Medical Linac Research Articles

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

Gamma Radiation Simulations of the Medical Therapy Linac’s Target

Gamma Radiation Simulations of the Medical Therapy Linac’s Target

Monte Carlo evaluation of uncertainties in photon and electron TG-51 absorbed dose calibration.

The accuracy of dose delivery to all patients treated with medical linacs depends on the accuracy of beam calibration. Dose delivery cannot be any more accurate than this. Given the importance of this, it seems worthwhile taking another look at the expected uncertainty in TG-51 photon dose calibration and a first look at electron calibration. This work builds on the 2014 addendum to TG-51 for photons and adds to it by also considering electrons. In that publication, estimates were made of the uncertainty in the dose calibration. In this paper, we take a deeper look at this important issue. The methodology used here is more rigorous than previous determinations as it is based on Monte Carlo simulation of uncertainties. It is assumed that mechanical QA has been performed following TG-142 prior to beam calibration and that there are no uncertainties that exceed the tolerances specified by TG-142. Despite the different methodology and assumptions, the estimated uncertainty in photon beam calibration is close to that in the addendum. The careful user should be able to easily reach a 95% confidence interval (CI) of±2.3% for photon beam calibration with standard instrumentation. For electron beams calibrated with a Farmer chamber, the estimated uncertainties are slightly larger, and the 95% CI is ±2.6% for 6 MeV and slightly smaller than this for 18 MeV. There is no clear energy dependence in these results. It is unlikely that the user will be able to improve on these uncertainties as the dominant factor in the uncertainty resides in the ion chamber dose calibration factor . For both photons and electrons, reduction in the ion chamber depth uncertainty below about 0.5mm and SSD uncertainty below 1mm have almost no effect on the total dose uncertainty, as uncertainties beyond the user's control totally dominate under these circumstances.

Effect of thermoplastic mask hole diameter on dose distribution in the build-up region of medical LINAC

This study aims to investigate the effect of variations in the hole diameter of unstretched thermoplastic masks offered by a single company on the dose distribution in the build-up region. 2, 2.5, 3 and 4 mm hole diameters masks with the same properties were irradiated with 6 MV X-rays and dose distribution was analyzed according to the open field. Dose measurements were obtained with a parallel plane ionization chamber. All irradiations were repeated for 5 × 5, 8 × 8, 10 × 10 and 15 × 15 cm2 beam fields. It was observed that the mask had a bolus effect in all measurements. It was observed that this bolus effect varied especially depending on the hole diameter in the mask. In a 10 × 10 cm2 field, the dmax depth without mask was 15 mm, while the dmax for masks with 2, 2.5, 3 and 4 mm hole diameters was 11, 12, 13 and 13 mm, respectively. It was observed that the mask hole diameter affected the dmax depth. Thermoplastic masks with large hole diameters increase the surface dose less than those with small hole diameters.

Radiation safety and shielding evaluation of newly installed medical LINAC facility in Bangladesh

Medical linear accelerator (Linac) is widely used for radiotherapy purposes which is one of the modalities for radiation therapy. When a Linac is being installed, an initial survey is required during the first beam and a complete shielding survey must be carried out after the Linac has been operational and calibration is completed. Based on the detailed survey report, the facility license is issued. Worldwide several organizations provide various recommendations regarding shielding design and radiation safety issues. The purpose of this paper is to show how to assess or validate the radiation safety and shielding status of the newly installed Linac facility in Bangladesh. In this study, one of the Linac facilities in Dhaka city has been chosen which was installed in 2021. The dose rate meter RadEYE PRD and tele survey meter TELE-STTC are used for x-ray/gamma-ray dose measurement and REM 12/4 count rate meter for neutron measurement. A maximum photon dose of 5.56 μSvh−1 was observed at a gantry angle of 900 along the center front of the door, while a maximum neutron dose of 1 μSvh−1 was found at three different locations. The obtained results are in line with the recommendations by controlling organizations. This study found that the Linac can be operated safely for both conventional and IMRT modes, and the same shielding procedure can be followed all over the country for the same Linac setting.

Assessment of radio-activation using spectroscopy in medical linear accelerators.

In radiotherapy, when photon energy exceeding 8 MV is utilized, photoneutrons can activate the components within the gantry of the linear accelerator (linac). At the end of the linac's lifecycle, radiation workers are tasked with its dismantling and disposal, potentially exposing them to unintentional radiation. This study aims to identify and measure the radioisotopes generated by this activation through spectroscopy, and to evaluate the effective dose rate. We selected nine medical linacs, considering various factors such as manufacturer (Siemens, Varian, and Elekta), model, energy, period of operation, and workload. We identified the radionuclides in the linac head by employing an in situ high-purity germanium (HPGe) detector. Spectroscopy and dose-rate measurements were conducted post-shutdown. We also measured the dose rates at the beam-exit window following irradiation with 10 MV and 15 MV photon beams. As a result of the spectroscopy, we identified approximately 20 nuclides including those with half-lives of 100 days or longer, such as 54Mn, 60Co, 65Zn, 122Sb, and 198Au. The dose rate measurements after 10 MV irradiation decreased to the background level in 10 min. By contrast, on 15 MV irradiation, the dose rate was 628 nSv/h after 10 min and decreased to 268 nSv/h after 1.5 hours. It was confirmed that the difference in the level of radiation and the type of nuclide depends on the period of use, energy, and workload. However, the type of nuclide does not differ significantly between the linacs. It is necessary to propose appropriate guidelines for the safety of workers, and disposal/move-install should be planned while taking into consideration the equipment's energy usage rate.

Matrix metal collimators studies for the spatially fractionated radiation therapy

Traditional radiation therapy is hindered by limitations in dose delivery and the risk of over-irradiation, which can harm healthy tissues surrounding tumors. Spatially fractionated and intensity-modulated radiation therapies have emerged as promising techniques to mitigate these issues by reducing damage to healthy tissues. This study explores the potential of enhancing radiation therapy's therapeutic efficacy through spatial fractionation of X-ray beams, a technique that can significantly increase radiation efficiency by reducing the dose load on healthy tissues. This research hypothesizes that microscopic lesions within the paths of micro/mini-beams can be repaired by minimally irradiated cells adjacent to the irradiated tissue slices, based on observations with high-energy (MV) photons. Spatial fractionation is particularly valuable for X-ray and electron radiation therapy, which is more widespread and cost-effective than proton therapy. The study introduces a new type of cost-effective metal matrix collimators designed for beam fractionation. These collimators are constructed using lead plates for thickness variation implementation and employ a 5x5 hole matrix with 1 mm diameter and 3 mm pitch, covering an area of 14x14 mm². To visualize beam distribution, a Timepix detector, capable of providing real-time 2D beam profiles, was employed. Experiments were conducted using a medical LINAC with typical therapeutic energy ranging from 6 to 18 MeV. Results indicate that spatial fractionation can be achieved for X-ray radiation, with a PVDR of approximately 3 for a 3 cm lead collimator. However, issues with hole uniformity and blurring in the peak area prompted a shift to using copper collimators due to their superior manufacturing properties. This study presents a novel matrix collimator design made from various materials for shaping mini beams, aimed at improving the efficiency of spatial dose fractionation for different ionizing radiation types. Geant4 simulations have been instrumental in optimizing collimator features. The findings suggest that high levels of dose fractionization can be achieved for X-rays, electrons, and proton beams. These results warrant further biological studies to evaluate the effects of fractionation on normal and tumoral tissues, supporting the practical implementation of collimation in radiation therapy.

Additive Manufacturing of Side-Coupled Cavity Linac Structures from Pure Copper: A First Concept

Compared to conventional manufacturing, additive manufacturing (AM) of radio frequency (RF) cavities has the potential to reduce manufacturing costs and complexity and to enable higher performance. This work evaluates whether normal conducting side-coupled linac structures (SCCL), used worldwide for a wide range of applications, can benefit from AM. A unit cell geometry (SC) optimized for 75 MeV protons was developed. Downskins with small downskin angles α were avoided to enable manufacturing by laser powder bed fusion without support structures. SCs with different α were printed and post-processed by Hirtisation (R) (an electrochemical process) to minimize surface roughness. The required accuracy for 3 GHz SCCL (medical linacs) is achieved only for α>45∘. After a material removal of 140 µm due to Hirtisation (R), a quality factor Q0 of 6650 was achieved. This corresponds to 75% of the Q0 simulated by CST®. A 3 GHz SCCL concept consisting of 31 SCs was designed. The effective shunt impedance ZT2 simulated by CST corresponds to 60.13MΩm and is comparable to the ZT2 of SCCL in use. The reduction in ZT2 expected after Hirtisation (R) can be justified in practice by up to 70% lower manufacturing costs. However, future studies will be conducted to further increase Q0.

Neutron dosimetry with a pair of TLDs for the Elekta Precise medical linac and the evaluation of optimum moderator thickness for the conversion of fast to thermal neutrons

IntroductionIn this study, TLD 600 and TLD 700 pairs were used to measure the neutron dose of Elekta Precise medical linac. To this end, the optimum moderate thickness for the conversion of fast to thermal neutrons were evaluated. Materials and methods241Am-Be and 252Cf sources were simulated to calculate the optimum thicknesses of the moderator for the conversion of maximum fast neutrons (FN) into thermal neutrons (TN). Pair TLDs were used to measure F&TN doses for three different field sizes at four depths of the medical linac. ResultsThe maximum thickness of the moderator was optimized at 6 cm. The measurement results demonstrated that the TN dose increased with the expansion of field size and depth. The FN dose, which was converted TN, exhibits behaviors comparable to the TN due to its nature. ConclusionThis study presents the optimum thickness for the moderator to convert FN into TN and measure F&TN using TLDs.

Dosimetric Study of Total Marrow and Lymphoid Irradiation on a Ring Gantry-Based Medical Linac with a Two-Layer Multi-Leaf Collimator

In this study, we aimed to evaluate dosimetric quality of total marrow and lymphoid irradiation (TMLI) plans for a ring gantry-based medical Linac with a two-layer multi-leaf collimator. We retrospectively retrieved treatment planning CT images, structure sets, and plan dose for four adult patients, two male and two female, who previously received TMLI treatments on helical tomotherapy (HT) at our institution. TMLI plans were optimized for a ring gantry-based medical Linac with a two-layer multi-leaf collimator (Halcyon, Varian Medical Systems, Inc., Palo Alto, CA). A prescription dose of 12 Gy in 8 fractions was prescribed to the skeletal bones from the skull to mid-thigh, spleen, spinal canal, and lymphoid volume. Five or six isocenters were placed with equal spacing along the patient's longitudinal direction in each TMLI plan with two 6-MV flattening filter-free volumetric modulated arc therapy (VMAT) fields at each isocenter. Isocenter separation ranged from 15 cm to 16.5 cm. Each VMAT field has a field size of 28 cm to 28 cm with the collimator at 90° and a full gantry rotation. The nominal dose rate was 800 MU/minute, and the maximum gantry rotation speed was 24°/sec. Institutional dosimetric constraints were used for optimization including a mean lung dose limit of less than 8 Gy. All the plans were normalized so that 85% the primary planning target volume received the prescription dose. The average mean doses to the target volumes ranged from 12.2 to 12.6 Gy in the Halcyon TMLI plans, while they ranged from 12.1 to 12.5 Gy in the HT TMLI plans. Relative to the prescription dose, the average mean dose for normal organs ranged from 21.3% to 56.6% in the Halcyon TMLI plans, while it ranged from 10.1% to 68.4% in the clinical HT plans. The difference in the average mean dose to normal organs was less than 0.5 Gy except two organs between the Halcyon and HT TMLI plans. The average median dose for normal organs ranged from 18.2% to 48.8% relative to the prescription dose in the Halcyon TMLI plans. The mean lung dose (MLD) in the Halcyon TMLI plans met the institutional limit with an average dose of 6.75±0.42 Gy (range: 6.44 - 7.36 Gy), while the average MLD was 6.54±0.77 Gy (range: 6.24 - 7.22 Gy) in the HT plans (p-value = 0.71 in the paired t-test). The average total monitor unit in the Halcyon TMLI plans was 4,425±906 MU (range: 3,470 - 5,575 MU) with an average beam-on time of 5.1±1.3 minutes (range: 4.1 - 7.0 minutes), which excludes isocenter setup time, while the average beam-on time was 22.2±3.2 minutes (range: 19.6 - 26.1 minutes) with the HT plans. Halcyon TMLI plans met our institutional dosimetric constraints with adequate normal organ sparing and target dose coverage. The beam-on time with the Halcyon plans was significantly shorter than that with the HT plans, which could lead to shorter treatment time and increased patient comfort. This study showed the feasibility of TMLI treatments on the Halcyon machine. The same method could be used for total body irradiation on Halcyon.

GEANT4 Simulation of Photoneutron Spectrum from Medical Linear Accelerator

Photons with energy totaling more than 10 MeV provide efficient treatment for deeply seated tumors but interact with the nuclei of high-Z materials constituting a head of the linac. These interactions result in photoneutrons that deliver an additional out-of-field dose to the patient, which increases the risk of radiation-induced cancer. Monte Carlo simulation is an accurate strategy for estimating the effective photoneutron dose for a patient. In the current study, the possibility of using GEANT4 to calculate the photoneutron spectrum from the medical linac was investigated. The free-in-air photoneutron spectrum from a head of the linac was simulated using the NeutronHP experimental package. Validation of the simulated model was carried out based on a comparison of simulated and measured percentage depth–dose curves from photons in the water phantom. The obtained photoneutron spectrum was compared with the previously measured spectrum at the Varian Thilogy linac. GEANT4 may improve the accuracy of calculations of the effective dose based on photoneutrons. However, the simulated model should be improved and optimized. In the future, this model may constitute a physical basis for the prediction of the risk of radiation-induced cancer at our clinical center.

Development of a new MC Fast Dose Engine for small compact radiation fields generated by the CyberKnifeⓇ Iris medical linac

MC-based computerized TPS is an essential tool for improving treatment quality in external radiation therapy chains. However, incorporating these systems into clinical TPS is challenging due to their long computation time. To address this problem, Phase Spaces (PhSp) can be utilized, which reduce the computation time of such simulations while maintaining accuracy levels comparable to full simulations. Since PhSps can only generate a finite number of particles, recycling the same PhSp several times during a simulation will increase statistical uncertainty. To carry out this disadvantage, the present study employs a numerical reconstruction model to develop a novel generation of MC Fast Dose Engine (MC-FDE) that relies on the Cyberknife linac parameters publicly available in the IAEA database. The MC-FDE model’s performance is evaluated based on the precision of the reconstructed parameters and the accuracy of dose calculation in the 3D BEAMSCAN water phantom and patient CT datasets. In contrast to the recently published Wasserstein Generative Adversarial Networks (WGAN), the MC-FDE reconstruction model produces significant agreement with measured data, IAEA PhSp files and TPS Accuray, as determined by marginal distributions, correlation metrics, 2-3D gamma indexes and the Dose Volume Histogram (DVH) test. Regarding calculation speed, MC-FDE has a very fast process and can generate an infinite number of particles, unlike the limited number of particles recorded in IAEA PhSp files. According to the findings of this study, the proposed MC-FDE model is considered suitable for use as part of clinical TPS.

Comparison and estimation of photoneutron dose produce between 10 MV flattened and unflattened beam in Elekta Versa HD™ medical linac.

In a high-energy medical linear accelerator (linac), if the interaction of photon energy is higher than the neutron binding energy of high atomic material, it emits a neutron field through photonuclear (γ, n) reaction. The current study, evaluates the photoneutron dose equivalent (PNDE) produced between the 10 MV flattened and unflattened beams as a function of field sizes in the Elekta Versa HD™ linac. The PNDE produced from Versa linac was recorded along the patient plane using the bubble detector personal neutron dosimeter and from the measured PNDE values, the theoretical PNDE values were simulated for various field sizes using nonlinear least-squares curve-fitting as a function of a polynomial. The percentage of deviation (PoD) and Chi-square (χ2) tests were performed between the measured and simulated PNDE values to study the reliability and validity. The results show that the mean PoD between the measured and simulated PNDE values for respective positions of a field size of FF beam was found to be -1.99% for 0.3×0.3, -4.39% for 5×5, -3.868% for 10×10, 0.590% for 15×15, 9.18% for 20×20, -4.133% for 25×25, and 0.467% for 30×30 cm2. Similarly, the mean PoD between the measured and simulated PNDE values for flattening filter-free (FFF) beam was found to be 1.36% for 0.3×0.3, -1.39% for 5×5, -5.38% for 10×10, 4.41% for 15×15, 3.84% for 20×20, 5.69% for 25×25, and -1.75% for 30×30 cm2. The maximum deviation between the measured and simulated PNDE values lies within the range ± 5%. From the study, it is observed that the FFF beam produces lesser neutron contamination than the FF beam.

Measurement of neutron yield for a medical linear accelerator below 10 MV.

The recent trend toward 10MV for volumetric radiotherapy treatment such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), and stereotactic ablative body radiotherapy (SABR) introduces photoneutron production, with implications for non-therapeutic patient dose and additional shielding requirements for treatment room design. The sharply nonlinear drop-off in photoneutron production below 10MV to negligible at 6MV has scarcely been characterized quantitatively, yet can elucidate important practical insights. To measure photoneutron yields in a medical linac at 8MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6MV while reducing photoneutron production which is present at 10MV. A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15MV by calibrating the bending magnet and adjusting other beam generation parameters. Neutron dose within the treatment room was measured using an Anderson-Braun type detector over a continuum of intermediate energies. The photoneutron production for energies below 10MV was measured, adding to data that is otherwise scarce in the literature. Our results are consistent with previously published results for neutron yield. We found that the photoneutron production at 8MV was about 1/10 of the value at 10MV, and about 10 times higher than detector background at 6MV. Photoneutron production drops off below 10MV, but is still present at 8MV. An 8MV beam is more penetrating than a 6MV beam, and may offer a suitable tradeoff for modern radiotherapy techniques such as VMAT, SRS, and SABR. Further studies are needed to better understand the impact on treatment plan quality between 8 and 10MV beams considering the benefits to facility requirements and non-therapeutic patient dose.

Implementation and validation of a beam-current transformer on a medical pulsed electron beam LINAC for FLASH-RT beam monitoring

Implementation and validation of a beam-current transformer on a medical pulsed electron beam LINAC for FLASH-RT beam monitoring

RF Design and Measurements of a C-Band Prototype Structure for an Ultra-High Dose-Rate Medical Linac

In this paper, we illustrate the RF design and measurements of a C-band prototype structure for an Ultra High Dose Rate medical linac. (1) Background: FLASH Radiotherapy (RT) is a revolutionary new technique for cancer cure. It releases ultra-high radiation dose rates (above 100 Gy/s) in microsecond short pulses. In order to obtain a high dose in a very short time, accelerators with high-intensity currents (the order of 100 mA peak currents) have to be developed. In this contest, Sapienza University, in collaboration with SIT-Sordina IORT Technology spa, is developing a new C-band linac to achieve the FLASH regime. (2) Methods: We performed the RF electromagnetic design of the prototype of the C band linac using CST STUDIO Suite Code and the RF low power RF test at Sapienza University of Rome. The measurements of the field in the cavity have been done with the bead-pull technique. (3) Results: This device is a nine-cell structure operating on the π/2 mode at 5.712 GHz (C-band). We report and discuss the test measurement results on a full-scale copper prototype, showing good agreement with CST RF simulations. A tuning procedure has been implemented in order to ensure proper operating frequency and to reach a field profile flatness of the order of a few percent. (4) Conclusions: The prototype of a C-band linac for FLASH applications was successfully tested with low RF power at Sapienza University. The fabrication and ad hoc tuning procedures have been optimized and discussed in the paper.

Investigation of Radiation Shielding in Shanghai Medical Linac Machine Room

Investigation of Radiation Shielding in Shanghai Medical Linac Machine Room

Activity Evaluation Methodology for the Disposed Medical Linear Accelerators

For the medical linear accelerators (linac) that utilize more than 10 MV of photon energy, components inside the linac head become radioactivate during the 10–15-year operating cycle. Prior to disposal, radioactive waste must be evaluated for activity, and the same procedure should be followed for medical linacs. In the Republic of Korea, regulation and methodology for the radioactivity evaluation for the medical linac is not established yet. In this study, we employed gamma spectroscopy and a survey meter for evaluating the radioactivity of medical linac components. The components of the Siemens linac considered in this study were classified after decommissioning, and dose rates were measured to up to a 5 cm distance from the component surfaces by using a survey meter. Radionuclides from components were detected using an in situ HPGe detector. Based on the type of radionuclides and dose rate, we estimated the radioactivity of the components. We studied the feasibility of the methodology for disposing of radioactive components by using the in situ HPGe detector.

Comparison of PRIMO Monte Carlo code and Eclipse treatment planning system in calculation of dosimetric parameters in brain cancer radiotherapy.

It is important to evaluate the dose calculated by treatment planning systems (TPSs) and dose distribution in tumor and organs at risk (OARs). The aim of this study is to compare dose calculated by the PRIMO Monte Carlo code and Eclipse TPS in radiotherapy of brain cancer patients. PRIMO simulation code was used to simulate a Varian Clinac 600C linac. The simulations were validated for the linac by comparison of the simulation and measured results. In the case of brain cancer patients, the dosimetric parameters obtained by the PRIMO code were compared with those calculated by Eclipse TPS. Gamma function analysis with 3%, 3 mm criteria was utilized to compare the dose distributions. The evaluations were based on the dosimetric parameters for the planning target volume (PTV) and OAR including D min, D mean, and D max, homogeneity index (HI), and conformity index (CI). The gamma function analysis showed a 98% agreement between the results obtained by the PRIMO code and measurement for the percent depth dose (PDD) and dose profiles. The corresponding value in comparing the dosimetric parameters from PRIMO code and Eclipse TPS for the brain patients was 94%, on average. The results of the PRIMO simulation were in good agreement with the measured data and Eclipse TPS calculations. Based on the results of this study, the PRIMO code can be utilized to simulate a medical linac with good accuracy and to evaluate the accuracy of treatment plans for patients with brain cancer.

Evaluation of the effectiveness of testicular shielding in rectal cancer radiotherapy

This study aimed to investigate the effect of local shielding on the reduction of the dose received by the male sexual organs in pelvic radiation therapy. For this purpose, a clinical case, and its corresponding simulation with the Monte Carlo Geant4 toolkit were studied. Here, the 15 MV photon mode of a medical LINAC was used to irradiate the pelvic region of three patients with rectal cancer. Using thermoluminescent dosimeter (TLD) 100, the doses received by the testes, seated out of the field, were measured in two situations, i.e., with and without a testicular shield. After benchmarking the simulation setup by comparing the simulated and experimental PDDs (Percentage Depth Dose) and lateral profiles in a water phantom, the simulation corresponding to the clinical case has been done. Afterward, the desired quantities such as absorbed dose, total equivalent dose, and neutron equivalent dose have been calculated. The results indicate that the local placement of a spherical lead shield leads to a decrease in the absorbed dose received by the testes with a coefficient greater than 2. It was also observed that shielding had a minimal contribution to the production of secondary neutrons and was not seen any sensible change in neutron equivalent dose, whereas a decrease in the total equivalent dose by a factor of 2.2 for the testes was observed. These observations imply that the risk of secondary cancers decreased proportionately, as well as a reduction in the risk of permanent infertility for young men.