-XD is desirable for measuring the changing entrance dose rate, as it enables the reduction of the response time, dimensions, and cost of the dosimeter. The dosimeter was connected to a microcomputer (mbed), and the output voltages from the dosimeter were measured using an analog-digital converter in the mbed. The output voltages were proportional to the tube currents at a constant tube voltage of 100 kV using an industrial x-ray tube, and the calibrated dose rates corresponded well to those obtained using a commercially available semiconductor dosimeter. However, owing to the energy dependence of the dosimeter, the calibrated dose rate was ∼10% higher than that of a commercially available semiconductor dosimeter at the lower tube voltage. In the angular dependence of the dosimeter, the flatness measured from 60° to 120° was ∼103% in this study. A fundamental study for measuring the dose-rate variations with rotation was performed. The results showed a different profile than the angular dependence due to the distance from the source and the complex factors of the scattered radiation.

Characterization of an under-development capacitor dosimeter equipped with a silicon x-ray diode.

Herein, we evaluated a capacitor dosimeter under development by a manufacturer, which is designed to monitor the entrance dose in x-ray diagnosis and comprises a silicon x-ray diode (Si-XD), a 0.1 µF capacitor, and a dosimeter dock. The Si-XD is a high-sensitivity photodiode optimized for x-ray detection. The dosimeter was charged to 3.30 V using the dock before x-ray irradiation. The charging voltage was reduced by photocurrents flowing through the Si-XD during irradiation, and the discharging voltage was measured. For the fundamental characterization of this capacitor dosimeter, we investigated the x-ray tube-current and tube-voltage dependences of the measured dose using an industrial x-ray tube; the angular dependence was also investigated. A commercially available semiconductor dosimeter (RaySafe ThinX) was used for dose calibration. The doses were proportional to the tube current at a constant tube voltage of 100 kV and increased with increasing tube voltage at a constant tube current of 1.0 mA. The dose difference with respect to the commercially available semiconductor dosimeter was within 1.0% when the tube current was varied and it was within 3.0% when the tube voltage was varied. In the angular dependence measurement, a difference of up to 6.0% was observed as the angle varied from 0° to 355° in steps of 5°. The dose-calibration results indicated that the determination of the initial charging voltage was important for dose conversion using the capacitor dosimeter.

Evaluation of four-dimensional cone beam computed tomography ventilation images acquired with two different linear accelerators at various gantry speeds using a deformable lung phantom.

We evaluated four-dimensional cone beam computed tomography (4D-CBCT) ventilation images (VICBCT) acquired with two different linear accelerator systems at various gantry speeds using a deformable lung phantom. The 4D-CT and 4D-CBCT scans were performed using a computed tomography (CT) scanner, an X-ray volume imaging system (Elekta XVI) mounted in Versa HD, and an On-Board Imager (OBI) system mounted in TrueBeam. Intensity-based deformable image registration (DIR) was performed between peak-exhale and peak-inhale images. VICBCT- and 4D-CT-based ventilation images (VICT) were derived by DIR using two metrics: one based on the Jacobian determinant and one on changes in the Hounsfield unit (HU). Three different DIR regularization values (λ) were used for VICBCT. Correlations between the VICBCT and VICT values were evaluated using voxel-wise Spearman's rank correlation coefficient (r). In case of both metrics, the Jacobian-based VICBCT with a gantry speed of 0.6 deg/sec in Versa HD showed the highest correlation for all the gantry speeds (e.g., λ = 0.05 and r = 0.68). Thus, the r value of the Jacobian-based VICBCT was greater or equal to that of the HU-based VICBCT. In addition, the ventilation accuracy of VICBCT increased at low gantry speeds. Thus, the image quality of VICBCT was affected by the change in gantry speed in both the imaging systems. Additionally, DIR regularization considerably influenced VICBCT in both the imaging systems. Our results have the potential to assist in designing CBCT protocols, incorporating VICBCT imaging into the functional avoidance planning process.

The impact of 4DCT-ventilation imaging-guided proton therapy on stereotactic body radiotherapy for lung cancer.

Functional lung avoidance during radiotherapy can help reduce pulmonary toxicity. This study assessed the potential impact of four-dimensional computed tomography (4DCT)-ventilation imaging-guided proton radiotherapy (PT) on stereotactic body radiotherapy (SBRT) by comparing it with three-dimensional conformal radiotherapy (3D-CRT) and volumetric modulated arc therapy (VMAT), which employ photon beams. Thirteen lung cancer patients who received SBRT with 3D-CRT were included in the study. 4DCT ventilation was calculated using the patients' 4DCT data, deformable image registration, and a density-change-based algorithm. Three functional treatment plans sparing the functional lung regions were developed for each patient using 3D-CRT, VMAT, and PT. The prescribed doses and dose constraints were based on the Radiation Therapy Oncology Group 0618 protocol. We evaluated the region of interest (ROI) and functional map-based dose-function metrics for 4DCT ventilation and the irradiated dose. Using 3D-CRT, VMAT, and PT, the percentages of the functional lung regions that received ≥ 5 Gy (fV5) were 26.0%, 21.9%, and 10.7%, respectively; the fV10 were 14.4%, 11.4%, and 9.0%, respectively; and fV20 were 6.5%, 6.4%, and 6.6%, respectively, and the functional mean lung doses (fMLD) were 5.6 Gy, 5.2 Gy, and 3.8 Gy, respectively. These results indicated that PT resulted in a significant reduction in fMLD, fV5, and fV10, but not fV20. The use of PT reduced the radiation to highly functional lung regions compared with those for 3D-CRT and VMAT while meeting all dose constraints.

In silico comparison of the dosimetric impacts of a greater omentum spacer for abdominal and pelvic tumors in carbon-ion, proton and photon radiotherapy.

PURPOSE: The purpose of this study was to compare carbon-ion (C-ion), proton and photon radiotherapy (RT) plans with regard to dose reduction of the gastrointestinal (GI) tract by using a greater omentum spacer (GO spacer). METHODS: We retrospectively retrieved data for ten patients who received the GO spacer as surgical spacer placement for abdominal and pelvic tumors. Simulation plans were created on pre-spacer Computed Tomography (CT) and post-spacer CT for C-ion RT, proton RT and photon RT to compare the dose of the GI tract. The plans were normalized so that at least 95% of the planning target volume (PTV) received 70 Gy (relative biological effectiveness equivalent) delivered in 35 fractions. All plans were created with the lowest possible dose to the GI tract under conditions that meet the dose constraints for the PTV and spinal cord (maximum dose < 45 Gy). The part of the GI tract to be evaluated was defined as that most adjacent to the PTV. C-ion RT plans and proton RT plans were calculated by a spot scanning technique, and photon RT plans were calculated employing by fixed-field intensity-modulated radiation therapy. RESULTS: D2 cc and V10-70 of the GI tract were significantly lower on post-spacer plans than on pre-spacer plans for all three RT modalities. Regarding post-spacer plans, D2 cc of the GI tract was significantly lower on C-ion RT plans and proton RT plans than on photon RT plans (C-ion vs photon p = 0.001, proton vs photon p = 0.002). However, there was no significant difference between C-ion RT plans and proton RT plans for D2 cc of the GI tract (C-ion vs proton p = 0.992). In the photon RT plan for one patient, D2 cc of the GI tract did not meet < 50 Gy. CONCLUSIONS: The GO spacer shows a significant dose reduction effect on the GI tract.

Evaluation of a 3D-printed heterogeneous anthropomorphic head and neck phantom for patient-specific quality assurance in intensity-modulated radiation therapy.

We evaluated an anthropomorphic head and neck phantom with tissue heterogeneity, produced using a personal 3D printer, with quality assurance (QA), specific to patients undergoing intensity-modulated radiation therapy (IMRT). Using semi-automatic segmentation, 3D models of bone, soft tissue, and an air-filled cavity were created based on computed tomography (CT) images from patients with head and neck cancer treated with IMRT. For the 3D printer settings, polylactide was used for soft tissue with 100% infill. Bone was reproduced by pouring plaster into the cavity created by the 3D printer. The average CT values for soft tissue and bone were 13.0 ± 144.3 HU and 439.5 ± 137.0 HU, respectively, for the phantom and 12.1 ± 124.5 HU and 771.5 ± 405.3 HU, respectively, for the patient. The gamma passing rate (3%/3 mm) was 96.1% for a nine-field IMRT plan. Thus, this phantom may be used instead of a standard shape phantom for patient-specific QA in IMRT.

Evaluation of functionally weighted dose-volume parameters for thoracic stereotactic ablative radiotherapy (SABR) using CT ventilation.

For the purpose of reducing radiation pneumontisis (RP), four-dimensional CT (4DCT)-based ventilation can be used to reduce functionally weighted lung dose. This study aimed to evaluate the functionally weighted dose-volume parameters and to investigate an optimal weighting method to realize effective planning optimization in thoracic stereotactic ablative radiotherapy (SABR). Forty patients treated with SABR were analyzed. Ventilation images were obtained from 4DCT using deformable registration and Hounsfield unit-based calculation. Functionally-weighted mean lung dose (fMLD) and functional lung fraction receiving at least x Gy (fVx) were calculated by two weighting methods: thresholding and linear weighting. Various ventilation thresholds (5th-95th, every 5th percentile) were tested. The predictive accuracy for CTCAE grade ≧ 2 pneumonitis was evaluated by area under the curve (AUC) of receiver operating characteristic analysis. AUC values varied from 0.459 to 0.570 in accordance with threshold and dose-volume metrics. A combination of 25th percentile threshold and fV30 showed the best result (AUC: 0.570). AUC values with fMLD, fV10, fV20, and fV40 were 0.541, 0.487, 0.548 and 0.563 using a 25th percentile threshold. Although conventional MLD, V10, V20, V30 and V40 showed lower AUC values (0.516, 0.477, 0.534, 0.552 and 0.527), the differences were not statistically significant. fV30 with 25th percentile threshold was the best predictor of RP. Our results suggested that the appropriate weighting should be used for better treatment outcomes in thoracic SABR.