Percentage Depth Dose Distributions Research Articles

On the gamma radiation response of commercially available 3D printing materials for medical dosimetry

3D printing technology has rapidly spread for decades, allowing the fabrication of medical implants and human phantoms and revolutionizing healthcare. The objective of this study is to evaluate some radiological properties of commercially available 3D printing materials as potential tissue mimicking materials. Among fifteen materials, we compared their properties with nine human tissues. In all materials and tissues, exposure and energy absorption buildup factors were calculated for photon energies between 0.015 and 15 MeV and penetration depths up to 40 mean free path. Furthermore, the Geant4 Monte Carlo toolkit (version 10.5) was used to simulate their percentage depth dose distributions. In addition, equivalent atomic numbers, effective atomic numbers, attenuation coefficients, and CT numbers have been examined. All parameters were considered in calculating the average relative error (σ), which was used as a statistical comparison tool. With σ between 6 and 7, we found that Polylactic Acid (PLA) was capable of simulating eye lenses, blood, soft tissue, lung, muscle, and brain tissues. Moreover, Polymethacrylic Acid (PMAA) material has a σ value of 4 when modeling adipose and breast tissues, respectively. Aside from that, variations in 3D printing materials’ infilling percentage can affect their CT numbers. We therefore suggest the PLA for mimicking soft tissue, muscle, brain, eye lens, lung and blood tissues, with an infill of between 92.7 and 94.3 percent. We also suggest an 89 percent infill when simulating breast tissue. Furthermore, with a 96.7 percent infill, the PMAA faithfully replicates adipose tissue. Additionally, we found that a 59 percent infill of Fe-PLA material is comparable to cortical bone. Due to the benefits of creating individualized medical phantoms and equipment, the results might be seen as an added value for both patients and clinicians.

Integrating X-ray kV millimetric field dosimetry with a synthetic diamond detector into the treatment planning system commissioning of a preclinical irradiator.

Small animal irradiators are equipped with x-ray beams and cone collimators with millimeter dimensions to be used in preclinical research. The use of small fields in the kV energy range may require the application of energy-dependent, field size-dependent, or depth-dependent correction factors to the dosimetric data acquired for treatment planning system (TPS) commissioning purposes to obtain accurate dose values. Considering that these corrections are also detector dependent, the suitability of a synthetic single-crystal diamond detector for small-field relative dosimetry in a preclinical irradiator (220-kVp) was evaluated to avoid the necessity of applying correction factors during TPS commissioning. The detector response was assessed during the transition for field sizes ranging from 20×20mm2 to 3×3mm2 , using the small animal radiation research platform (SARRP). The percentage depth dose distributions (PDDs), lateral profiles and output factors (OFs) were measured. The PDDs for the synthetic diamond detector were compared to the distributions acquired using a small-volume microchamber (0.016cm3 ) and with Monte Carlo calculations using the MC3D in-house software package. The profiles and OFs were compared to the data from a silicon solid-state detector and to radiochromic film data provided by the manufacturer; for the OF determination, measurements made using a microchamber were added for comparison. The performance of several detectors used as references was previously validated for relative dosimetry in preclinical irradiators. A commercial TPS was commissioned for the factor-based algorithm, using the data acquired with the diamond detector, and no additional correction factors were applied. To verify the performance of the TPS and the accuracy of the dosimetric methodology, radiochromic film irradiation in water was conducted, and two-dimensional (2D) dose distributions in the coronal and axial planes were compared under different gamma criteria. Compared with the microchamber and MC3D distributions, the agreement of the PDDs using the synthetic diamond detector was better than 2%. The profile data exhibited very good agreement compared with the data from the silicon detector, with an average and a maximum difference of 0.31 and 0.39mm in the penumbras, respectively. Compared with the data from the radiochromic film, the average and maximum differences were equal to 0.77 and 0.89mm, respectively. Very good agreement, within 1%, was obtained between the OFs measured with the synthetic diamond detector and the radiochromic film, compared only for the cone collimators. The validation of the TPS commissioning using gamma criteria compared to film showed an average passing rate of 100% and 93.2% with a global gamma criterion of 1mm/3% for the coronal and axial planes, respectively, including the 3×3mm2 field size and penumbra regions. Synthetic diamond is a suitable detector for the complete relative dosimetry of small x-ray fields. The commissioning of the TPS with its own beam dosimetric data exhibited encouraging results even in a 3×3mm2 field and penumbra region. This methodology allows for the prediction of 2D dose distributions with an accuracy in water ranging from 3 to 5% compared to the 2D distribution from film dosimetry.

Dosimetric characterization of a small-scale (Zn,Cd)S:Ag inorganic scintillating detector to be used in radiotherapy.

In modern radiotherapy techniques, to ensure an accurate beam modeling process, dosimeters with high accuracy and spatial resolution are required. Therefore, this work aims to propose a simple, robust, and a small-scale fiber-integrated X-ray inorganic detector and investigate the dosimetric characteristics used in radiotherapy. The detector is based on red-emitting silver-activated zinc-cadmium sulfide (Zn,Cd)S:Ag nanoclusters and the proposed system has been tested under 6 MV photons with standard dose rate used in the patient treatment protocol. The article presents the performances of the detector in terms of dose linearity, repeatability, reproducibility, percentage depth dose distribution, and field output factor. A comparative study is shown using a microdiamond dosimeter and considering data from recent literature. We accurately measured a small field beam profile of 0.5×0.5cm2 at a spatial resolution of 100µm using a LINAC system. The dose linearity at 400 MU/min has shown less than 0.53% and 1.10% deviations from perfect linearity for the regular and smallest field. Percentage depth dose measurement agrees with microdiamond measurements within 1.30% and 2.94%, respectively for regular to small field beams. Besides, the stem effect analysis shows a negligible contribution in the measurements for fields smaller than 3x3 cm2. This study highlights the drastic decrease of the convolution effect using a point-like detector, especially in small dimension beam characterization. Field output factor has shown a good agreement while comparing it with the microdiamond dosimeter. All the results presented here anticipated that the developed detector can accurately measure delivered dose to the region of interest, claim accurate depth dose distribution hence it can be a suitable candidate for beam characterization and quality assurance of LINAC system.

In-line MRI-LINAC depth dose measurements using an in-house plastic scintillation dosimeter

Plastic scintillation dosimeters (PSDs) have many properties that make them desirable for relative dosimetry with MRI-LINACs. An in-house PSD, Farmer ionisation chamber and Gafchromic EBT3 film were used to measure central axis percentage depth dose distributions (PDDs) at the Australian MRI-LINAC Mean errors were calculated between each detector’s responses, where the in-house PSD was on average within 0.7% of the Farmer chamber and 1.4% of film, while the Farmer chamber and film were on average within 1.1% of each other. However, the PSD systematically over-estimated the dose as depth increased, approaching a maximum overestimation of the order of 3.5% for the smallest field size measured. This trend was statistically insignificant for all other field sizes measured; further investigation is required to determine the source of this effect. The calculated values of mean absolute error are comparable to the those of trusted dosimeters reported in the literature. These mean absolute errors, and the ubiquity of desirable dosimetric qualities inherent to PSDs suggest that PSDs in general are accurate for relative dosimetry with the MRI-LINAC. Further investigation is required into the source of the reported systematic trends dependent on field-size and depth of measurement.

Analysis of central axis and build up region dose calculated by different dose calculation algorithms used for radiation treatment planning

Introduction: Computerized Treatment planning system (TPS) is an integral part of radiation treatment procedure. The main part of treatment planning system is a calculation algorithm incorporated in the Treatment Planning System. Various algorithms used in the TPS have capabilities and limitation on specific situations. Objective: The objective of the work is to analyse the depth dose distribution generated by different dose calculation algorithms used in TPS in regions of low and high dose gradient. Materials and Methods: Four different dose calculation algorithms used for treatment planning such as Modified Clarkson, Sector integration, Fast Fourier Transform, Convolution- Superposition algorithm were used to generate the percentage depth dose distribution in a water phantom up to 25 cm depth. The values are compared with experimental results using Radiation Field Analyser. The PDD values also generated at build up region and the values were compared with experimental results using a Parallel plate ionization chamber. Results: The results show that the dose calculated by different algorithms shows difference at build up region and at larger depths.

A new proposal of utilizing intraoperative electron radiation therapy on the surface of liver to prevent postoperative liver metastasis of pancreatic cancer

Pancreatic cancer is a lethal cancer with high rate of liver metastasis worldwide, whereas its treatment choices are limited to a large extent. The limitation of current therapeutic strategies calls for an effective approach which can lower the postoperative liver metastasis rate in order to improve the overall prognosis and survival rate. Comprehensively considering the basic knowledge and clinical practice of tumor treatment worldwide, we proposed three points of hypotheses. Basically, the existing evidences indicated that tumor cells shedding from pancreatic cancer localized in the marginal liver preferentially through the Portal vein. Then, the percentage depth dose distribution of electron radiation is consistent with the marginal distribution of liver metastasis from pancreatic cancer. Based on the characteristics of liver metastasis of pancreatic cancer and the percentage depth dose of electron radiation, we provide a new propose of preventing postoperative liver metastasis in a way of prophylactic intraoperative electron radiation therapy on the surface of liver. Intraoperative electron radiation is relatively easy to control radiation dose and treatment area under direct vision, effectively inhibiting the metastasis and growth of cancer cells and preventing further deterioration of pancreatic cancer patients’ condition. Therefore, this hypothesis has an important clinical significance for postoperative rehabilitation and improvement of patients’ survival.

Precise EBT3 Gafchromic film dosimetry for Grid therapy

ObjectivesGrid therapy is a radiation therapy technique for the treatment of bulky tumours. The high dose gradients and non-uniformity of dose distributions within the target lead to a challenge in the dosimetry of the Grid radiation fields. The aim of this study is to perform a precise EBT3 Gafchromic film dosimetry for Grid therapy fields using a commercially available flatbed colour scanner. MethodsIn this project, samples of the EBT3 Gafchromic films are exposed to Grid radiation fields. The irradiated EBT3 films were read using a flatbed Microtek scanner. The responses of these films (i.e. films from the same batch) as functions of the absorbed dose values are calibrated by irradiation under a fixed standard technique (i.e. 10 × 10 cm2 filed, 100 cm SSD, and depth of the maximum dose). These films are also read with the same scanner using the red, green, and blue channels. Four different approaches were used to evaluate film dosimetry for the Grid therapy applications: 1) single channel film dosimetry method (SCM), 2) dual channel film dosimetry method (DCM), 3) linearized dose-response curve method (LRCM), and 4) triple channel film dosimetry method (TCM). A dose of 20 Gy was delivered to the point along the central axis of the grid hole at the depth of maximum dose (dmax) for a 20 × 20 cm2 Grid field size. Beam profiles and percentage depth dose distributions of the Grid radiation have been measured in water-equivalent phantom material, using EBT3 films. The accuracy of the relative and the absolute dosimetry of the films were examined by comparison of the TLD measured data with the Monte Carlo simulated values. ResultsThe results of these investigations show that for a gamma index criterion of 5%/3 mm, the agreements between the MC calculations dose profiles and the SCM, DCM, LRCM, and TCM film dosimetry approaches the passing rates of 91%, 92%, 95%, and 95%, respectively. A much closer agreement was observed for using a linearized dose-response curve and triple-channel methods. ConclusionsSelection of an appropriate methodology in Gafchromic film dosimetry may lead to an accurate dose-response in a high dose gradient radiation field such as Grid therapy.

A comparative study of measurement of small field data and calculation based on Monte Carlo method

Objective To compare the relative dose of small fields measured by clinically common detectors and those obtained from Monte Carlo simulation in order to obtain the accurate measurement method, and to modify the inappropriate detectors. Methods The percentage depth-dose distribution curve and profile (flatness and symmetry) curves were collected at 2 cm×2 cm, 3 cm×3 cm, and 4 cm×4 cm under 6-MV X-ray of Trilogy linear accelerator by CC13, PFD, SFD, and blue phantom. The results were compared with the stimulation results from Monte Carlo method (the current gold standard). The correction factors for the detectors with large error were calculated to provide reference data for clinical practice. Results The results measured by SFD detector were most close to the results from Monte Carlo simulation. The measurement errors of CC13 and PFD detectors were large. The correction factor in the penumbra for CC13 and PFD detector was 0.664-1.499. Conclusions SFD detector is better than CC13 and PFD detector in the measurement of small fields, but CC13 and PFD detector can provide reference data for clinical practice after the corresponding correction. Key words: Small field; Detectors; Monte Carlo

Experimental verification of Acuros XB in the presence of lung-equivalent heterogeneities

Acuros XB (AXB) is a deterministic algorithm that directly accounts for the effects of heterogeneities and improves the accuracy of photon dose calculations in radiotherapy. Nevertheless, the accuracy of AXB inside lung has not been yet fully examined by means of experimental measurements. The aim of this study was to evaluate the accuracy of AXB in the presence of lung using 7Li-based thermoluminescent dosimeters (TLD-700). Percentage depth-dose distributions (PDD) obtained by AXB on a slab water-equivalent phantom with a lung-equivalent heterogeneity were compared with the TLD-700 measurements for 6 MV and 18 MV photon beams and a set of field sizes ranging from 2×2cm2to20×20cm2. Dose distributions obtained by the Anisotropic Analytical Algorithm (AAA) were also included in the comparison as a reference to a non-deterministic dose calculation algorithm. The agreement between AXB and the TLD results was kept within clinical tolerance levels (3%) for all beam configurations. On the contrary, AAA failed at correctly predicting the absorbed dose when the lateral electronic equilibrium was lost. AXB is capable of providing reliable dose estimations in the presence of lung and clearly outperforms AAA.

Percentage Depth Dose Comparative Study of 6 MV Photon Beam of Both Linear Accelerators Varian Clinac 2100 and Varian Clinac 2300 Using Gamma Index Method and Investigation of the Varian Technology

This work presents a comparative study of percentage depth dose (PDD), and especially the build-up region, on 6 MV photon beam in water phantom for Varian linear accelerators: Varian Clinac 2100 and Varian Clinac 2300. The build-up region depends on the photon beam energy, the field size, the incidence angle of the beam, the air gap, and the beam modifier devices. In our study, we have investigated the Varian technology evolution and we have clarified how Varian had optimized the delivered dose in its recent medical linear accelerator technology. The PDDs were compared for both Varian medical linear accelerators; we have use the relative dose parameter and the gamma index for this comparison study. The measurements were carried out for a field size of 10×10 cm 2 , for both Varian linear accelerators, it is the reference field size for this study, the photon beam energy of 6 MV and source-to-surface distance (SSD) is the 100 cm. The relative dose was less than 1 and gamma index criterion acceptance was 100% for gamma index < 1,5 beyond the depth of dose maximum(D max ), and the PDD difference was in the build-up region and especially in the shallower depth beneath the surface dose, this difference due to the beam modifier enhancement and the technological development . The recent Varian technology was more developed in the build-up region due to amelioration in the photon beam modifiers, and Varian Clinac 2300 is more recent than Varian Clinac 2100, thus the technology has the focus in the photon beam modifiers. Our work contributes in dosimetry study of Varian linear accelerator and to make an investigation of Varian technology. The aim of this work was to investigate the Varian technology via a comparative study of PDD distributions, and especially the build-up region, for accelerators Varian Clinac 2300 and Varian Clinac 2100, and to highlight the features of this technological evolution from Varian Clinac 2100 to Varian Clinac 2300.

Benchmarking Monte Carlo simulations against experimental data in clinically relevant passive scattering proton therapy beamline configurations

Monte Carlo simulations are widely used to calculate secondary neutron doses in proton therapy. When using the passive scattering technique, a small variation in the tumor size or location requires a full adjustment of the beamline configuration. This work thus focuses on modeling all possible beamline elements of the local facility with the MCNPX code while covering the entire clinical range of proton beam energies. This paper documents the experimental validation of realistic beamline configurations dedicated to intracranial tumor treatments, with different sources and beamline elements. Simulations were compared against measurements and treatment planning system (TPS) data considering percentage depth dose distributions (PDD) and relative lateral dose profiles. The results show that simulated and measured PDD distributions differ by no more than 1.6 mm on the proton beam range (tolerance set at 2 mm/2%) for the 162 MeV configuration. Meanwhile, for the modulated lateral dose profiles, the 219 MeV configuration presented the largest difference for the in-plane field width at 90% with 2 mm difference in measurements. The modeled proton sources and beamline elements were validated, and the new realistic geometries can be used to calculate secondary neutron doses to healthy tissues.

Quality assurance of proton beams using a multilayer ionization chamber system

The measurement of percentage depth-dose (PDD) distributions for the quality assurance of clinical proton beams is most commonly performed with a computerized water tank dosimetry system with ionization chamber, commonly referred to as water tank. Although the accuracy and reproducibility of this method is well established, it can be time-consuming if a large number of measurements are required. In this work the authors evaluate the linearity, reproducibility, sensitivity to field size, accuracy, and time-savings of another system: the Zebra, a multilayer ionization chamber system. The Zebra, consisting of 180 parallel-plate ionization chambers with 2 mm resolution, was used to measure depth-dose distributions. The measurements were performed for scattered and scanned proton pencil beams of multiple energies delivered by the Hitachi PROBEAT synchrotron-based delivery system. For scattered beams, the Zebra-measured depth-dose distributions were compared with those measured with the water tank. The principal descriptors extracted for comparisons were: range, the depth of the distal 90% dose; spread-out Bragg peak (SOBP) length, the region between the proximal 95% and distal 90% dose; and distal-dose fall off (DDF), the region between the distal 80% and 20% dose. For scanned beams, the Zebra-measured ranges were compared with those acquired using a Bragg peak chamber during commissioning. The Zebra demonstrated better than 1% reproducibility and monitor unit linearity. The response of the Zebra was found to be sensitive to radiation field sizes greater than 12.5 × 12.5 cm; hence, the measurements used to determine accuracy were performed using a field size of 10 × 10 cm. For the scattered proton beams, PDD distributions showed 1.5% agreement within the SOBP, and 3.8% outside. Range values agreed within -0.1 ± 0.4 mm, with a maximum deviation of 1.2 mm. SOBP length values agreed within 0 ± 2 mm, with a maximum deviation of 6 mm. DDF values agreed within 0.3 ± 0.1 mm, with a maximum deviation of 0.6 mm. For the scanned proton pencil beams, Zebra and Bragg peak chamber range values demonstrated agreement of 0.0 ± 0.3 mm with a maximum deviation of 1.3 mm. The setup and measurement time for all Zebra measurements was 3 and 20 times less, respectively, compared to the water tank measurements. Our investigation shows that the Zebra can be useful not only for fast but also for accurate measurements of the depth-dose distributions of both scattered and scanned proton beams. The analysis of a large set of measurements shows that the commonly assessed beam quality parameters obtained with the Zebra are within the acceptable variations specified by the manufacturer for our delivery system.

Poster — Wed Eve—18: Investigating Responses of Three Commercial p‐Type Silicon Diodes on Percentage Depth Dose (PDD) Distributions of Megavoltage LINAC Photon Beams

Percentage depth dose (PDD) distributions measured with three commercial p‐type silicon diode detectors in a water phantom show significant discrepancies from ionization‐chamber‐measured data for megavoltage (4‐ and 18‐MV) photon beams for large square field sizes ranging from 10×10 to . For a given large photon field, PDD data measured with a standard and energy‐shielded photon‐field diode (PFD) shows the least disagreement with the reference data, while the PDD data measured with a small and unshielded stereotactic‐field diode (SFD) shows the largest. The over‐responses of all diodes shown in the measured PDD distributions for the large photon square fields as well as the systematic errors observed among the three different diode detector types suggest: (1) the photon beam softening on the beam's central axis increases with field sizes and depths in water, and (2) the presence of energy‐shielding mixture (for filtering out low‐energy, backscattered photons in reaching the silicon chip of the diode) as well as the lateral dimension of the diode's sensitive area may be critical factors affecting the accuracy of the PDDs measured with the diodes for these fields. Monte Carlo simulations will be used to investigate these potential effects and correct for them when confirmed.

Influence of focal spot on characteristics of very small diameter radiosurgical beams

Percentage depth dose (PDD) distributions and beam profiles of very small diameter (1.5-5 mm) megavoltage radiosurgical beams calculated with Monte Carlo (MC) technique critically depend on the diameter of the circular focal spot used in the simulation: The smaller is the field diameter, the larger is the effect. Thus, in simulations of radiosurgical fields that have diameters of the order of the focal spot size, an accurate focal spot geometry should be used. We used a simplified moving slit technique in conjunction with a diode detector for evaluation of the focal spot size and shape of a megavoltage 6 MV linac as well as for determination of the equivalent focal spot diameter of the linac for use in MC simulations. The measured total diode signal contains three components: A direct focal spot signal, a background signal, and an extra-focal radiation signal. A single profile scan of the focal spot signal is Gaussian like in shape, and its full width at half maximum is used to define the focal spot dimension for this scan. The focal spot of our 6 MV linac is approximated with a Gaussian circle, and when the geometry of the effective focal spot circle is used in MC simulations, the agreement between MC-calculated and measured PDD distributions as well as beam profiles is good even for radiosurgical fields as small as 1.5 mm in diameter. Our results also confirm that matching the penumbral areas of accurately measured large-field beam profiles to the same areas of the MC-calculated beam profiles reliably leads to a realistic effective focal spot size for use in MC simulations of very small diameter beams.

SU-FF-T-284: Characterising An Electron Pencil Beam: Monte Carlo vs Measurement

Purpose: Electron beams from linear accelerators have widely varying characteristics in terms of spectral properties, mixtures of electrons and photons, spatial and angular characteristics. Characterizing pencil beams for pencil-beam based calculations and for complex electron optimization calculations is thus necessary for individual electron beams. We evaluated an extrapolation method for characterizing an electron pencil beam and compared a Monte Carlo based method with measurement. Method and Materials: Radiographic film was used to measure narrow 12 MeV electron beams from a Varian 21EX accelerator. A series of circular beams was considered, collimated with LMPA material between diameters of 1.5 and 32 millimeters. Beam profiles were measured at discrete depths, and the presence of photon contamination from the LMPA was accounted for by subtracting measurements for a solid piece of LMPA. From these measurements, an extrapolation was made to estimate the distribution for a 0.1 mm diameter beam. A similar process was carried out using EGSnrc Monte Carlo, with electron beam characteristics determined using BEAM. Results: A large amount of similarity was found between pencil beam distributions extrapolated from film-measured and Monte Carlo derived data. Given the amount of noise present in the measured and simulated results, the curve-fitting procedure yielded relatively smooth and appropriate pencil beam distributions. It was also found that the parameters from the curve-fitting could be used to extract central axis percentage depth dose distributions which were consistent (past dmax) with measured broad beam data. Conclusion: This study showed that the extrapolation method is appropriate for derivation of dose distributions for very narrow beams. A Monte Carlo simulated narrow beam distribution would normally be sufficient for characterizing such a distribution, though if high resolution is required, the extrapolation method would be useful.

The effect of the nasopharyngeal air cavity on x-ray interface doses

We investigated the impact of air cavities in head and neck cancer patients treated by photon beams based on clinical set-ups. The phantom for investigation was constructed with a cubic air cavity of located at the centre of a solid water slab. The cavity cube was used to resemble an extreme case for the nasal cavity. Apart from measuring the dose profiles and central axis percentage depth dose distribution, the dose values in voxels at regions around the air cavity were obtained by Monte Carlo simulations. A mean dose value was taken over the voxels of interest at each depth for evaluation. Single-field results were added to study parallel opposed field effects. For parallel opposed fields at 4, 6 and 8 MV, the mean dose at regions near the lateral interfaces of the cavity cube were decreased by 1 to 2% due to the lack of lateral scatter, while the mean dose near the proximal and distal interfaces was increased by 2 to 4% due to the greater transmission through air. Secondary build-up effects at points immediately beyond the air cavity cube are negligible using field sizes greater than . For most head and neck treatment, the field sizes are usually or greater, and most cavity volumes are smaller than our chosen dimensions. Therefore, the influence of closed air cavities on photon interface doses is not significant in clinical treatment set-ups.

Calculation of x-ray spectra for radiosurgical beams.

As conformal radiosurgery using micromultileaf collimators gains feasibility, dose calculation algorithms based on Monte Carlo or convolution techniques may become necessary. These require radiosurgical x-ray spectra. The most accurate method currently available to estimate clinical radiosurgery spectra is the Monte Carlo method. In this study the EGS4 Monte Carlo system was used to simulate the thick target of a 6 MV linear accelerator used for radiosurgery in our center. The calculated spectrum was attenuated through any significant mass thickness of material downstream from the target. The attenuated thick-target spectral distributions calculated both with and without the flattening filter were compared to the attenuated, thin target spectrum based on the small angle Schiff analytical spectrum calculated for the same target and attenuator material, as well as with a published spectrum from a full Monte Carlo simulation of a treatment head with a flattener in place. The Schiff spectrum neglects contributions from lower-energy scattered electrons that significantly degrade the quality of the beam. The flattener is removed from our accelerator during radiosurgery to increase the dose rate to approximately 750 cGy/min for a 10 x 10 cm2 field at the depth of dose maximum. This leaves a substantial fluence of photons below 1 MeV that are not observed in published spectra calculated for accelerators with flattening filters. Removal of the flattening filter has a measurable effect on the central axis depth dose, reducing the percentage dose at 10 cm depth from 59.2% to 54.3% for a 10 mm diam field. Radiosurgical off-axis ratios and percentage depth dose distributions calculated from these spectra with the EGS4 Monte Carlo code were compared to measured data. Measured and calculated dose distributions both with and without flattener were in good agreement. The dose distributions were found to be insensitive to the differences in the various calculated spectral distributions. Thus, although the attenuated Schiff spectrum is significantly harder than the clinical beam, it is adequate for dose calculations of radiosurgical beams.

The influence of phantom size on output, peak scatter factor, and percentage depth dose in large-field photon irradiation.

Machine outputs, peak scatter factors, and central axis percentage depth dose distributions were measured for various phantom sizes in large radiation fields produced at extended distances by cobalt, 6-MV, and 10-MV photon beams. The results can be applied to practical total body irradiation procedures which usually involve treatment volumes smaller than the actual field sizes in order to provide a uniform total body exposure to radiation. Our study addresses the question of the appropriate phantom dimension to be used in the calibration of photon beams employed in total body irradiations. The measurements show that the machine outputs are only slightly dependent on phantom size; the percentage depth dose distributions, however, are strongly dependent on the phantom size, suggesting that machine data for total body irradiations should be measured in phantoms whose dimensions approximate the patient during the total body irradiation. Peak scatter factors measured in large-field/small-phantom configurations link up well with the published small-field/large-phantom data. The finite patient thickness lowers the dose to points close to the beam exit surface by a few percent, when compared to dose measured at the same depths in infinitely thick phantoms. The surface doses in large radiation fields are essentially independent of phantom cross sections and range from 40% for the 10-MV beam, to 65% for the 6-MV beam and 80% for the cobalt beam.

Physical aspects of a rotational total skin electron irradiation.

A technique for rotational total skin electron irradiation is presented in which the patient stands on a slowly rotating platform (SSD = 285 cm) in a large uniform linear accelerator electron field (Eo = 3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special lead/aluminum scattering filter, and then degraded by a sheet of Lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within +/- 5% for dimensions of 180 X 40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an x-ray contamination of approximately 4%. The surface dose rate is about 25 cGy/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tissue, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an x-ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple-beam techniques.

Depth-dose distribution for fission neutron spectra degraded through low Z shields

Distributions of dose, dose equivalent and quality factor in depth of a cylindrical phantom for neutron spectra moderated by concrete, polyethylene, water and beryllium are presented. These include fission neutron spectra transmitted through 10, 20, 60 and 100 cm concrete; 5, 10, 20 and 40 cm polyethylene; 10 and 40 cm water; and 2.5, 5, 10 and 20 cm thick Be shields. Although average dose per neutron at 1 cm depth depends on the average energy of the neutron spectrum, percentage depth dose distributions for most of the spectra are almost similar. The spectra degraded by 40 cm water and 40 cm PE stand out from others in giving higher percentage depth doses, also the effective quality factor ( QF ) values remain nearly constant with depth for these spectra. QF values (1 cm depth) range from 7.6 to 8.7 for the spectra studied. They decrease with depth up to about 15 cm and then remain almost constant. Maximum reduction in their values is obtained for a highly degraded spectrum through a 20 cm thick Be shield.