Mikoto Tamura

The effect of gantry and collimator angles on leaf limited velocity and position in dynamic multileaf collimator intensity-modulated radiation therapy

The purpose of the study is to evaluate the limiting velocity (LV) of a multileaf collimator and the leaf position in various collimator and gantry angles. Both leading leaves and trailing leaves began to move with a constant acceleration from 0 to 4 cm s(-1). When the beam hold occurred, the leaf velocity was defined as the leaf LV. Dynamic irradiation was performed at eight gantry angles of every 45 degrees with three different collimator angles. The analysis of the LV and the leaf position was performed with a log file from a leaf motion controller. The mean LVs for Varian Clinac 21EX (21EX) ranged from 2.51 to 3.10 cm s(-1). The mean LVs for Clinac 600C ranged from 2.91 to 3.12 cm s(-1). When only central 5 mm leaves of 21EX moved, LVs were significantly higher than those when all 60 pairs of leaf moved, while the leaf position inconsistencies of the two accelerators were within 1 mm at the leaf velocities from 0.5 to 2.0 cm s(-1). It was recognized that the LV was affected by gravity. This measurement method can be utilized as routine quality assurance for a dynamic multileaf collimator (DMLC) is and easily reproducible.

Dosimetric comparison of RapidPlan and manually optimized plans in volumetric modulated arc therapy for prostate cancer

PURPOSE This study evaluated whether RapidPlan based plans (RP plans) created by a single optimization, are usable in volumetric modulated arc therapy (VMAT) for patients with prostate cancer. METHODS We used 51 previously administered VMAT plans to train a RP model. Thirty RP plans were created by a single optimization without planner intervention during optimization. Differences between RP plans and clinical manual optimization (CMO) plans created by an experienced planner for the same patients were analyzed (Wilcoxon tests) in terms of homogeneity index (HI), conformation number (CN), D95%, and D2% to planning target volume (PTV), mean dose, V50Gy, V70Gy, V75Gy, and V78Gy to rectum and bladder, monitor unit (MU), and multi-leaf collimator (MLC) sequence complexity. RESULTS RP and CMO values for PTV D95%, PTV D2%, HI, and CN were significantly similar (p<0.05 for all). RP mean dose, V50Gy, and V70Gy to rectum were superior or comparable to CMO values; RP V75Gy and V78Gy were higher than in CMO plans (p<0.05). RP bladder dose-volume parameter values (except V78Gy) were lower than in CMO plans (p<0.05). MU values were RP: 730±55MU and CMO: 580±37MU (p<0.05); and MLC sequence complexity scores were RP: 0.25±0.02 and CMO: 0.35±0.03 (p<0.05). CONCLUSIONS RP plans created by a single optimization were clinically acceptable in VMAT for patient with prostate cancer. Our simple model could reduce optimization time, independently of planners skill and knowledge.

Feasibility of tungsten functional paper in electron grid therapy: a Monte Carlo study.

Electron grid therapy is expected to be a valid treatment for bulky superficial tumors. It is difficult, however, to fit irradiation fields to bulky superficial tumor shapes for conventional electron grid therapy with a cerrobend grid collimator. In this study, we investigated whether a grid collimator using tungsten functional paper (TFP), with its radiation shielding ability, could be used for electron grid therapy. Dose distributions were measured using 9 MeV electron grid beams from a cerrobend grid collimator. For the simulation study, the same grid irradiation fields were shaped using a TFP grid collimator (thicknesses of 0.15, 0.3, 0.6, 0.9, and 1.2 cm) by laying them on a phantom. We then determined the dose distributions using Monte Carlo calculations and compared the cerrobend and TFP electron grid beams regarding dose distributions, including the depths of the maximum dose (d max), 90% dose (d 90), and 80% dose (d 80), and the ratios of the doses in the areas with and without shielding (valley to peak ratios). The equivalent dosimetric thickness was obtained with the TFP grid collimator that was equivalent to the dose distribution of the cerrobend grid collimator. For the cerrobend electron grid beams, the d max, d 90, and d 80 were 1.0, 2.1, and 2.5 cm, respectively, and the valley to peak ratios at those depths were 0.48, 0.66, and 0.73, respectively. The equivalent dosimetric thickness of TFP was 0.52 cm. The d max, d 90, and d 80 for the 0.52 cm thick TFP electron grid beams were 1.1, 1.9, and 2.3 cm, respectively, and the valley to peak ratios at those depths were 0.49, 0.63, and 0.71, respectively. The TFP grid collimator flexibly delivered excellent dose distributions by simply attaching it to the patients skin. It could thus be used for electron grid therapy instead of the cerrobend grid collimator.

A novel radiation protection device based on tungsten functional paper for application in interventional radiology

&NA; Tungsten functional paper (TFP), which contains 80% tungsten by weight, has radiation‐shielding properties. We investigated the use of TFP for the protection of operators during interventional or therapeutic angiography. The air kerma rate of scattered radiation from a simulated patient was measured, with and without TFP, using a water‐equivalent phantom and fixed C‐arm fluoroscopy. Measurements were taken at the level of the operators eye, chest, waist, and knee, with a variable number of TFP sheets used for shielding. A Monte Carlo simulation was also utilized to analyze the dose rate delivered with and without the TFP shielding. In cine mode, when the number of TFP sheets was varied through 1, 2, 3, 5, and 10, the respective reduction in the air kerma rate relative to no TFP shielding was as follows: at eye level, 24.9%, 29.9%, 41.6%, 50.4%, and 56.2%; at chest level, 25.3%, 33.1%, 34.9%, 46.1%, and 44.3%; at waist level, 45.1%, 57.0%, 64.4%, 70.7%, and 75.2%; and at knee level, 2.1%, 2.2%, 2.1%, 2.1%, and 2.1%. In fluoroscopy mode, the respective reduction in the air kerma rate relative to no TFP shielding was as follows: at eye level, 24.8%, 30.3%, 34.8%, 51.1%, and 58.5%; at chest level, 25.8%, 33.4%, 35.5%, 45.2%, and 44.4%; at waist level, 44.6%, 56.8%, 64.7%, 71.7%, and 77.2%; and at knee level, 2.2%, 0.0%, 2.2%, 2.8%, and 2.5%. The TFP paper exhibited good radiation‐shielding properties against the scattered radiation encountered in clinical settings, and was shown to have potential application in decreasing the radiation exposure to the operator during interventional radiology.

Evaluation of lung toxicity risk with computed tomography ventilation image for thoracic cancer patients

Background Four-dimensional computed tomography (4D-CT) ventilation is an emerging imaging modality. Functional avoidance of regions according to 4D-CT ventilation may reduce lung toxicity after radiation therapy. This study evaluated associations between 4D-CT ventilation-based dosimetric parameters and clinical outcomes. Methods Pre-treatment 4D-CT data were used to retrospectively construct ventilation images for 40 thoracic cancer patients retrospectively. Fifteen patients were treated with conventional radiation therapy, 6 patients with hyperfractionated radiation therapy and 19 patients with stereotactic body radiation therapy (SBRT). Ventilation images were calculated from 4D-CT data using a deformable image registration and Jacobian-based algorithm. Each ventilation map was normalized by converting it to percentile images. Ventilation-based dosimetric parameters (Mean Dose, V5 [percent lung volume receiving ≥5 Gy], and V20 [percent lung volume receiving ≥20 Gy]) were calculated for highly and poorly ventilated regions. To test whether the ventilation-based dosimetric parameters could be used predict radiation pneumonitis of ≥Grade 2, the area under the curve (AUC) was determined from the receiver operating characteristic analysis. Results For Mean Dose, poorly ventilated lung regions in the 0–30% range showed the highest AUC value (0.809; 95% confidence interval [CI], 0.663–0.955). For V20, poorly ventilated lung regions in the 0–20% range had the highest AUC value (0.774; 95% [CI], 0.598–0.915), and for V5, poorly ventilated lung regions in the 0–30% range had the highest AUC value (0.843; 95% [CI], 0.732–0.954). The highest AUC values for Mean Dose, V20, and V5 were obtained in poorly ventilated regions. There were significant differences in all dosimetric parameters between radiation pneumonitis of Grade 1 and Grade ≥2. Conclusions Poorly ventilated lung regions identified on 4D-CT had higher AUC values than highly ventilated regions, suggesting that functional planning based on poorly ventilated regions may reduce the risk of lung toxicity in radiation therapy.

A novel radiation-shielding undergarment using tungsten functional paper for patients with permanent prostate brachytherapy

Abstract Tungsten functional paper (TFP) is a paper-based radiation-shielding material, which is lead-free and easy to cut. We developed a radiation protection undergarment using TFP for prostate cancer patients treated with permanent 125I seed implantation (PSI). The aim of this study was to evaluate the shielding ability of the undergarment with respect to household contacts and members of the public. Between October 2016 and April 2017, a total of 10 prostate cancer patients treated with PSI were enrolled in this prospective study. The external radiation exposure from each patient 1 day after PSI was measured with and without the undergarment. Measurements were performed using a survey meter at 100 cm from the surface of the patient’s body. The exposure rates were measured from five directions: anterior, anteriorly oblique, lateral, posteriorly oblique, and posterior. The measured radiation exposure rates without the undergarment, expressed as mean ± standard deviation, from the anterior, anteriorly oblique, lateral, posteriorly oblique, and posterior directions were 1.28 ± 0.43 μSv/h, 0.70 ± 0.34 μSv/h, 0.21 ± 0.062 μSv/h, 0.65 ± 0.33 μSv/h and 1.24 ± 0.41 μSv/h, respectively. The undergarment was found to have (mean ± standard deviation) shielding abilities of 88.7 ± 5.8%, 44.0 ± 42.1%, 50.6 ± 15.9%, 72.9 ± 27.0% and 90.4 ± 10.7% from the anterior, anteriorly oblique, lateral, posteriorly oblique, and posterior directions, respectively. In conclusion, this shielding undergarment is a useful device that has the potential to reduce radiation exposure for the general public and the patient’s family.

Minimizing dose variation from the interplay effect in stereotactic radiation therapy using volumetric modulated arc therapy for lung cancer

Abstract It is important to improve the magnitude of dose variation that is caused by the interplay effect. The aim of this study was to investigate the impact of the number of breaths (NBs) to the dose variation for VMAT‐SBRT to lung cancer. Data on respiratory motion and multileaf collimator (MLC) sequence were collected from the cases of 30 patients who underwent radiotherapy with VMAT‐SBRT for lung cancer. The NBs in the total irradiation time with VMAT and the maximum craniocaudal amplitude of the target were calculated. The MLC sequence complexity was evaluated using the modulation complexity score for VMAT (MCSv). Static and dynamic measurements were performed using a cylindrical respiratory motion phantom and a micro ionization chamber. The 1 standard deviation which were obtained from 10 dynamic measurements for each patient were defined as dose variation caused by the interplay effect. The dose distributions were also verified with radiochromic film to detect undesired hot and cold dose spot. Dose measurements were also performed with different NBs in the same plan for 16 patients in 30 patients. The correlations between dose variations and parameters assessed for each treatment plan including NBs, MCSv, the MCSv/amplitude quotient (TMMCSv), and the MCSv/amplitude quotient × NBs product (IVS) were evaluated. Dose variation was decreased with increasing NBs, and NBs of >40 times maintained the dose variation within 3% in 15 cases. The correlation between dose variation and IVS which were considered NBs was shown stronger (R 2 = 0.43, P < 0.05) than TMMCSv (R 2 = 0.32, P < 0.05). The NBs is an important factor to reduce the dose variation. The patient who breathes >40 times during irradiation of two partial arcs VMAT (i.e., NBs = 16 breaths per minute) may be suitable for VMAT‐SBRT for lung cancer.

The Shielding Ability of Novel Tungsten Rubber Against the Electron Beam for Clinical Use in Radiation Therapy

Background/Aim: A newly-introduced tungsten containing rubber (TCR) is a potentially useful shielding material in electron radiotherapy because it is lead-free, containing as much as 90% fine tungsten powder by weight. This study aimed to investigate the shielding ability of TCR against electron beams. Materials and Methods: Transmission of TCR was measured for energies of 4, 6, 9 and 12 MeV. Dose profiles were measured to compare the TCR and lead. The electron backscatter factor (EBF) was also compared. Results: The transmission of equivalent thickness for 4, 6, 9 and 12 MeV with TCR (0.78%, 1.34%, 2.16% and 3.08%, respectively) were lower than that with lead (0.81%, 1.44%, 2.19% and 3.16%, respectively) (p<0.05). The dose profiles were not significantly different for TCR and lead. The EBF with TCR was up to 17% lower than that with lead. Conclusion: TCR has adequate radiation shielding ability for electron beams and could be employed instead of lead.