Investigations of different kilovoltage x-ray energy for three-dimensional converging stereotactic radiotherapy system: Monte Carlo simulations with CT data

We are investigating three-dimensional converging stereotactic radiotherapy (3DCSRT) with suitable medium-energy x rays as treatment for small lung tumors with better dose homogeneity at the target. A computed tomography (CT) system dedicated for non-coplanar converging radiotherapy was simulated with BEAMnrc (EGS4) Monte-Carlo code for x-ray energy of 147.5, 200, 300, and 500 kilovoltage (kVp). The system was validated by comparing calculated and measured percentage of depth dose in a water phantom for the energy of 120 and 147.5 kVp. A thorax phantom and CT data from lung tumors (<20 cm3) were used to compare dose homogeneities of kVp energies with MV energies of 4, 6, and 10 MV. Three non-coplanar arcs (0 degrees and +/-25 degrees ) around the center of the target were employed. The Monte Carlo dose data format was converted to the XiO RTP format to compare dose homogeneity, differential, and integral dose volume histograms of kVp and MV energies. In terms of dose homogeneity and DVHs, dose distributions at the target of all kVp energies with the thorax phantom were better than MV energies, with mean dose absorption at the ribs (human data) of 100%, 85%, 50%, 30% for 147.5, 200, 300, and 500 kVp, respectively. Considering dose distributions and reduction of the enhanced dose absorption at the ribs, a minimum of 500 kVp is suitable for the lung kVp 3DCSRT system.

A national survey on the medical physics workload of external beam radiotherapy in Japan†

Several staffing models are used to determine the required medical physics staffing, including radiotherapy technologists, of radiation oncology departments. However, since Japanese facilities tend to be smaller in scale than foreign ones, those models might not apply to Japan. Therefore, in this study, we surveyed workloads in Japan to estimate the optimal medical physics staffing in external beam radiotherapy. A total of 837 facilities were surveyed to collect information regarding radiotherapy techniques and medical physics specialists (RTMPs). The survey covered facility information, staffing, patient volume, equipment volume, workload and quality assurance (QA) status. Full-time equivalent (FTE) factors were estimated from the workload and compared with several models. Responses were received from 579 facilities (69.2%). The median annual patient volume was 369 at designated cancer care hospitals (DCCHs) and 252 across all facilities. In addition, the median FTE of RTMPs was 4.6 at DCCHs and 3.0 at all sites, and the average QA implementation rate for radiotherapy equipment was 69.4%. Furthermore, advanced treatment technologies have increased workloads, particularly in computed tomography simulations and treatment planning tasks. Compared to published models, larger facilities (over 500 annual patients) had a shortage of medical physics staff. In very small facilities (about 140 annual patients), the medical physics staffing requirement was estimated to be 0.5 FTE, implying that employing a full-time medical physicist would be inefficient. However, ensuring the quality of radiotherapy is an important issue, given the limited number of RTMPs. Our study provides insights into optimizing staffing and resource allocation in radiotherapy departments.

Contrast enhancement for portal images by combination of subtraction and reprojection processes for Compton scattering

For patient setup of the IGRT technique, various imaging systems are currently available. MV portal imaging is performed in identical geometry with the treatment beam so that the portal image provides accurate geometric information. However, MV imaging suffers from poor image contrast due to larger Compton scatter photons. In this work, an original image processing algorithm is proposed to improve and enhance the image contrast without increasing the imaging dose. Scatter estimation was performed in detail by MC simulation based on patient CT data. In the image processing, scatter photons were eliminated and then they were reprojected as primary photons on the assumption that Compton interaction did not take place. To improve the processing efficiency, the dose spread function within the EPID was investigated and implemented on the developed code. Portal images with and without the proposed image processing were evaluated by the image contrast profile. By the subtraction process, the image contrast was improved but the EPID signal was weakened because 15.2% of the signal was eliminated due to the contribution of scatter photons. Hence, these scatter photons were reprojected in the reprojection process. As a result, the tumor, bronchi, mediastinal space and ribs were observed more clearly than in the original image. It was clarified that image processing with the dose spread functions provides stronger contrast enhancement while maintaining a sufficient signal‐to‐noise ratio. This work shows the feasibility of improving and enhancing the contrast of portal images.

Direct energy spectrum measurement of X-ray from a clinical linac

A realistic X‐ray energy spectrum is essential for accurate dose calculation using the Monte Carlo (MC) algorithm. An energy spectrum for dose calculation in the radiation treatment planning system is modeled using the MC algorithm and adjusted to obtain acceptable agreement with the measured percent depth dose (PDD) and off‐axis ratio. The simulated energy spectrum may not consistently reproduce a realistic energy spectrum. Therefore, direct measurement of the X‐ray energy spectrum from a linac is necessary to obtain a realistic spectrum. Previous studies have measured low photon fluence directly, but the measurement was performed with a nonclinical linac with a thick target and a long target‐to‐detector distance. In this study, an X‐ray energy spectrum from a clinical linac was directly measured using a NaI(Tl) scintillator at an ultralow dose rate achieved by adjusting the gun grid voltage. The measured energy spectrum was unfolded by the Gold algorithm and compared with a simulated spectrum using statistical tests. Furthermore, the PDD was calculated using an unfolded energy spectrum and a simulated energy spectrum was compared with the measured PDD to evaluate the validity of the unfolded energy spectrum. Consequently, there was no significant difference between the unfolded and simulated energy spectra by nonparametric, Wilcoxon's rank‐sum, chi‐square, and two‐sample Kolmogorov–Smirnov tests with a significance level of 0.05. However, the PDD calculated from the unfolded energy spectrum better agreed with the measured compared to the calculated PDD results from the simulated energy spectrum. The adjustment of the incident electron parameters using MC simulation is sensitive and takes time. Therefore, it is desirable to obtain the energy spectrum by direct measurement. Thus, a method to obtain the realistic energy spectrum by direct measurement was proposed in this study.

Bremsstrahlung and Photoneutron leakage from steel shielding board impinged by 12-24 MeV electrons beams

Many medical linear accelerators generate not only high-energy photons, but also high-energy electrons, and they are no longer equipped with beam stoppers. Therefore, shields might be necessary against bremsstrahlung and photoneutron generated by high-energy electron beams. However, there are few physical studies, and no recommendations are made about shields nowadays. In this report, the leakage doses of bremsstrahlung and photoneutron were calculated by the use of Monte Carlo simulation. To verify the calculated results, the photoneutron leakage dose was measured with a rem counter. The results clearly show that the bremsstrahlung and photoneutron leakage dose generated by electron beams of 24 MeV or below is negligible.

Depth scaling of solid phantom for intensity modulated radiotherapy beams

To reduce the uncertainty of absorbed dose for high energy photon beams, water has been chosen as a reference material by the dosimetry protocols. However, solid phantoms are used as media for absolute dose verification of intensity modulated radiotherapy (IMRT). For the absorbed dose measurement, the fluence scaling factor is used for converting an ionization chamber reading in a solid phantom to absorbed dose to water. Furthermore the depth scaling factor is indispensable in determining the fluence scaling factor. For IMRT beams, a photon energy spectrum is varied by transmitting through a multileaf collimator and attenuating in media. However, the effects of spectral variations on depth scaling have not been clarified yet. In this study, variations of photon energy spectra were determined using the EGS Monte Carlo simulation. The depth scaling factors for commercially available solid phantoms were determined from effective mass attenuation coefficients using photon energy spectra. The results clarified the effect of spectral variation on the depth scaling and produced an accurate scaling method for IMRT beams.

Bremsstrahlung and photoneutron production in a steel shield for 15-22-MeV clinical electron beams

The physical data regarding bremsstrahlung and neutrons produced in a steel shield by high-energy electron beams from a medical linear accelerator were investigated. These data are required to allow the accurate prediction of shielding performance for high-energy electron beams and in the design of radiotherapy facilities. A Monte Carlo code was used to develop Monte Carlo beam models for clinical electron beams and to directly simulate bremsstrahlung and secondary neutron production in a steel shield. The effective dose and dose equivalent of bremsstrahlung X rays and secondary neutrons outside a vault were determined using a realistic radiation source. The accuracy of Monte Carlo simulations was validated experimentally by comparing the measured and calculated physical quantities. In validating the Monte Carlo simulation, the measured and calculated values showed reasonable agreement, indicating that bremsstrahlung and photoneutron production and transport were simulated accurately. The bremsstrahlung X-ray dose was the main component of the total dose outside a vault. The secondary neutron dose was 1-20 % of the bremsstrahlung X-ray dose, but the neutron dose was also at a non-negligible level. The calculated neutron dose outside the vault differed from the McGinley's reported data. These results indicate that McGinley's method overestimates the neutron dose beyond the steel shield. The physical data used here will be useful in the accurate estimation of bremsstrahlung X-ray and neutron doses for high-energy electron beams.

Dose calculation system for remotely supporting radiotherapy

The dose calculation system IMAGINE is being developed keeping in mind remotely supporting external radiation therapy using photon beams. The system is expected to provide an accurate picture of the dose distribution in a patient body, using a Monte Carlo calculation that employs precise models of the patient body and irradiation head. The dose calculation will be performed utilising super-parallel computing at the dose calculation centre, which is equipped with the ITBL computer, and the calculated results will be transferred through a network. The system is intended to support the quality assurance of current, widely carried out radiotherapy and, further, to promote the prevalence of advanced radiotherapy. Prototypes of the modules constituting the system have already been constructed and used to obtain basic data that are necessary in order to decide on the concrete design of the system. The final system will be completed in 2007.

[Evaluation of relative electrometer calibration factor with user beam.]

To establish traceability of absorbed dose to water, a cobalt calibration coefficient is transferred to a reference ionization chamber by the standard dosimetry laboratory in the radiotherapy field. In Japan, the calibration is done against a set of an ionization chamber and an electrometer as a system. Nowadays, solely electrometer calibration is desirable to measure absorbed dose with more than one combination of ionization chamber and electrometer. Unfortunately, there is no domestic electrometer calibration service for nano-ampere range appropriate for ionization current measurement in the radiotherapy field. In this report, a relative electrometer calibration factor was determined by comparison between a calibrated combination and a non-calibrated electrometer under identical irradiation conditions at the user site. To estimate uncertainties of user electrometer calibration, comparison was made between calibration factors obtained by ionization current under a linac photon beam and DC current with a precision DC source instrument. It was found that the variation of electrometer readings to pulse ionization current is negligible under the steady photon beam output and dose monitor system. Therefore relative electrometer calibration under identical irradiation conditions at the user site was judged valid until a domestic nano-ampere electrometer calibration service becomes available.

Current status of the working environment of brachytherapy in Japan: a nationwide survey-based analysis focusing on radiotherapy technologists and medical physicists

Brachytherapy (BT), especially in high dose rate (HDR), has become increasingly complex owing to the use of image-guided techniques and the introduction of advanced applicators. Consequently, radiotherapy technologists and medical physicists (RTMPs) require substantial training to enhance their knowledge and technical skills in image-guided brachytherapy. However, the current status of the RTMP workload, individual abilities and quality control (QC) of BT units in Japan remains unclear. To address this issue, we conducted a questionnaire survey from June to August 2022 in all 837 radiation treatment facilities in Japan involving RTMPs. This survey focused on gynecological cancers treated with HDR-BT (GY-HDR) and permanent prostate implantation using low-dose-rate BT (PR-LDR). The responses revealed that the average working time in the overall process for HDR varied: 120 min for intracavitary BT and 180 min for intracavitary BT combined with interstitial BT. The QC implementation rate, in accordance with domestic guidelines, was 65% for GY-HDR and 44% for PR-LDR, which was lower than the 69% observed for external beam radiation therapy (EBRT). Additionally, the implementation rate during regular working hours was low. Even among RTMP working in facilities performing BT, the proportion of those able to perform QC for BT units was ~30% for GY-HDR and <20% for PR-LDR, significantly lower than the 80% achieved for EBRT. This study highlights the vulnerabilities of Japan's BT unit QC implementation structure. Addressing these issues requires appropriate training of the RTMP staff to safely perform BT tasks and improvements in practical education and training systems.

Effects of density changes in the chest on lung stereotactic radiotherapy

To experimentally and theoretically evaluate dose distribution during lung stereotactic radiotherapy, we investigated the relative electron densities in lung and tumor tissues using X-ray computed tomography images obtained from 30 patients in three breathing states: free breathing, inspiration breath-hold, and expiration breath-hold. We also calculated dose distribution using Monte Carlo simulation for lung tissue with two relative electron densities. The effect of changes in relative electron density on dose distribution in lung tissue was evaluated using calculated differential and integral dose volume histograms. The relative electron density of lung tissue was 0.22 in free breathing, 0.23 in shallow expiration, and 0.17 in shallow inspiration, and there was a tendency for relative electron density to decrease with age. The relative electron density of tumor tissue was approximately 0.9, with little variation due to differences in breathing state. As the relative electron density of lung tissue decreases, the low-dose region expands and leads to changes in the marginal dose.