Survey on utilization of flattening filter-free photon beams in Japan.

To understand the current state of flattening filter-free (FFF) beam implementation in C-arm linear accelerators (LINAC) in Japan, the quality assurance (QA)/quality control (QC) 2018-2019 Committee of the Japan Society of Medical Physics (JSMP) conducted a 37-question survey, designed to investigate facility information and specifications regarding FFF beam adoption and usage. The survey comprised six sections: facility information, devices, clinical usage, standard calibration protocols, modeling for treatment planning (TPS) systems and commissioning and QA/QC. A web-based questionnaire was developed. Responses were collected between 18 June and 18 September 2019. Of the 846 institutions implementing external radiotherapy, 323 replied. Of these institutions, 92 had adopted FFF beams and 66 had treated patients using them. FFF beams were used in stereotactic radiation therapy (SRT) for almost all disease sites, especially for the lungs using 6 MV and liver using 10 MV in 51 and 32 institutions, respectively. The number of institutions using FFF beams for treatment increased yearly, from eight before 2015 to 60 in 2018. Farmer-type ionization chambers were used as the standard calibration protocol in 66 (72%) institutions. In 73 (80%) institutions, the beam-quality conversion factor for FFF beams was calculated from TPR20,10, via the same protocol used for beams with flattening filter (WFF). Commissioning, periodic QA and patient-specific QA for FFF beams also followed the procedures used for WFF beams. FFF beams were primarily used in high-volume centers for SRT. In most institutions, measurement and QA was conducted via the procedures used for WFF beams.

Evaluation of differences and dosimetric influences of beam models using golden and multi-institutional measured beam datasets in radiation treatment planning systems.

PURPOSE: The beam model in radiation treatment planning systems (RTPSs) plays a crucial role in determining the accuracy of calculated dose distributions. The purpose of this study was to ascertain differences in beam models and their dosimetric influences when a golden beam dataset (GBD) and multi-institution measured beam datasets (MBDs) are used for beam modeling in RTPSs. METHODS: The MBDs collected from 15 institutions, and the MBDs' beam models, were compared with a GBD, and the GBD's beam model, for Varian TrueBeam linear accelerator. The calculated dose distributions of the MBDs' beam models were compared with those of the GBD's beam model for simple geometries in a water phantom. Calculated dose distributions were similarly evaluated in volumetric modulated arc therapy (VMAT) plans for TG-119 C-shape and TG-244 head and neck, at several dose constraints of the planning target volumes (PTVs), and organs at risk. RESULTS: The agreements of the MBDs with the GBD were almost all within ±1%. The calculated dose distributions for simple geometries in a water phantom also closely corresponded between the beam models of GBD and MBDs. Nevertheless, there were considerable differences between the beam models. The maximum differences between the mean energy of the energy spectra of GBD and MBDs were -0.12 MeV (-10.5%) in AcurosXB (AXB, Eclipse) and 0.11 MeV (7.7%) in collapsed cone convolution (CCC, RayStation). The differences in the VMAT calculated dose distributions varied for each dose region, plan, X-ray energy, and dose calculation algorithm. The ranges of the differences in the dose constraints were -5.6% to 3.0% for AXB and -24.1% to 2.8% for CCC. In several VMAT plans, the calculated dose distributions of GBD's beam model tended to be lower in high-dose regions and higher in low-dose regions than those of the MBDs' beam models. CONCLUSIONS: We found that small differences in beam data have large impacts on the beam models, and on calculated dose distributions in clinical VMAT plan, even if beam data correspond within ±1%. GBD's beam model was not a representative beam model. The beam models of GBD and MBDs and their calculated dose distributions under clinical conditions were significantly different. These differences are most likely due to the extensive variation in the beam models, reflecting the characteristics of beam data. The energy spectrum and radial energy in the beam model varied in a wide range, even if the differences in the beam data were <±1%. To minimize the uncertainty of the calculated dose distributions in clinical plans, it was best to use the institutional MBD for beam modeling, or the beam model that ensures the accuracy of calculated dose distributions.

Evaluation of beam-on time and number of breath-holds using a flattening-filter-free beam with the deep inspiration breath-hold method in left-sided breast cancer.

We performed a dosimetric study to evaluate the benefits of using a flattening-filter-free (FFF) beam with the deep inspiration breath-hold (DIBH) method for left-breast cancer. We used data from 30 previous patients with treatment plans that included DIBH for left-breast cancer with a flattened beam. FFF beam plans were calculated from previous treatment plan images and compared to the original plans in terms of monitor units (MU), number of segments, beam-on time, and breath-holds. Beam-on time was calculated by adding the traveling time of 1.5 second between segments to the time calculated from the MU and dose rate. Breath-holds were calculated based on the beam-on time, assuming 15 s per hold. The FFF beam had increased MU in all cases (mean ± SD: flattened beam, 122.4 ± 9.8 MU; FFF beam, 160.2 ± 17.5 MU). Furthermore, the number of segments increased with the FFF beam in all cases (median [range]: flattened beam, 2 [1 to 3]; FFF beam, 5 [3 to 7]). However, in most cases, the beam-on time was reduced using the FFF beam (mean ± SD: flattened beam, 27.8 ± 7.4 seconds; FFF beam, 13.2 ± 1.7 seconds), although when a 6 MV flattened beam was used there was not a large increase. There were fewer breath-holds in most cases with the FFF beam. Cases using a 4 MV flattened beam also had fewer breath-holds; however, the number of breath-holds was consistent or increased in cases that used a 6 MV flattened beam (median [range]: flattened beam, 3 [1 to 3]; FFF beam, 1 [1 to 2]).

Questionnaire survey on treatment planning techniques for lung stereotactic body radiotherapy in Japan.

This study aimed to obtain details regarding treatment planning techniques for lung stereotactic body radiation therapy (SBRT) employed at each institution in Japan by using a questionnaire survey. An Internet questionnaire survey on SBRT procedures performed in 2016 was conducted by the QA/QC committee of the Japan Society of Medical Physics from April to June 2017. The questionnaire assessed two aspects: the environment for SBRT at each institution and the treatment planning techniques with and without respiratory motion management techniques (RMMT). Of the 309 evaluated responses, 218 institutions had performed SBRT. A total of 186 institutions performed SBRT without RMMT and 139 institutions performed SBRT with RMMT. When respiratory motion was ≥10 mm, 69 institutions applied RMMT. The leading RMMT were breath holding (77 institutions), respiratory gating (49 institutions) and real-time tumor tracking (11 institutions). The most frequently used irradiation technique was 3D conformal radiotherapy, which was used in 145 institutions without RMMT and 119 institutions with RMMT. Computed tomography (CT) images acquired under free breathing were mostly used for dose calculation for patients treated without RMMT. The usage ratio of IMRT/VMAT to SBRT is low in Japan, compared to elsewhere in the world (<20% vs ≥70%). Among the available dose calculation algorithms, superposition convolution was the most frequently used regardless of RMMT; however, 2% of institutions have not yet made heterogeneity corrections. In the prescription setting, about half of the institutions applied point prescriptions. The survey results revealed the most frequently used conditions, which may facilitate standardization of treatment techniques in lung SBRT.

Verification of the dose attenuation of a newly developed vacuum cushion for intensity-modulated radiation therapy of prostate cancer.

This study measured the dose attenuation of a newly developed vacuum cushion for intensity-modulated radiation therapy (IMRT) of prostate cancer, and verified the effect of dose-correction accuracy in a radiation treatment planning system (RTPS). The new cushion was filled with polystyrene foams inflated 15-fold (Sφ â‰’ 1 mm) to reduce contraction caused by air suction and was compared to normal polystyrene foam inflated to 50-fold (Sφ â‰’ 2 mm). The dose attenuation at several thicknesses of compression bag filled with normal and low-inflation materials was measured using an ionization chamber; and then the calculated RTPS dose was compared to ionization chamber measurements, while the new cushion was virtually included as region of interest in the calculation area. The dose attenuation rate of the normal cushion was 0.010 %/mm (R (2) = 0.9958), compared to 0.031 %/mm (R (2) = 0.9960) in the new cushion. Although the dose attenuation rate of the new cushion was three times that of the normal cushion, the high agreement between calculated dose by RTPS and ionization chamber measurements was within approximately 0.005 %/mm. Thus, the results of the current study indicate that the new cushion may be effective in clinical use for dose calculation accuracy in RTPS.

Accuracy of positional correction for the floor-mounted kV X-ray IGRT system in angled couch positions.

Stereotactic irradiation (STI) requires high geometric accuracy. We evaluated the positional correction accuracy after treatment couch rotation for non-coplanar STI with a frameless mask. A steel ball was embedded as a virtual target in a head phantom with a human cranial bone structure, and the head phantom was placed in the isocenter of the treatment-planning system with the image-guide system. The Winston-Lutz test at treatment couch angles of ±90°, ±45°, and 0° was performed, and the amount of displacement from the center position at the treatment couch angle of 0° was calculated. After treatment couch rotation through each treatment couch angle, the amount of center displacement was compared between cases with and without a positional correction by the image-guide system, and then the accuracy of the positional correction after treatment couch rotation was examined. The maximum amount of three-dimensional displacement without and with positional correction after treatment couch rotation was 0.52 mm at a treatment couch angle of -90° and 0.49 mm at a treatment couch angle of -45°. These results indicate that the image-guide system provides accuracy within about 0.50 mm regardless of the positional correction even after rotation of the treatment couch.

Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9MeV electron beams.

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0-15mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5cm depth occurred at 4, 9, and 15mm with 4, 6, and 9MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.

Effects of interportal error on dose distribution in patients undergoing breath-holding intensity-modulated radiotherapy for pancreatic cancer: evaluation of a new treatment planning method.

In patients with pancreatic cancer, intensity-modulated radiotherapy (IMRT) under breath holding facilitates concentration of the radiation dose in the tumor, while sparing the neighboring organs at risk and minimizing interplay effects between movement of the multileaf collimator and motion of the internal structures. Although the breath-holding technique provides high interportal reproducibility of target position, dosimetric errors caused by interportal breath-holding positional error have not been reported. Here, we investigated the effects of interportal breath-holding positional errors on IMRT dose distribution by incorporating interportal positional error into the original treatment plan, using random numbers in ten patients treated for pancreatic cancer. We also developed a treatment planning technique that shortens breath-holding time without increasing dosimetric quality assurance workload. The key feature of our proposed method is performance of dose calculation using the same optimized fluence map as the original plan, after dose per fraction in the original plan was cut in half and the number of fractions was doubled. Results confirmed that interportal error had a negligible effect on dose distribution over multiple fractions. Variations in the homogeneity index and the dose delivered to 98%, 2%, and 50% of the volume for the planning target volume, and the dose delivered to 1 cc of the volume for the duodenum and stomach were ±1%, on average, in comparison with the original plan. The new treatment planning method decreased breath-holding time by 33%, and differences in dose-volume metrics between the original and the new treatment plans were within ± 1%. An additional advantage of our proposed method is that interportal errors can be better averaged out; thus, dose distribution in the proposed method may be closer to the planned dose distribution than with the original plans.

Simulation for improvement of system sensitivity of radiochromic film dosimetry with different band-pass filters and scanner light intensities.

The delivered dose of high-energy photon beams is measured with radiochromic film. Previous studies sought to improve the system sensitivity of radiochromic film dosimetry by use of band-pass filters. However, band-pass filters reduce the scanning light intensity. To avoid a reduction of the signal-to-noise ratio, one must increase the scanner light intensity. Our purposes in this study were to develop an optical system model of GAFCHROMIC EBT2 radiochromic film dosimetry, and to estimate the system sensitivity characteristics by employing a combination of band-pass filters and scanner light intensities. The spectra of the scanner light source, band-pass filter, and irradiated EBT2 films were measured with a spectrometer. Meanwhile, the intensity of a light path from the scanner light source to the scanner detector was simulated. Then, the dose-response curves were computed with six simulated virtual band-pass filters of varying bandwidth. The simulated dose-response curves were in good agreement with the experimental values. The slope of the simulated dose-response curve was steeper when a filter of narrower bandwidth was used; however, at the same time, saturation was observed at a lower dose. For achieving the same dose response as was observed without a band-pass filter, it was necessary to increase the scanner light intensity. We proved that our proposed optical system model was valid, suggesting that a realistic simulation may be feasible with the proposed model. For improvement of the system sensitivity of radiochromic film dosimetry, it is necessary to select a well-balanced combination of band-pass filter and scanner light intensity.

[Evaluation of a method for correcting systematic setup error in external-beam radiation therapy].

BACKGROUND AND PURPOSE: We verified the propriety of our systematic error reduction strategy by means of a computer simulation based on our data of position error with a prone fixation device for prostate IMRT. MATERIALS AND METHODS: Computer simulations of the off-line correction method for systematic setup errors based on the portal imaging taken on the first several days of the treatment session were performed. Using the computer simulations, an optimal number of portal images were evaluated for the SD value, from 0.5 mm to 1.5 mm at a 0.25 mm interval, and the respective required setup margins were calculated. RESULTS: The value of systematic error was reduced as the frequency of data obtained increased. Moreover, the reduction rate was so remarkable that random error was large.

[Evaluation of setup error and adequate setup margins in patients with prostate cancer treated by IMRT and fixed in the prone position using a set of immobilization devices].

PURPOSE: Positional reproducibility in patients with prostate cancer fixed in the prone position with a set of immobilization devices for external-beam intensity-modulated radiation therapy (IMRT) was evaluated. In addition, the adequacy of our positional error reduction strategy and current planning target volume (PTV) margins was also evaluated. RESULTS: Systematic error was corrected by the positional correction that we executed at the first stage of irradiation. The setup margin that we had calculated was 1.1 mm in the L-R direction, 1.3 mm in the A-P direction, and 2.7 mm in the C-C direction. CONCLUSION: We determined that the effectiveness of the method of correcting the error margin and the setup accuracy of the fixed method were well maintained.

[Evaluation of radiation therapy for keloid using electron beam].

In radiation therapy for keloid, electron beams are delivered to the skin through a lead shield hollowed into the shape of the keloid. The shape of a postoperative keloid scar is linear, causing the irradiation shield to be long and narrow. This lead shield is put on the surface of the skin. Therefore, it is considered that beam data used in general external irradiation are not applicable to irradiation for keloid. Therefore, we used a water equivalent phantom and measured beam data by using chambers or film dosimeters. Experimental conditions were the same as those of actual radiotherapy for keloid. As a result of this procedure, the radiation technique was optimized. Electron energy and thickness of the bolus, thickness of the lead shield, margins such that the planning target volume would receive the necessary dose, and the method of MU calculation all were determined. It was suggested that these experiments were useful to establish the appropriate technique in irradiation for keloid.