An evaluation of epoxy resin phantom materials for megavoltage photon dosimetry

Investigation of the effects of spinal surgical implants on radiotherapy dosimetry: A study of 3D printed phantoms

PURPOSE

The use of three-dimensional (3D) printing to develop custom phantoms for dosimetric studies in radiotherapy is increasing. The process allows production of phantoms designed to evaluated specific geometries, patients, or patient groups with a defining feature. The ability to print bone-equivalent phantoms has, however, proved challenging. The purpose of this work was to 3D print a series of three similar spine phantoms containing no surgical implants, implants made of titanium, and implants made of carbon fiber, for future dosimetric and imaging studies. Phantoms were evaluated for (a) tissue and bone equivalence, (b) geometric accuracy compared to design, and (c) similarity to one another.

METHODS

Sample blocks of PLA, HIPS, and StoneFil PLA-concrete with different infill densities were printed to evaluate tissue and bone equivalence. The samples were used to develop CT to physical (PD) and effective relative electron density (RED_eff ) conversion curves and define the settings for printing the phantoms. CT scans of the printed phantoms were obtained to assess the geometry and densities achieved. Mean distance to agreement (MDA) and DICE coefficient (DSC) values were calculated between contours defining the different materials, obtained from design and like phantom modules. HU values were used to determine PD and RED_eff and subsequently evaluate tissue and bone equivalence.

RESULTS

Sample objects showed linear relationships between HU and both PD and RED_eff for both PLA and StoneFil. The PD and RED_eff of the objects calculated using clinical CT conversion curves were not accurate and custom conversion curves were required. PLA printed with 90% infill density was found to have a PD of 1.11 ± 0.03 g.cm^-3 and RED_eff of 1.04 ± 0.02 and selected for tissue- equivalent phantom elements. StoneFil printed with 100% infill density showed a PD of 1.35 ± 0.03 g.cm^-3 and RED_eff of 1.24 ± 0.04 and was selected for bone-equivalent elements. Upon evaluation of the final phantoms, the PLA elements displayed PD in the range of 1.10 ± 0.03 g.cm^-3 -1.13 ± 0.03 g.cm^-3 and RED_eff in the range of 1.02 ± 0.03-1.06 ± 0.03. The StoneFil elements showed PD in the range of 1.43 ± 0.04 g.cm^-3 -1.46 ± 0.04 g.cm^-3 and RED_eff in the range of 1.31 ± 0.04-1.33 ± 0.04. The PLA phantom elements were shown to have MDA of ≤1.00 mm and DSC of ≥0.95 compared to design, and ≤0.48 mm and ≥0.91 compared like modules. The StoneFil elements displayed MDA values of ≤0.44 mm and DSC of ≥0.98 compared to design and ≤0.43 mm and ≥0.92 compared like modules.

CONCLUSIONS

Phantoms which were radiologically equivalent to tissue and bone were produced with a high level of similarity to design and even higher level of similarity of one another. When used in conjunction with the derived CT to PD or RED_eff conversion curves they are suitable for evaluating the effects of spinal surgical implants of varying material of construction.

Dosimetric properties of a Solid Water High Equivalency (SW557) phantom for megavoltage photon beams

The dosimetric properties of the recently developed SW557 phantom have been investigated by comparison with those of the existing SW457 phantom in megavoltage photon beams. The electron fluence ratio φ_pl^w, and chamber ionization ratio k_pl, of water to SW457 and water to SW557 for 4-15MV photons were calculated as a function of depth using Monte Carlo simulations, and compared with measured values. Values of φ_pl^w for SW457 were in the range of 1.004-1.014 for 4MV, and 1.014-1.018 for 15MV photons. The φ_pl^w for SW557 ranged from 1.005 to 1.008 for 4MV and from 1.010 to 1.015 for 15MV photons and the variation of φ_pl^w with depth for each beam energy was within ±0.5%. Values of k_pl were obtained with a PTW 30013 Farmer-type ionization chamber. The k_pl for SW457 ranged from 0.997 to 1.011 for 4-15MV photons. Values of k_pl for SW557 were almost unity for 4 and 6MV photons, while in the case of 10 and 15MV photons they were less than 1.006, excepting the build-up region. The measured and calculated k_pl values of water to SW557 were in the range of 0.997-1.002 and 1.000-1.006, respectively, for 4-15MV photons, at a depth of 10cm with a source-to-axis distance of 100cm. The measured and calculated k_pl values were in agreement within their uncertainty ranges. As a water-equivalent phantom, SW557 can be used with a dosimetric difference within±0.6%, for 4-15MV photons, and is more water-equivalent than SW457 in megavoltage photon beams.

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

Dosimetric impact of density variations in Solid Water 457 water-equivalent slabs

The purpose of this study was to determine the dosimetric impact of density variations observed in water-equivalent solid slabs. Measurements were performed using two 30 cm × 30 cm water-equivalent slabs, one being 4 cm think and the other 5 cm thick. The location and extent of density variations were determined by computed tomography (CT) scans. Additional imaging measurements were made with an amorphous silicon megavoltage portal imaging device and an ultrasound unit. Dosimetric measurements were conducted with a 2D ion chamber array, and a scanned diode in water. Additional measurements and calculations were made of small rectilinear void inhomogeneities formed with water-equivalent slabs, using a 2D ion chamber array and the convolution superposition algorithm. Two general types of density variation features were observed on CT images: 1) regions of many centimeters across, but typically only a few millimeters thick, with electron densities a few percent lower than the bulk material, and 2) cylindrical regions roughly 0.2 cm in diameter and up to 20 cm long with electron densities up to 5% lower than the surrounding material. The density variations were not visible on kilovoltage, megavoltage or ultrasound images. The dosimetric impact of the density variations were not detectable to within 0.1% using the 2D ion chamber array or the scanning photon diode at distances 0.4 cm to 2 cm beyond the features. High-resolution dosimetric calculations using the convolution-superposition algorithm with density corrections enabled on CT-based datasets showed no discernable dosimetric impact. Calculations and measurements on simulated voids place the upper limit on possible dosimetric variations from observed density variations at much less than 0.6%. CT imaging of water-equivalent slabs may reveal density variations which are otherwise unobserved with kV, MV, or ultrasound imaging. No dosimetric impact from these features was measureable with an ion chamber array or scanned photon diode. Consequently, they were determined to be acceptable for all clinical use.

Dosimetric evaluation of Plastic Water Diagnostic-Therapy

High‐precision radiotherapy planning and quality assurance require accurate dosimetric and geometric phantom measurements. Phantom design requires materials with mechanical strength and resilience, and dosimetric properties close to those of water over diagnostic and therapeutic ranges. Plastic Water Diagnostic Therapy (PWDT: CIRS, Norfolk, VA) is a phantom material designed for water equivalence in photon beams from 0.04 MeV to 100 MeV; the material has also good mechanical properties. The present article reports the results of computed tomography (CT) imaging and dosimetric studies of PWDT to evaluate the suitability of the material in CT and therapy energy ranges.

We characterized the water equivalence of PWDT in a series of experiments in which the basic dosimetric properties of the material were determined for photon energies of 80 kVp, 100 kVp, 250 kVp, 4 MV, 6 MV, 10 MV, and 18 MV. Measured properties included the buildup and percentage depth dose curves for several field sizes, and relative dose factors as a function of field size. In addition, the PWDT phantom underwent CT imaging at beam qualities ranging from 80 kVp to 140 kVp to determine the water equivalence of the phantom in the diagnostic energy range. The dosimetric quantities measured with PWDT agreed within 1.5% of those determined in water and Solid Water (Gammex rmi, Middleton, WI). Computed tomography imaging of the phantom was found to generate Hounsfield numbers within 0.8% of those generated using water. The results suggest that PWDT material is suitable both for regular radiotherapy quality assurance measurements and for intensity‐modulated radiation therapy (IMRT) verification work. Sample IMRT verification results are presented.

PACS number: 87.53Dq

Dosimetric evaluation of a MOSFET detector for clinical application in photon therapy

Dosimetric characteristics of a metal oxide-silicon semiconductor field effect transistor (MOSFET) detector are studied with megavoltage photon beams for patient dose verification. The major advantages of this detector are its size, which makes it a point dosimeter, and its ease of use. In order to use the MOSFET detector for dose verification of intensity-modulated radiation therapy (IMRT) and in-vivo dosimetry for radiation therapy, we need to evaluate the dosimetric properties of the MOSFET detector. Therefore, we investigated the reproducibility, dose-rate effect, accumulated-dose effect, angular dependence, and accuracy in tissue-maximum ratio measurements. Then, as it takes about 20 min in actual IMRT for the patient, we evaluated fading effect of MOSFET response. When the MOSFETs were read-out 20 min after irradiation, we observed a fading effect of 0.9% with 0.9% standard error of the mean. Further, we applied the MOSFET to the measurement of small field total scatter factor. The MOSFET for dose measurements of small field sizes was better than the reference pinpoint chamber with vertical direction. In conclusion, we assessed the accuracy, reliability, and usefulness of the MOSFET detector in clinical applications such as pinpoint absolute dosimetry for small fields.

Characterization of the phantom material Virtual Water™ in high-energy photon and electron beams

The material Virtual Water has been characterized in photon and electron beams. Range-scaling factors and fluence correction factors were obtained, the latter with an uncertainty of around 0.2%. This level of uncertainty means that it may be possible to perform dosimetry in a solid phantom with an accuracy approaching that of measurements in water. Two formulations of Virtual Water were investigated with nominally the same elemental composition but differing densities. For photon beams neither formulation showed exact water equivalence-the water/Virtual Water dose ratio varied with the depth of measurement with a difference of over 1% at 10 cm depth. However, by using a density (range) scaling factor very good agreement (<0.2%) between water and Virtual Water at all depths was obtained. In the case of electron beams a range-scaling factor was also required to match the shapes of the depth dose curves in water and Virtual Water. However, there remained a difference in the measured fluence in the two phantoms after this scaling factor had been applied. For measurements around the peak of the depth-dose curve and the reference depth this difference showed some small energy dependence but was in the range 0.1%-0.4%. Perturbation measurements have indicated that small slabs of material upstream of a detector have a small (<0.1% effect) on the chamber reading but material behind the detector can have a larger effect. This has consequences for the design of experiments and in the comparison of measurements and Monte Carlo-derived values.

Dosimetric verification of a commercial collapsed cone algorithm in simulated clinical situations

BACKGROUND AND PURPOSE

This work reports a detailed study carried out in two UK radiotherapy centres of the dosimetric accuracy of the collapsed cone algorithm of a commercial treatment planning system (Helax-TMS) in simulated clinical situations.

MATERIALS AND METHODS

Initially the accuracy of the collapsed cone algorithm in homogeneous media is evaluated for an extensive set of simple and complex fields. Water, lung and bone substitute epoxy resin material were then used to assess the algorithm in inhomogeneous media and compare its accuracy with the pencil beam algorithm currently in clinical use. Finally a semi-anatomic phantom and an anthropomorphic phantom were employed to assess the dosimetric accuracy using simulated clinical set ups. Thermoluminescence dosimeter (TLD) measurements were made with the anthropomorphic phantom and ionisation chambers otherwise. Nominal 4, 6 and 15 MV photon beams were studied.

RESULTS

For most homogeneous cases agreement between measured and calculated dose is within +/-2% or +/-2 mm. In cases with heterogeneities and simulated clinical situations it is observed that the accuracy is also generally within +/-2% or +/-2 mm. Specific instances where the difference between measured and calculated values exceed this are highlighted.

CONCLUSIONS

It can be concluded that in clinical treatment planning situations where lung is present the collapsed cone algorithm should be considered in preference to pencil beam algorithms normally used but that there may still be some discrepancy between calculations and measurement.

The effect of the Varian amorphous silicon electronic portal imaging device on exit skin dose

Measurements have been made of the increase in exit surface dose resulting from backscattered radiation generated by the Varian amorphous silicon electronic portal imaging device (EPID). An increase of < or = 14% was demonstrated at both 6 MV and 10 MV, in a manner which suggests that backscatter from the EPID acts to re-establish electronic equilibrium at the exit surface, normally absent in the build-down region. The magnitude of this effect was influenced by field size, measurement depth and exit surface to EPID distance. Assuming typical constraints of portal imaging frequency and geometry, the results suggest that EPID generated backscatter is unlikely to alter the frequency or severity of exit skin reactions. However, the results do suggest that a limit on the minimum separation between the EPID and the exit surface should be set, and that similar investigations should be made for other EPID models.

Design of a multiblock phantom for radiotherapy dosimetry applications

This note describes the design of a multiblock semi-anatomic phantom, which lends itself to a variety of radiotherapy dosimetry applications, in particular, the audit of external beam treatment planning and delivery. The basic building blocks of the phantom were formed from a variety of tissue substitute materials and could be assembled in many ways to model different cross-sections through the body.

Breast radiotherapy phantom design for the START trial. START trial management group

The design of phantoms for use in radiotherapy involves a number of complex issues. This paper describes breast and chest wall phantoms that have been designed and constructed for the START trial. Four phantoms have been manufactured, including two two-dimensional phantoms used on the first round of audit visits to assess the ability of departments to plan with the required accuracy. Two further phantoms have been constructed and will be used in the second round of audit visits; one is a water-filled three-dimensional phantom for investigating off-axis dosimetry, the other is to be used to assess dose in the junction region between the tangential fields and the supraclavicular fossa field. The manufacturing and design process for each of the phantoms is discussed.

A comparative description of three multipurpose phantoms (MPP) for external audits of photon beams in radiotherapy: the water MPP, the Umeå MPP and the EC MPP

AIM

To present a technical description and intercomparison of three multipurpose phantoms (MPP) developed for mailed dosimetry checks of therapeutic photon beams in reference and non-reference conditions.

MATERIALS

The W-MPP is a water MPP, whereas the Umeâ-MPP, made of perspex (PMMA, Plexiglas), and the EC-MPP, made of polystyrene, are solid MPPs. The W-MPP uses only thermoluminescent dosimeters (TLD) for dosimetry checks, the EC MPP uses film and TLD; the Umeâ phantom uses film and TLD, and offers in addition the possibility for ionization chamber measurements. Either using TLD or films, the MPPs have been designed to check on-axis and off-axis the following irradiation conditions: square and rectangular fields, asymmetric fields, wedged beams, oblique incidence and, for the solid MPPs, also the influence of inhomogeneities.

RESULTS AND DISCUSSION

The MPPs have been compared for different aspects going from their dosimetric performance (number of dosimetric parameters that can be checked) to some practical consideration in the use of the different MPPs (set-up time, stability, instruction sheets, etc.). From a comparison between the solid multi-purpose phantoms, it turns out that the EC-MPP is capable of checking the largest number of dosimetric parameters per beam, but has the longest set-up time ( approximately 2 h) per beam according to the users. The Umeå-MPP has a smaller number of set-ups (hence a smaller average time) and also includes some parameters not checked with the EC-MPP (e.g. SSD accuracy). The major drawback however of the Umeå-MPP is considered to be its high density (>1.1 g/cm(3)) which increases the difficulty of the analysis with the treatment planning system. The W-MPP checks the smallest number of parameters, but is the fastest in set-up time, the easiest for mailing, and is water equivalent, which is advantageous for the TPS checks. The major drawback of this MPP is however the inability to check complete dose distribution (film) or inhomogeneities.