Dosimetric Characteristics of the Siemens IGRT Carbon Fiber Tabletop

In this work, the dosimetric characteristics of a new commercial carbon fiber treatment table are investigated. The photon beam attenuation properties of the Siemens image-guided radiation therapy (IGRT) tabletop were studied in detail. Two sets of dosimetric measurements were performed. In the first experiment a polystyrene slab phantom was used: the central axis attenuation and the skin-sparing detriment were investigated. In the second experiment, the off-axis treatment table transmission was investigated using a polystyrene cylindrical phantom. Measurements were taken at the isocenter for a 360° rotation of the radiation beam. Our results show that the photon beam attenuation of the Siemens IGRT carbon fiber tabletop varies from a minimum of 2.1% (central axis) to a maximum of 4.6% (120° and 240° beam incidence). The beam entrance dose increases from 82% to 97% of the dose at the depth of maximum for a clinical 6-MV radiation field. The depth of maximum also decreases by 0.4 cm. Despite the wedge cross section of the table the beam attenuation properties of the IGRT tabletop remain constant along the longitudinal direction. American Association of Medical Dosimetrists.     

Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

Purpose  The dose changes in the buildup region and beam attenuation by a carbon fiber tabletop were investigated for 6-and 18-MV photon beams. Materials and methods  Measurements were performed for 2 × 2 cm to 40 × 40 cm field sizes. The surface dose and percentage depth doses (PDD) were measured by a Markus parallel plate chamber. Attenuation measurements were made at the cylindrical phantom for 180° rotation of the beam. Results  A carbon fiber tabletop increases the surface dose from 7.5% to 63.0% and from 4% to 43% for small fields at 6 and 18 MV, respectively. The increase was nearly fivefold for the 10 × 10 cm field and nearly twofold for the 40 × 40 cm field. Beam attenuation of the tabletop varies from 3.0% to 5.6% for 180° and 120° gantry angles for 6 MV. Conclusion  The carbon fiber tabletop significantly decreases the skin-sparing effect. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage.     

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

PURPOSE: To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. MATERIAL AND METHODS: Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. RESULTS: Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). CONCLUSION: Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.     

The effect of titanium stabilization rods on spinal cord radiation dose.

The purpose of this study was to investigate the dosimetric effect of a titanium-rod spinal stabilization system on surrounding tissue, especially the spinal cord. Ion chamber dosimetry was performed for 6- and 18-MV photon beams in a water phantom containing a titanium-rod spinal stabilization system. Isodose curves were obtained in the phantom with and without rods. To assess the ability of a treatment planning system to reproduce the effects of the stabilization system on the radiation dose delivered to surrounding tissue, dose distributions were calculated after appropriate modifications were made in the computed tomography number-to-density conversion table to account for the increased density of the titanium rods. The resultant heterogeneity-corrected plans were compared with uncorrected plans. At a 7-cm depth in the water phantom, corresponding to the depth of the spinal cord, the beam was attenuated by 4% under the rods alone and by 13% rods under the rods with screws for the 6-MV photon beam as compared with curves generated in the absence of rods. The beam was attenuated by 3% and 11%, respectively, for the 18-MV beam. Using anteroposterior (18-MV) and posteroanterior (6-MV) photon beams, with and without heterogeneity correction for the rods, the corrected isodose plan showed an approximately 2% beam attenuation 4 cm anterior to the rods as compared with the uncorrected plan. No significant difference in the spinal cord dose was observed between the 2 plans, however. The titanium-rod spinal stabilization system tested in this study caused a decrease in the dose delivered distal to the rods but did not significantly affect the dose delivered to the spinal cord.     

Accuracy of numerically produced compensators.

A feasibility study is performed to assess the utility of a computer numerically controlled (CNC) mill to produce compensating filters for conventional clinical use and for the delivery of intensity-modulated beams. A computer aided machining (CAM) software is used to assist in the design and construction of such filters. Geometric measurements of stepped and wedged surfaces are made to examine the accuracy of surface milling. Molds are milled and filled with molten alloy to produce filters, and both the molds and filters are examined for consistency and accuracy. Results show that the deviation of the filter surfaces from design does not exceed 1.5%. The effective attenuation coefficient is measured for CadFree, a cadmium-free alloy, in a 6 MV photon beam. The effective attenuation coefficients at the depth of maximum dose (1.5 cm) and at 10 cm in solid water phantom are found to be 0.546 cm-1 and 0.522 cm-1, respectively. Further attenuation measurements are made with Cerrobend to assess the variations of the effective attenuation coefficient with field size and source-surface distance. The ability of the CNC mill to accurately produce surfaces is verified with dose profile measurements in a 6 MV photon beam. The test phantom is composed of a 10 degrees polystyrene wedge and a 30 degrees polystyrene wedge, presenting both a sharp discontinuity and sloped surfaces. Dose profiles, measured at the depth of compensation (10 cm) beneath the test phantom and beneath a flat phantom, are compared to those produced by a commercial treatment planning system. Agreement between measured and predicted profiles is within 2%, indicating the viability of the system for filter production.     

Electron depth absorbed doses for small phantom depths. Comparison between different accelerators.

Depth dose distributions at small phantom depths in polystyrene were measured with a liquid ionization chamber at six different accelerators. The absorbed dose at 0.5 mm depth relative to the absorbed dose maximum in the central beam varied between 0.77 and 0.92 for 10 MeV electrons. Generally the surface absorbed dose increased with increasing energy or decreasing field size. Results from measurements off axis indicate small differences from the data available for the central beam.     

Dependences of mucosal dose on photon beams in head-and-neck intensity-modulated radiation therapy: a Monte Carlo study

Dependences of mucosal dose in the oral or nasal cavity on the beam energy, beam angle, multibeam configuration, and mucosal thickness were studied for small photon fields using Monte Carlo simulations (EGSnrc-based code), which were validated by measurements. Cylindrical mucosa phantoms (mucosal thickness = 1, 2, and 3 mm) with and without the bone and air inhomogeneities were irradiated by the 6- and 18-MV photon beams (field size = 1 × 1 cm²) with gantry angles equal to 0°, 90°, and 180°, and multibeam configurations using 2, 4, and 8 photon beams in different orientations around the phantom. Doses along the central beam axis in the mucosal tissue were calculated. The mucosal surface doses were found to decrease slightly (1% for the 6-MV photon beam and 3% for the 18-MV beam) with an increase of mucosal thickness from 1–3 mm, when the beam angle is 0°. The variation of mucosal surface dose with its thickness became insignificant when the beam angle was changed to 180°, but the dose at the bone-mucosa interface was found to increase (28% for the 6-MV photon beam and 20% for the 18-MV beam) with the mucosal thickness. For different multibeam configurations, the dependence of mucosal dose on its thickness became insignificant when the number of photon beams around the mucosal tissue was increased. The mucosal dose with bone was varied with the beam energy, beam angle, multibeam configuration and mucosal thickness for a small segmental photon field. These dosimetric variations are important to consider improving the treatment strategy, so the mucosal complications in head-and-neck intensity-modulated radiation therapy can be minimized.     

ElektaPrecise直线加速器新型全碳素纤维治疗床床板对放疗剂量的影响

目的 探讨Elekta Precise直线加速器新型全碳素纤维治疗床床板对放疗剂量的影响.方法 应用等中心技术( SAD)测量,设2组射野,均为等中心对穿野(一组穿过床板,另一组不穿过床板),用PTW剂量仪0.6 cm3 Farmer电离室比对测量,计算出治疗床主床板、体部延长板、头颈肩延长板不同位置及其衔接处对放疗剂量的衰减.结果 6 MV能量时,治疗床主床板对剂量的衰减为1.4％ ～ 7.2％,主床板衔接头1、4、7和8 cm处对剂量的衰减分别为2.8％～38.7％、1.4％～30.1％、1.5％ ～20.8％和1.4％ ～11.2％;体部延长板对剂量的衰减为0.5％ ～5.0％,体部延长板衔接头1cm位置对剂量的衰减为4.7％ ～15.4％;头颈肩延长板颈部位置对剂量的衰减在0.5％～3.3％,头颈肩延长板肩部位置对剂量的衰减在5.3％～l6.7％;延长板和主床板衔接处对剂量的衰减在6.8％～30.4％.结论 新型全碳素纤维治疗床床板不同部位对剂量衰减不同,床板衔接部位标记区域对剂量衰减比较大.     

BNCT dose calculation in irregular fields using the sector integration method.

Irregular fields for boron neutron capture therapy (BNCT) have been already proposed to spare normal tissue in the treatment of superficial tumors. This added dependence would require custom measurements and/or to have a secondary calculation system. As a first step, we implemented the sector-integration method for irregular field calculation in a homogeneous medium and on the central beam axis. The dosimetric responses (fast neutron and photon dose and thermal neutron flux), are calculated by sector integrating the measured responses of circular fields over the field boundary. The measurements were carried out at our BNCT facility, the RA-6 reactor (Argentina). The input data were dosimetric responses for circular fields measured at different depths in a water phantom using ionisation and activation techniques. Circular fields were formed by shielding the beam with two plates: borated polyethilene plus lead. As a test, the dosimetric responses of a 7x4 cm(2) rectangular field, were measured and compared to calculations, yielding differences less than 3% in equivalent dose at any depth indicating that the tool is suitable for redundant calculations.     

Dosimetric characteristics of unflattened 6 MV photon beams of a clinical linear accelerator: a Monte Carlo study.

The purpose of this study is to evaluate the dosimetric properties of a flattening filter free 6 MV photon beam. The 6 MV photon beam of a Varian Clinac 21 EX linac was modeled using the MCNP4C Monte Carlo (MC) code. Dosimetric features including central axis absorbed doses, beam profiles and photon energy spectra were calculated for flattened and unflattened 6 MV photon beams. A substantial increase in the dose rate was seen for the unflattened beam, which was decreased with field size and depth. The penumbra width was decreased less than 0.2 mm (about 5%) and a 25% decrease in out-of-field dose was observed for the unflattened beam. The photon energy spectra were softer for the unflattened beam and the mean energies of spectra were higher for smaller field size. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV beams.     

Leakage-Penumbra effect in intensity modulated radiation therapy step-and-shoot dose delivery

AIM: To study the leakage-penumbra (LP) effect with a proposed correction method for the step-and-shoot intensity modulated radiation therapy (IMRT). METHODS: Leakage-penumbra dose profiles from 10 randomly selected prostate IMRT plans were studied. The IMRT plans were delivered by a Varian 21 EX linear accelerator equipped with a 120-leaf multileaf collimator (MLC). For each treatment plan created by the Pinnacle³ treatment planning system, a 3-dimensional LP dose distribution generated by 5 coplanar photon beams, starting from 0^o with equal separation of 72^o, was investigated. For each photon beam used in the step-and-shoot IMRT plans, the first beam segment was set to have the largest area in the MLC leaf-sequencing, and was equal to the planning target volume (PTV). The overshoot effect (OSE) and the segment positional errors were measured using a solid water phantom with Kodak (TL and X-OMAT V) radiographic films. Film dosimetric analysis and calibration were carried out using a film scanner (Vidar VXR-16). The LP dose profiles were determined by eliminating the OSE and segment positional errors with specific individual irradiations. RESULTS: A non-uniformly distributed leaf LP dose ranging from 3% to 5% of the beam dose was measured in clinical IMRT beams. An overdose at the gap between neighboring segments, represented as dose peaks of up to 10% of the total BP, was measured. The LP effect increased the dose to the PTV and surrounding critical tissues. In addition, the effect depends on the number of beams and segments for each beam. Segment positional error was less than the maximum tolerance of 1 mm under a dose rate of 600 monitor units per minute in the treatment plans. The OSE varying with the dose rate was observed in all photon beams, and the effect increased from 1 to 1.3 Gy per treatment of the rectal intersection. As the dosimetric impacts from the LP effect and OSE may increase the rectal post-radiation effects, a correction of LP was proposed and demonstrated for the central beam profile for one of the planned beams. CONCLUSION: We concluded that the measured dosimetric impact of the LP dose inaccuracy from photon beam segment in step-and-shoot IMRT can be corrected.     

Dosimetric properties of a flattening filter-free 6-MV photon beam: a Monte Carlo study

Purpose The dosimetric features of an unflattened 6-MV photon beam of an Elekta SL-25 linac was calculated by the Monte Carlo (MC) method. Material and methods The head of the Elekta SL-25 linac was simulated using the MCNP4C MC code. The accuracy of the model was evaluated using measured dosimetric features, including depth dose values and dose profiles in a water phantom. The flattening filter was then removed, and beam dosimetric properties were calculated by the MC method and compared with those of the flattened photon beam. Results Our results showed a significant (twofold) increase in the dose rate for all field sizes. Also, the photon beam spectra for an unflattened beam were softer, which led to a steeper reduction in depth doses. The decrease in the out-of-field dose and increase in the contamination electrons and a buildup region dose were the other consequences of removing the flattening filter. Conclusion Our study revealed that, for recent radiotherapy techniques, the use of multileaf collimators for beam shaping removing the flattening filter could offer some advantages, including an increased dose rate and decreased out-of-field dose.     

In-phantom characterisation studies at the Birmingham Accelerator-Generated epIthermal Neutron Source (BAGINS) BNCT facility.

A broad experimental campaign to validate the final epithermal neutron beam design for the BNCT facility constructed at the University of Birmingham concluded in November 2003. The final moderator and facility designs are overviewed briefly, followed by a summary of the dosimetric methods and presentation of a small subset of the results from this campaign. The dual ionisation chamber technique was used together with foil activation to quantify the fast neutron, photon, and thermal neutron beam dose components in a large rectangular phantom exposed to the beam with a 12 cm diameter beam delimiter in place. After application of a normalisation factor, dose measurements agree with in-phantom MCNP4C predictions within 10% for the photon dose, within 10% for thermal neutron dose, and within 25% for the proton recoil dose along the main beam axis.     

Carcinogenic effect of radiation on the mouse embryo; physical aspects of radiation dosimetry.

The doses received by the embryos from abdominal irradiation of the females at the time of pregnancy may adversely affect the offspring. The dose to the embryo will depend on the depth of the embryo from the surface, the distance of the embryo from the central axis of the beam and on the photon energy. A knowledge of the dose as well as the percentage depth dose in this region will be necessary for the meaningful correlation between the radiation dose and the neoplasm formation due to prenatal irradiation. This paper presents the results of our experimental investigation of the percentage depth doses in a mouse phantom. The results of theoretical computation of percentage depth doses to off-axis points are found to be in good agreement with the experimental values.     

A Pelvic Phantom for Modeling Internal Organ Motions

A pelvic phantom was developed for use in testing image-guided radiation therapy (IGRT) and adaptive applications in radiation therapy (ART) with simulating the anterior-posterior internal organ motions during prostate radiotherapy. Measurements could be done with an ionization chamber (IC) in the simulated prostate. The rectum was simulated by air-equivalent material (AEM). The volume superior to the IC placement was considered as the bladder. The extension of AEM volume could be varied. The vertical position of the IC placement could be shifted by ±1 cm to simulate the prostate motion parallel to the changes in bladder volume. The reality of the simulation was inspected. Three-millimeter-slice-increment computed tomography (CT) scans were taken for irradiation planning. The structure set was adapted to the phantom from a treated patient. Planning target volume was delineated according to the RTOG 0126 study. IMRT and 3D conformal radiation therapy (3D-CRT) plans were made. Prostate motion and rectum volume changes were simulated in the phantom. IC displacement was corrected by phantom shifting. The delivered dose was measured with IC in 7 cases using intensity-modulated radiation therapy (IMRT) and 3D-CRT fractions, and single square-shaped beams: anteroposterior (AP), posteroanterior (PA), and lateral (LAT). Variations from the calculated doses were slightly below 1% at IMRT and around 1% at 3D-CRT; below 4.5% at square AP beam; up to 9% at square PA beam; and around 0.5% at square LAT beam. Other authors have already shown that by using planning systems and ultrasonic and cone beam CT guidance, correction of organ motions in a real patient during prostate cancer IGRT does not have a significant dosimetric effect. The inspection of our phantom—as described here—ended with similar results. Our team suggested that our model is sufficiently realistic and can be used for IGRT and ART testing.     

A phantom for dosimetric characterization of small radiation fields: design and use

An acrylic phantom was designed and constructed for the acquisition and verification of basic dosimetric data of narrow fields in stereotactic radiotherapy/radiosurgery (SRT/SRS) using thermoluminescent (TL) dosimetry. An array of holes to accommodate up to 426 dosimeters was used to allow the assessment of dose distribution in circular fields with a 1-mm spatial dose resolution with minimal field perturbation. It was found experimentally that there must be a minimum gap of 1 mm between neighboring dosimeters in 6-MV photon fields. Most of the dosimetric characteristics of a 6-MV x-ray SRS/SRT unit assessed using TL dosimetry and ion chamber dosimetry were in good agreement when the longitudinal axis of the chamber was parallel to the central beam axis. TL dosimetry showed that the penumbra width increased with increasing collimator aperture. The low cost of the phantom and the widespread use and familiarity of TL dosimetry in radiotherapy departments offer a significant advantage in the use of the proposed methodology.     

Dosimetric properties and commissioning of cone-beam CT image beam line with a carbon target

Background and purpose

Accurate patient positioning before radiotherapy is often verified using advanced imaging techniques such as cone-beam computed tomography (CBCT). Even for dedicated imaging beam lines, the applied dose is not necessarily negligible with respect to the treatment dose and should be considered in the treatment plan.

Materials and methods

This study presents measurements of the beam properties of the Siemens kView (Siemens AG, Munich, Germany) image beam line (IBL) and the commissioning in the Philips Pinnacle³ treatment planning system (TPS; Philips, Amsterdam, Netherlands).

Results

The percent depth dose curve reaches its maximum at a depth of 10 mm, with a surface dose of 44?%. The IBL operates in flattening filter-free mode, showing the characteristic dose falloff from the central axis. Stability over several days to months is within less than 2?% dose deviation or 1 mm distance-to-agreement. Modelling of the IBL beam line was performed using the Pinnacle³ automatic modelling routine, with absolute dosimetric verification and film measurements of the fluence distribution.

Conclusion

After commissioning of the IBL beam model, the dose from the imaging IBL CBCT can be calculated. Even if the absolute dose deposited is small, repeated imaging doses may sum up to significant amounts and can shift the position of the dose maximum by several centimetres.     

Physical-dosimetric characterization of a multi-leaf collimator system for clinical implementation in conformational radiotherapy

INTRODUCTION: In the present paper we discuss the main dosimetric characteristics of the multileaf collimator (MLC) installed on the Elekta SLi Precise accelerators. To evaluate the effectiveness of the MLC in conformal radiotherapy, beam transmission through leaves and/or diaphragms, leakage between the leaves, central axis depth dose, surface dose, effective penumbra, scalopping effect and field size factors were measured. MATERIALS AND METHODS: The MLC installed on the dual energy (4 and 6 MV) linear accelerator Elekta SLi Precise consists of 40 opposed pairs of 75 mm thick tungsten leaves, set in two raws mounted in place of the upper collimator. Each leaf has a nominal projected width of 10 mm. The maximum field size attainable is 40 x 40 cm2 at 100 cm SAD. Beam transmission through leaves and/or diaphragms and field size factors were measured in RW3 phantom with a ionization chamber, leakage between the leaves and effective penumbra were instead evaluated with radiographic films (X-Omat-V) and a laser scanning photodensitometer. Percentage depth doses were measured in an automatic water phantom. RESULTS: For both energies, approximately 1% of the incident radiation on the multileaf collimator is transmitted through the backup collimator, while the transmission through the different combinations of leaves and collimators is between 0.03 and 0.14%. These values show a good agreement with literature data and are in general lower than the peak values specified by the manufacturer. The peak value of the leakage between the leaves was about 2% for both energies, without significative variation with gantry or collimator angle or distance from the axis. MLC shaped fields show a skin dose less (about 3%) than the one of cerrobend block shaped fields, because of the electronic contamination due to the plexiglass tray of the cerrobend blocks; in both cases, the depth doses are similar, as are flatness and symmetry of irradiation fields. The effective penumbra increases with field dimension, depth and leaves positioning, with a mean value of about 9 mm for both energies. The different beam configurations do not significantly affect the values of the field size factors. CONCLUSIONS: The dosimetric characteristics and the case of use of the Elekta multileaf collimator make its application to conformal radiotherapy convenient and reliable, able to improve the accuracy and the effectiveness of radiation therapy and to develop new kinds of treatments. However, because of the complexity of the MLC, its implementation in radiotherapic practice requires careful dosimetric characterization to evaluate those parameters (transmission, penumbra and output factors) that play a fundamental role in the accuracy of the treatment.     

Monte Carlo study on a flattening filter-free 18-MV photon beam of a medical linear accelerator

PURPOSE: Several studies on the dosimetric properties of unflattened photon beams have shown some advantages for radiotherapy. In this study, the effect of removing the flattening filter from an 18-MV photon beam was investigated using the Monte Carlo method. MATERIALS AND METHODS: The 18-MV photon beam of an Elekta SL25 linear accelerator was simulated using the MCNP4C Monte Carlo (MC) code. Beam dosimetric features, including central axis absorbed doses, beam profiles, and photon energy spectra, were calculated for flattened and unflattened 18-MV photon beams. RESULTS: A 4.24-fold increase in the dose rate was seen for the unflattened beam with a field size of 10 x 10 cm(2). A decrease in the out-of-field dose up to 30% was seen for the unflattened beam. For the unflattened beam, photon energy spectra were softer, and the mean energies of the spectra were higher for a smaller field size. CONCLUSION: Our study showed that the increase in dose rate and lower out-of-field dose can be possible advantages for an unflattened 18-MV beam.