Minsoo Chun

A Patient-Specific Polylactic Acid Bolus Made by a 3D Printer for Breast Cancer Radiation Therapy.

Purpose The aim of this study was to assess the feasibility and advantages of a patient-specific breast bolus made using a 3D printer technique. Methods We used the anthropomorphic female phantom with breast attachments, which volumes are 200, 300, 400, 500 and 650 cc. We simulated the treatment for a right breast patient using parallel opposed tangential fields. Treatment plans were used to investigate the effect of unwanted air gaps under bolus on the dose distribution of the whole breast. The commercial Super-Flex bolus and 3D-printed polylactic acid (PLA) bolus were applied to investigate the skin dose of the breast with the MOSFET measurement. Two boluses of 3 and 5 mm thicknesses were selected. Results There was a good agreement between the dose distribution for a virtual bolus generated by the TPS and PLA bolus. The difference in dose distribution between the virtual bolus and Super-Flex bolus was significant within the bolus and breast due to unwanted air gaps. The average differences between calculated and measured doses in a 200 and 300 cc with PLA bolus were not significant, which were -0.7% and -0.6% for 3mm, and -1.1% and -1.1% for 5 mm, respectively. With the Super-Flex bolus, however, significant dose differences were observed (-5.1% and -3.2% for 3mm, and -6.3% and -4.2% for 5 mm). Conclusion The 3D-printed solid bolus can reduce the uncertainty of the daily setup and help to overcome the dose discrepancy by unwanted air gaps in the breast cancer radiation therapy.

Dosimetric effects of sectional adjustments of collimator angles on volumetric modulated arc therapy for irregularly-shaped targets

Purpose To calculate an optimal collimator angle at each of sectional arcs in a full-arc volumetric modulated arc therapy (VMAT) plan and evaluate dosimetric quality of these VMAT plans comparing full-arc VMAT plans with a fixed collimator angle. Methods Seventeen patients who had irregularly-shaped target in abdominal, head and neck, and chest cases were selected retrospectively. To calculate an optimal collimator angle at each of sectional arcs in VMAT, integrated MLC apertures which could cover all shapes of target determined by beam’s-eye view (BEV) within angular sections were obtained for each VMAT plan. The angular sections were 40°, 60°, 90° and 120°. When the collimator settings were rotated at intervals of 2°, we obtained the optimal collimator angle to minimize area size difference between the integrated MLC aperture and collimator settings with 5 mm-margins to the integrated MLC aperture. The VMAT plans with the optimal collimator angles (Colli-VMAT) were generated in the EclipseTM. For comparison purposes, one full-arc VMAT plans with a fixed collimator angles (Std-VMAT) were generated. The dose-volumetric parameters and total MUs were evaluated. Results The mean dose-volumetric parameters for target volume of Colli-VMAT were comparable to Std-VMAT. Colli-VMAT improved sparing of most normal organs but for brain stem, compared to Std-VMAT for all cases. There were decreasing tendencies in mean total MUs with decreasing angular section. The mean total MUs for Colli-VMAT with the angular section of 40° (434 ± 95 MU, 317 ± 81 MU, and 371 ± 43 MU for abdominal, head and neck, and chest cases, respectively) were lower than those for Std-VMAT (654 ± 182 MU, 517 ± 116 MU, and 533 ± 25 MU, respectively). Conclusions For an irregularly-shaped target, Colli-VMAT with the angular section of 40° reduced total MUs and improved sparing of normal organs, compared to Std-VMAT.

Sensitivity and stability of optically stimulated luminescence dosimeters with filled deep electron/hole traps under pre-irradiation and bleaching conditions

PURPOSE We aimed to evaluate the characteristics of optically stimulated luminescence dosimeters (OSLDs) with fully filled deep electron/hole traps, and determine the optimal bleaching conditions for these OSLDs to minimize the changes in dose sensitivity or linearity according to the accumulated dose. METHODS InLight nanoDots were used as OSLDs. The OSLDs were first pre-irradiated at a dose greater than 5kGy to fill the deep electron and hole traps, and then bleached (OSLDfull). OSLDfull characteristics were investigated in terms of the full bleaching, fading, regeneration of luminescence, dose linearity, and dose sensitivity with various bleaching conditions. For comparison, OSLDs with un-filled deep electron/hole traps (OSLDempty) were investigated in the same manner. RESULTS The fading for OSLDfull exhibited stable signals after 10min, for 1 and 10Gy. The mean supra-linear index values for OSLDfull were 1.001±0.001 for doses from 2 to 10Gy. Small variations in dose sensitivity were obtained for OSLDfull within standard deviations of 0.85% and 0.71%, whereas those of OSLDempty decreased by 2.3% and 4.2% per 10Gy for unfiltered and filtered bleaching devices, respectively. CONCLUSIONS Under the bleaching conditions determined in this study, clinical dosimetry with OSLDfull is highly stable, minimizing the changes in dose sensitivity or linearity for the clinical dose.

Monte Carlo simulation of neutron dose equivalent by photoneutron production inside the primary barriers of a radiotherapy vault

PURPOSE To evaluate the neutron dose equivalent produced by photoneutrons inside the primary barriers of a radiotherapy vault. METHODS Monte Carlo simulations were performed for investigating the production of photoneutrons as well as neutron shielding requirements. Two photon beams of 15 and 18 MV struck sheets of steel and lead, and the neutron doses were calculated at the isocenter (Piso) and at a distance of 50 cm from the inside wall (Pwall) while delivering 1 Gy to the patient. The proper thicknesses of borated polyethylene (BPE) and concrete were simulated to reduce neutron contamination. RESULTS When the primary barrier consisted of a concrete alone, the neutron doses at Piso were 0.5 μSv/Gy and 12.8 μSv/Gy for 15- and 18-MV, respectively. At Pwall, the neutron doses were 15.8 μSv/Gy and 318.4 μSv/Gy for 15- and 18-MV, respectively. When 15 MV photons interacted with metal sheets, the neutron doses were 0.4-22.2 μSv/Gy at Piso and 15.8-812.5 μSv/Gy at Pwall, depending on the thickness and material of the metal sheets and neutron shielding. In the case of 18 MV photons with the same configuration, the neutron doses were 0.9-59.5 μSv/Gy and 73.9-5006.1 μSv/Gy for Piso and Pwall, respectively. The neutron dose delivered to the patient was reduced to the level of the dose delivered with a concrete barrier by including a 10-cm-thick BPE for each beam. CONCLUSIONS When the primary barrier shielding is designed with a metal sheet inside for high energy, proper neutron shielding should be constructed to avoid undesirable photoneutron dose.

Dosimetric characteristics of a reusable 3D radiochromic dosimetry material

Purpose To investigate the dosimetric characteristics of PRESAGEREU dosimeters. Methods Commercially available PRESAGEREU dosimeters (size of 10 mm × 10 mm × 45 mm) were divided into two groups, with one of the groups placed at room temperature of 22°C (RT group) and another group placed at low temperature of 10°C (LT group). A total of 3 dosimeters (set of dosimeters) were irradiated at a time, with doses of 1 Gy, 2 Gy, 4 Gy, 8 Gy, 12 Gy, 16 Gy, and 20 Gy, at a nominal dose rate of 400 MU/min at temperature of 22°C. The dosimeters were irradiated three additional times by delivering the same doses as those during the initial irradiations (4 irradiation cycles). Optical density (OD) was assessed using optical CT scanning. Results Considering both linearity and sensitivity of the OD curves, R2 above 0.95 and sensitivity above 0.04 ΔOD/Gy were observed at the 1st irradiation (reading time ≤ 6 h) and 2nd irradiation (reading time = 0.5 h) for the RT group. For the LT group, those values were observed at the 1st irradiation (reading time ≤ 2 h), and the 3rd and 4th irradiations (both reading times = 0.5 h). Considering the reproducibility of signals in response to the same dose, dosimeters in the RT group showed average deviations among dosimeters less than 5% (the 1st and 2nd irradiations at the reading time of 0.5 h), while for dosimeters in the LT group showed average deviations among dosimeters less than 6% (the 3rd and 4th irradiations at the reading time of 0.5 h). For the rest, the OD curves were not linear, sensitivities of the dosimeters were lower than 0.04 ΔOD/Gy, and OD deviations at the same dose were larger than 6%. Conclusions At room temperature, PRESAGEREU dosimeters could be used for dose measurement only for up to two dose measurement sessions. At low temperatures, usage of PRESAGEREU dosimeters for dose measurement seems to be possible from the 3rd irradiation. When reusing PRESAGEREU dosimeters, the OD curve should be re-defined for every measurement session because the shape of this curve depends on the irradiation history.

Gamma analysis with a gamma criterion of 2%/1 mm for stereotactic ablative radiotherapy delivered with volumetric modulated arc therapy technique: a single institution experience

To report a single-institution experience of gamma evaluations with 2%/1 mm for stereotactic ablative radiotherapy (SABR) delivered with volumetric modulated arc therapy (VMAT) technique, from January 2014 to January 2016. A total of 168 SABR VMAT plans were analyzed with a gamma criterion of 2%/1 mm, a threshold value of 10%, and a tolerance level of 90%. Of the 168 cases, four cases failed with 2%/1 mm. The average passing rate was 97.0% ± 2.5%. Three of the four failed cases showed passing rates higher than 90%, which was achieved by shifting the measuring device by 1 mm in the left-to-right or anterior-to-posterior directions. One failed case showed a passing rate higher than 90%, which was achieved by changing the threshold value from 10% to 5%, leading to an increase in the number of tested points from 26 to 51. Concerns regarding the high susceptibility of the gamma criterion of 2%/1 mm to setup errors of the measuring device are unnecessary based on our two-year experience, since only four cases failed with the 2%/1 mm from a total of 168 clinical cases. Therefore, the gamma criterion of 2%/1 mm could be successfully applied in the clinic with its high sensitivity to detect errors in VMAT plans.To report a single-institution experience of gamma evaluations with 2%/1 mm for stereotactic ablative radiotherapy (SABR) delivered with volumetric modulated arc therapy (VMAT) technique, from January 2014 to January 2016. A total of 168 SABR VMAT plans were analyzed with a gamma criterion of 2%/1 mm, a threshold value of 10%, and a tolerance level of 90%. Of the 168 cases, four cases failed with 2%/1 mm. The average passing rate was 97.0% ± 2.5%. Three of the four failed cases showed passing rates higher than 90%, which was achieved by shifting the measuring device by 1 mm in the left-to-right or anterior-to-posterior directions. One failed case showed a passing rate higher than 90%, which was achieved by changing the threshold value from 10% to 5%, leading to an increase in the number of tested points from 26 to 51. Concerns regarding the high susceptibility of the gamma criterion of 2%/1 mm to setup errors of the measuring device are unnecessary based on our two-year experience, since only four cases failed with the 2%/1 mm from a total of 168 clinical cases. Therefore, the gamma criterion of 2%/1 mm could be successfully applied in the clinic with its high sensitivity to detect errors in VMAT plans.

Treatment plan comparison between Tri-Co-60 magnetic-resonance image-guided radiation therapy and volumetric modulated arc therapy for prostate cancer

To investigate the plan quality of tri-Co-60 intensity-modulated radiation therapy (IMRT) with magnetic-resonance image-guided radiation therapy compared with volumetric-modulated arc therapy (VMAT) for prostate cancer. Twenty patients with intermediate-risk prostate cancer, who received radical VMAT were selected. Additional tri-Co-60 IMRT plans were generated for each patient. Both primary and boost plans were generated with tri-Co-60 IMRT and VMAT techniques. The prescription doses of the primary and boost plans were 50.4 Gy and 30.6 Gy, respectively. The primary and boost planning target volumes (PTVs) of the tri-Co-60 IMRT were generated with 3 mm margins from the primary clinical target volume (CTV, prostate + seminal vesicle) and a boost CTV (prostate), respectively. VMAT had a primary planning target volume (primary CTV + 1 cm or 2 cm margins) and a boost PTV (boost CTV + 0.7 cm margins), respectively. For both tri-Co-60 IMRT and VMAT, all the primary and boost plans were generated that 95% of the target volumes would be covered by the 100% of the prescription doses. Sum plans were generated by summation of primary and boost plans. In sum plans, the average values of V70 Gy of the bladder of tri-Co-60 IMRT vs. VMAT were 4.0% ± 3.1% vs. 10.9% ± 6.7%, (p < 0.001). Average values of V70 Gy of the rectum of tri-Co-60 IMRT vs. VMAT were 5.2% ± 1.8% vs. 19.1% ± 4.0% (p < 0.001). The doses of tri-Co-60 IMRT delivered to the bladder and rectum were smaller than those of VMAT while maintaining identical target coverage in both plans.

TU‐F‐CAMPUS‐I‐04: Fully Automated Evaluation of CT AEC Performance Using a Novel Automated Noise Level Measurement Technique

Purpose: To assess the performance of CT automatic exposure control (AEC) system by using a newly developed automated noise level measurement technique. Methods: Twenty pediatric patients’ CT image data of chest routine protocol taken with AEC (CARE Dose4D, Siemens Healthcare) were collected. A newly developed automated noise level measurement technique was applied to assess the noise level consistency across the varying body size. Subcutaneous fat region was segmented by applying thresholding of predefined intensity range followed by morphology operation on noise filtered image. A novel structure feature was calculated by combining sum of gradient and directional entropy of gradient at each pixel within the segmented subcutaneous fat. Five ROIs were randomly selected which have feature values less than 10 percentiles. The representative noise level was determined as an average of the HU standard deviations on 5 ROIs. Variation of noise levels were evaluated for the 20 CT data with the above noise measurements. In addition, 3 body factors such as water-equivalent body diameter (WBD), AP, and LAT diameter were extracted on each slice, and Pearson correlation coefficients between the noise measurements and body factors were compared. Results: Most of the ROI placement (98%) by automated technique were accepted by an expert observer. Variation of noise standard deviation per patient ranged from 0.83 to 2.17 HU, whereas the mean noise ranged from 5.80 to 12.38 HU. Pearson correlation coefficient was highest between WBD and noise level by showing 0.76±0.06. Conclusion: Performance of CT AEC system could be assessed with a fully automated technique. Our proposed technique is a potentially useful tool for investigating various characteristics and optimal use of AEC system in clinical setting.

WE-D-18A-07: Development and Evaluation of Automated Noise Measurement Technique in CT Images.

PURPOSE To develop and evaluate an automated technique which measures the noise level in CT images. METHODS Candidate regions of interests (ROIs) with circular shape were selected with a criteria where the pixel intensities on Gaussian filtered image lie within a predefined range. The ROIs were further refined with the sum of absolute gradients within each candidate ROI being used as a ROI feature to distinguish homogeneous ROIs. A histogram of ROI feature was created, and the ROIs with feature values less than the mode of histogram were selected as homogeneous ROIs. Connected component analysis was then used to remove the non-clustered ROIs to restrict the location of ROI in desired organ. Standard deviation of pixel intensities was calculated for each cluster of homogeneous ROIs to represent the local noise level, and its mode value was used to represent the noise level. Fifty abdomen CT images were collected, and five observers participated to manually measure the noise level with a guidance to draw same size ROIs. Manual results were then averaged to establish a reference value, and a correlation coefficient(r) between a reference value and our automatic result. Intra- and inter-rater agreement were compared to demonstrate the performance of our proposed technique. RESULTS We confirmed successful operation of automatic homogeneous region localization and noise level measurement. Intra- and inter-rater agreement(r) ranged from 0.70 to 0.95, and from 0.79 to 0.94, respectively. Agreements between reference and manual measurements by raters ranged from 0.85 to 0.95, and those between a reference and an automated result by our proposed method was 0.90. CONCLUSION Our developed technique has successfully performed the noise measurement task in all the test images by showing an agreement of 0.90 with the reference measurement. Our proposed method enables the automated assessment of CT image noise.

Combined Homogeneous Region Localization and Automated Evaluation of Radiation Dose Dependent Contrast-to-Noise Ratio in Dual Energy Abdominal CT

This study presents a homogeneous region localization technique combined with automated evaluation of radiation dose-dependent contrast-to-noise ratio in dual energy abdominal CT. Patient body size was calculated using region growing segmentation and used to estimate size-specific dose estimate, and contrast-to-noise ratio are automatically evaluated by using proposed technique. Contrast-to-noise ratio turned out to be similar between low and high tube potential for both pre- and post-contrast phases, while radiation dose is remarkably lower at 80 kVp. Low tube potential can be recommended to reduce radiation dose while maintaining contrast-to-noise.