MV Photon Fields Research Articles

Camera-based radiotherapy dosimetry using dual-material 3D printed scintillator arrays.

To describe a methodology for the dual-material fused deposition modeling (FDM) 3D printing of plastic scintillator arrays, to characterize their light output under irradiation using an sCMOS camera, and to establish a methodology for the dosimetric calibration of planar array geometries. We have published an investigation into the fabrication and characterization of single element FDM printed scintillators intending to produce customizable dosimeters for radiation therapy applications. 1 This work builds on previous investigations by extending the concept to the production of a high-resolution (scintillating element size 3×3×3mm3 ) planar scintillator array. The array was fabricated using a BCN3D Epsilon W27 3D printer and composed of polylactic acid (PLA) filament and BCF-10 plastic scintillator. The array's response was initially characterized using a 20×20cm2 6 MV photon field with a source-to-surface (SSD) distance of 100cm and the beam incident on the top of the array. The light signals emitted under irradiation were imaged using 200ms exposures from a sCMOS camera positioned at the foot of the treatment couch (210cm from the array). The collected images were then processed using a purpose-built software to correct known optical artefacts and determine the light output for each scintillating element. The light output was then corrected for element sensitivity and calibrated to dose using Monte Carlo simulations of the array and irradiation geometry based on the array's digital 3D print model. To assess the accuracy of the array calibration both a 3D beam and a clinical VMAT plan were delivered. Dose measurements using the calibrated array were then compared to EBT3 GAFChromic film and OSLD measurements, as well as Monte Carlo simulations and TPS calculations. Our results establish the feasibility of dual-material 3D printing for the fabrication of custom plastic scintillator arrays. Assessment of the 3D printed scintillators response across each row of the array demonstrated a nonuniform response with an average percentage deviation from the mean of 2.1%±2.8%. This remains consistent with our previous work on individual 3D printed scintillators which showed an average difference of 2.3% and a maximum of 4.0% between identically printed scintillators.1 Array dose measurements performed following calibration indicate difficulty in differentiating the scintillator response from ambient background light contamination at low doses (<20-25cGy) and dose rates (≤100 MU/min). However, when analysis was restricted to exclude dose values less than 10% of the Monte Carlo simulated max dose the average absolute percentage dose difference between Monte Carlo simulation and array measurement was 5.3%±4.8% for the fixed beam delivery and 5.4%±5.2% for the VMAT delivery CONCLUSION: In this study, we developed and characterized a 3D printed array of plastic scintillators and demonstrated a methodology for the dosimetric calibration of a simple array geometry.

Dose-volume parameters comparison of organs at risk between the prone and supine positions in pelvic tumors using 3D-CRT.

Cancer is a major public health problem worldwide, the leading cause of death in developed countries. Radiotherapy is an important treatment for many malignancies. The main purpose of this study was to compare the two techniques of supine and prone in prostate and rectal cancers using DVH extraction parameters. Clinical and dosimetry data of 41 rectal and prostate cancer patients were evaluated in both the supine and prone positions with belly board. Administered dose was daily 180 cGy. The four box fields in the first phase and two lateral fields in the second phase with 18 MV photon fields were used. Each patient underwent CT scan, at both the positions using a contrast agent with a full bladder. By using IBM SPSS software v23, all the data were described. The normal distribution of the data was performed using the KS sample statistical test. For data analysis, paired t test was used in the normal data and the Wilcoxon test was used in the non-normal data. In patients with rectal cancer, there is no change in the received minimum dose by organs at risk. A significant decrease in received maximum dose, except for the prostate organ, could be due to the spatial proximity of the two organs to each other. Also, the received average dose in the small intestine was significantly reduced (P = 0.005). But in other organs, the dose reduction was not significant. In patients with prostate cancer, there is no change in the received minimum dose by OARs, except for the bladder organ (P = 0.003). Except the bladder organs (P = 0.011), there is no significant decrease in the received average dose by OARs. The maximum dose of the OARs is significantly reduced, except for the colon where there was not much overlap in the PTV, in addition to receiving the dose in the range. There was no significant relationship between CI in the rectal field and UI in the prostate field (P > 0.05), but there was a significant relationship between CI in the prostate field and UI in the rectal field with change in patient position. In the prone position, in both patients' groups, the OARs receive an optimal and better dose than the supine position, especially the small intestine organ in the rectal field and the bladder and rectum organs in the prostate field. However, it seems that this change in the position of rectal cancer patients is ineffective in reducing the dose of prostate and needs further investigation.

A phantom-based experimental and Monte Carlo study of the suitability of in-vivo diodes and TLD for entrance in-vivo dosimetry in small-to-medium sized 6 MV photon fields

In-vivo dosimetry is recommended for radiotherapy treatment verification. The main purpose of the study is to investigate the suitability of various dosimeters for performing in-vivo measurements in small 6 MV photon fields by measurement as well as Monte Carlo simulation. Two types of diode dosimeters designed for placement on skin for the purpose of in-vivo dosimetry (EDD-5 and EDP-10), 3.1 × 3.1 × 0.89 mm3 LiF TLDs, and an electron field diode (EFD) were used for dosimetric measurements in a custom-made PMMA phantom. An Elekta accelerator’s treatment head was Monte Carlo modeled in detail, which was used together with validated Monte Carlo models of the diodes. Dose perturbation effects of the dosimeters and their suitability for in-vivo measurements were investigated. In fields ≥2 × 2 cm2, differences in the measured response of EDD-5, EDP-10 and TLD from the Monte Carlo gold-standard were <2%. For the 1 × 1 cm2 field, however, the differences from the Monte Carlo gold-standard were all greater than 2% due to the influence of dosimeter size as well as any misalignment and set-up uncertainties. In fields ≥1 × 1 cm2, EDD-5, EDP-10, and TLD produced 2.2%–3.2%, 5.3%–6.6% and 0.8–1.1% dose perturbation (≤10 cm depths), respectively. A better than 2% dosimetric accuracy was not achieved by these diodes for field sizes smaller than 2 × 2 cm2. Routine patient in-vivo dosimetry is feasible with the EDD-5 diode and TLD for field sizes down to 2 × 2 cm2 without exceeding the 5% tolerance level for overall uncertainty. But the EDP-10 diode produces a higher perturbation making it much less suitable for this purpose.

The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation.

We have experimentally and computationally characterized the PTW microSilicon 60023-type diode's performance in 6 and 15 MV photon fields ≥5×5 mm2 projected to isocenter. We tested the detector on- and off-axis at 5 and 15cm depths in water, and investigated whether its response could be improved by including within it a thin airgap. Experimentally, detector readings were taken in fields generated by a Varian TrueBeam linac and compared with doses-to-water measured using Gafchromic film and ionization chambers. An unmodified 60023-type diode was tested along with detectors modified to include 0.6, 0.8, and 1.0 mm thick airgaps. Computationally, doses absorbed by water and detectors' sensitive volumes were calculated using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Detector response was characterized using , a factor that corrects for differences in the ratio of dose-to-water to detector reading between small fields and the reference condition, in this study 5cm deep on-axis in a 4×4 cm2 field. The greatest errors in measurements of small field doses made using uncorrected readings from the unmodified 60023-type detector were over-responses of 2.6%±0.5% and 5.3%±2.0% determined computationally and experimentally, relative to the reading-per-dose in the reference field. Corresponding largest errors for the earlier 60017-type detector were 11.9%±0.6% and 11.7%±1.4% over-responses. Adding even the thinnest, 0.6mm, airgap to the 60023-type detector over-corrected it, leading to under-responses of up to 4.8%±0.6% and 5.0%±1.8% determined computationally and experimentally. Further, Monte Carlo calculations indicate that a detector with a 0.3mm airgap would read correctly to within 1.3% on-axis. The ratio of doses at 15 and 5cm depths in water in a 6 MV 4×4 cm2 field was measured more accurately using the unmodified 60023-type detector than using the 60017-type detector, and was within 0.3% of the ratio measured using an ion chamber. The 60023-type diode's sensitivity also varied negligibly as dose-rate was reduced from 13 to 4Gy min-1 by decreasing the linac pulse repetition frequency, whereas the sensitivity of the 60017-type detector fell by 1.5%. The 60023-type detector performed well in small fields across a wide range of beam energies, field sizes, depths, and off-axis positions. Its response can potentially be further improved by adding a thin, 0.3mm, airgap.

Late Skeletal Effects in Sheep Treated as Lambs With Targeted vs. Wide Symmetrical Spinal Fields

<h3>Purpose/Objective(s)</h3> Craniospinal irradiation (CSI) is a standard treatment for pediatric medulloblastoma. The conventional CSI technique is associated with incidental radiotherapy (RT) to organs at risk (OARs). In growing children, RT can cause subsequent skeletal abnormalities such as curvature and bone growth retardation. However, these effects have not been sufficiently studied for children receiving symmetrical spinal fields. We tested the null hypothesis that narrow spinal fields (NSFs) typical of CSI would cause in 10 degrees increased curvature and 10 cm less spinal growth longitudinally than standard wide spinal fields (WSFs), with lambs as an animal model for children. <h3>Materials/Methods</h3> RT treatment plans were designed for 6 MV photon fields and delivered to lambs of ages 6 weeks or younger—four receiving NSFs that targeted the thecal sac, four receiving wide fields, and four not receiving radiation. Half of the lambs in each set were male and half were female. We chose lambs as a model for children because of their skeletal behavioral resemblance to that of humans. RT fields were expanded laterally with 3 mm margins to produce planning target volumes. RT dose was 27 Gy in 8 fractions, biologically equivalent to the standard total dose of 36 Gy in 20 fractions. Every 6 months and at 2 years, after full growth of lambs in all three arms of the study, thoracic scoliosis Cobb angle, thoracic and lumbar kyphosis Cobb angle, and vertebral hypoplasia in the thoracic spine were measured using computed tomography and compared. <h3>Results</h3> Although differences were observed between the experimental and control (no RT) sets, our hypothesis was found to be false for each spinal abnormality measure. Specifically, between the sheep receiving NSF and WSF, less than 10 degrees increased curvature and 10 cm shorter spinal growth was observed. Mean scoliosis Cobb angles were 0.7, 4.0 and 4.7 degrees for the control, WSF and NSF arms, respectively. Mean thoracic kyphosis Cobb angles were 10.7, 9.0 and 7.0 degrees for the control, WSF and NSF arms, respectively. Mean lumbar kyphosis Cobb angles were 12.3, 7.7 and 9.3 degrees for the control, WSF and NSF arms, respectively. Spinal lengths from occipital bone to sacrum were 78, 70 and 66 cm, for the control, WSF and NSF arms, respectively. <h3>Conclusion</h3> We found no differences in spinal curvature or length in fully-grown sheep between targeted and wide spinal RT characteristic of CSI that had been irradiated as young lambs. This suggests that targeted photon fields may be applied to children receiving CSI, potentially reducing their risks of long-term toxicities, including secondary malignancies. Studies on human subjects are needed to confirm the same findings with children receiving CSI.

The influence of various vascular access ports on MV photon beam uniformity examined on the PMMA phantom

The presence of a high-density material object such as the vascular access port made of titanium, can affect the homogeneity of dose distribution in underlying tissues. This influence depends on numerous factors but in the first place on the composition material of such an object and its geometry. In this work an influence of the various titanium-made vascular ports, placed in the 6 MV photon field, have been analyzed. The vascular ports of various sizes were placed on the top of thepolymethyl-methacrylate (PMMA) phantom which is then scanned on a computed tomography (CT) simulator to generate the digitized 3-dimensional images for the purpose of treatment planning. The treatment plans were prepared in matRad treatment planning toolkit. The beam profiles and the percentage depth doses have been analyzed. The observed maximum dose values, for ports A, B, C and D, relatively to the maximum dose value in PMMA phantom alone, were 102.25%, 100.62%, 101.78% and 102.48%, respectively. The titanium edges of the ports reduce the dose below them in amount of 8.52%, 8.64%, 10.01% and 10.04% observed for ports A, B, C and D, respectively, in comparison to the central axis dose value obtained in PMMA phantom for the port-free case. The established changes in PDD curves and beam profiles depend on the vascular access port dimensions, reservoir volumes as well as of the amount of titanium content.

Basic Properties of a New Polymer Gel for 3D-Dosimetry at High Dose-Rates Typical for FFF Irradiation Based on Dithiothreitol and Methacrylic Acid (MAGADIT): Sensitivity, Range, Reproducibility, Accuracy, Dose Rate Effect and Impact of Oxygen Scavenger.

The photon induced radical-initiated polymerization in polymer gels can be used for high-resolution tissue equivalent dosimeters in quality control of radiation therapy. The dose (D) distribution in radiation therapy can be measured as a change of the physical measurement parameter T2 using T2-weighted magnetic resonance imaging. The detection by T2 is relying on the local change of the molecular mobility due to local polymerization initiated by radicals generated by the ionizing radiation. The dosimetric signals R2 = 1/T2 of many of the current polymer gels are dose-rate dependent, which reduces the reliability of the gel for clinical use. A novel gel dosimeter, based on methacrylic acid, gelatin and the newly added dithiothreitol (MAGADIT) as an oxygen-scavenger was analyzed for basic properties, such as sensitivity, reproducibility, accuracy and dose-rate dependence. Dithiothreitol features no toxic classification with a difference to THPC and offers a stronger negative redox-potential than ascorbic acid. Polymer gels with three different concentration levels of dithiothreitol were irradiated with a preclinical research X-ray unit and MR-scanned (T2) for quantitative dosimetry after calibration. The polymer gel with the lowest concentration of the oxygen scavenger was about factor 3 more sensitive to dose as compared to the gel with the highest concentration. The dose sensitivity (α = ∆R2/∆D) of MAGADIT gels was significantly dependent on the applied dose rate (≈48% reduction between = 0.6 Gy/min and = 4 Gy/min). However, this undesirable dose-rate effect reduced between 4–8 Gy/min (≈23%) and almost disappeared in the high dose-rate range (8 ≤12 Gy/min) used in flattening-filter-free (FFF) irradiations. The dose response varied for different samples within one manufacturing batch within 3%–6% (reproducibility). The accuracy ranged between 3.5% and 7.9%. The impact of the dose rate on the spatial integrity is demonstrated in the example of a linear accelerator (LINAC) small sized 5 × 10 mm2 10 MV photon field. For MAGADIT the maximum shift in the flanks in this field is limited to about 0.8 mm at a FFF dose rate of 15 Gy/min. Dose rate sensitive polymer gels likely perform better at high dose rates; MAGADIT exhibits a slightly improved performance compared to the reference normoxic polymer gel methacrylic and ascorbic acid in gelatin initiated by copper (MAGIC) using ascorbic acid.

Comparison of deep inspiration breath hold and free breathing technique in left breast cancer irradiation: a dosimetric evaluation in 40 patients

To analyze the reduction of dose to organs at risk between deep inspiration breath hold (DIBH) and free breathing (FB) techniques in left breast irradiation. Forty left-sided breast cancer patients received adjuvant radiotherapy and were retrospectively analyzed. Treatment plans were generated on both DIBH and FB computed tomography (CT) scans. The patients were monitored by the Varian RPM™ respiratory gating system. The treatment planning was performed with conformal tangential fields by means of 6 and 10 MV photon fields. Treatment schedule were 50 Gy in 25 fractions (conventional schedule) or 40.05 Gy in 15 fractions (hypofractionated schedule), with or without sequential boost to tumor bed. Dose-volume histograms were compared for all plans. For the comparison, we considered cardiac and ipsilateral lung doses and volumes. Quantitative statistical analyses of plans dose differences were generated. In order to correlate the mean cardiac dose difference between the FB and DIBH techniques with maximum heart distance (MHD), a linear regression model was used. A statistically significant reduction of cardiac and pulmonary doses using the DIBH technique was achieved compared with FB plans, maintaining an equal coverage of planning target volume (PTV). A positive correlation was found between MHD and mean heart dose difference. Our study confirms the DIBH technique advantage in reducing cardiac and pulmonary doses for tangentially treated left-sided breast cancer patients. Further research is warranted to evaluate potential long-term clinical implications of these relevant dosimetry results.

Measurement of fast neutron contamination caused by the presence of wedge and block using CR-39 detector.

Undesired neutron contamination imposed to patients during treatment is among the main factors increasing the risk of secondary cancer in radiotherapy. This additional undesirable dose is due to neutron contamination production in high-energy accelerators. In this study, neutron contamination is investigated in the presence of wedge and block in 15 MV photon fields of Siemens Primus linear accelerator. Neutron production by 30°, 45°, and 60° wedges and cerrobend block was investigated. Measurements were conducted in a 10 cm × 10 cm field at the source to -surface distance of 100 cm at 0.5, 2, 3, and 4 cm depths of a 30 cm × 30 cm × 30 cm Perspex phantom using the CR-39 passive film detectors. Chemical etching was performed using sodium hydroxide solution with 6.25 M concentration as the etchant at 85°C for 3 h. The neutron dosimetry results reveal that the presence of wedge and block increases the neutron contamination. However, the 45° wedge is most effective in producing neutron contamination. The results also show that the fast neutron contamination is lower in the steeper depths. The presence of a wedge in a therapeutic high-energy photon field is a source of neutron contamination and may be of concern regarding clinical aspects. The results of this study show that superficial tissues such as skin will incur higher fast neutron contamination than the deep tissues.

Measuring Workload with Paired Detectors

Linear accelerator workloads for each available photon energy are important quantities to know for radiation safety considerations, and presented is a technique to measure the workload using paired detectors. The signals from the two detectors can give sufficient information to separate the signal contributions from 6 and 18 MV photon fields and, combined with a signal-per-monitor-unit calibration factor, yields the number of monitor units delivered for each energy. CR-39 NTD is a neutron detector chosen for its ability to discriminate between 6 MV and 18 MV radiation fields. TLD-100 is a detector responsive to both 6 MV and 18 MV fields. These appeared to be a good choice for a detector pair. This experiment had both failures and successes to report. The CR-39 NTD and TLD-100 were not a successful pairing. The CR-39 NTD signals saturated under this experiment's exposure conditions. The TLD-100 had a combination of detector noise and detector sensitivity that made extracting the 6 MV signal from the total signal impractical, unless the total exposure was overwhelmingly 6 MV. Nevertheless, the TLD-100 proved to be excellent for determining workloads when it was exposed to a single energy with 1% accuracy and 3% precision. The theory and data analysis showed the importance of understanding the noise contributions for the more general problem of pairing any two detector types. This experiment indicated the TLD-100 could be an excellent detector choice if paired with a suitable second detector.

Feasibility of a dual detector system to perform transit dosimetry and MV imaging in-vivo

In this work, a dual detector system for simultaneous in-vivo dosimetry and MV imaging was investigated. The ability to measure real-time water equivalent dose post transit for a heterogeneous phantom was demonstrated, illustrating its advantage over non-water equivalent EPID dosimetry. Identification of incorrect MU delivery as well as patient misalignment can be identified in the dose profiles and confirmed through co-registration with EPID images. The dual detector comprising of a silicon array detector “Magic Plate” (MP) and an EPID was positioned beneath the patient couch at a source to detector distance of 150 cm. Two pseudo lung phantoms were utilised in this study; identical, except for the inclusion of a water equivalent hidden target mimicking a tumour. The dosimetric accuracy of the MP was evaluated with EBT3 film and TPS calculations. The presence and position of the hidden target was detected and accurately co-registered between the dose profiles measured with MP and the EPID image. The system measured a difference in post transit doses through the two phantoms of (5.0 ±1.6) cGy due to the presence of the target, in agreement with the TPS calculation of 5.16 cGy. The capability of the system for error detection was confirmed when a deliberate 7 mm lateral shift of the phantom was observed in both the dose profiles measured with MP and the EPID images. The MP as a dosimetric real time detector is lucent in MV photon fields and does not obstruct the EPID field of view.

Fiber optically coupled radioluminescence detectors: A short review of key strengths and weaknesses of BCF-60 and Al2O3:C scintillating-material based systems in radiotherapy dosimetry applications

Radiotherapy technologies have improved for several decades aiming to effectively destroy cancerous tissues without overdosing surrounding healthy tissues. In order to fulfil this requirement, accurate and precise dosimetry systems play an important role. Throughout the years, ionization chambers have been used as a standard detector for basic linear accelerator calibrations and reference dosimetry in hospitals. However, they are not ideal for all treatment modalities: and limitations and difficulties have been reported in case of (i) small treatment fields, (ii) strong magnetic field used in the new hybrid MRI LINAC/cobalt systems, and (iii) in vivo measurements due to safety-issues related to the high operating voltage. Fiber optically coupled luminescence detectors provide a promising supplement to ionization chambers by offering the capability of real-time in vivo dose monitoring with high time resolution. In particular, the all-optical nature of these detectors is an advantage for in vivo measurements due to the absence of high voltage supply or electrical wire that could cause harm to the patient or disturb the treatment. Basically, fiber-coupled luminescence detector systems function by radiation-induced generation of radioluminescence from a sub-mm size organic/inorganic phosphor. A thin optical fiber cable is used for guiding the radioluminescence to a photomultiplier tube or similar sensitive light detection systems. The measured light intensity is proportional to dose rate. Throughout the years, developments and research of the fiber detector systems have undergone in several groups worldwide. In this article, the in-house developed fiber detector systems based on two luminescence phosphors of (i) BCF-60 polystyrene-based organic plastic scintillator and (ii) carbon-doped aluminum oxide crystal (Al2O3:C) are reviewed with comparison to the same material-based systems reported in the literature. The potential use of these detectors for reference-class dosimetry in radiotherapy will be discussed with a particular emphasis on uses in small and large MV photon fields.

PO-0784: Volume correction factors for alanine dosimetry in small MV photon fields

PO-0784: Volume correction factors for alanine dosimetry in small MV photon fields

Neutron dose in and out of 18 MV photon fields

In radiation therapy, neutron contamination is an undesirable side effect of using high energy photons to treat patients. Neutron contamination requires adjustments to the shielding requirements of the linear accelerator vault and contributes to the risk of secondary malignancies in patients by delivering dose outside of the primary treatment field. Using MCNPX, an established Monte Carlo code, manufacturer blueprints, and the most up to date ICRP neutron dose conversion factors, the neutron spectra, neutron/photon dose ratio, and the neutron capture gamma ray dose were calculated at different depths and off axis distances in a tissue equivalent phantom. Results demonstrated that the neutron spectra and dose are dependent on field size, depth in the phantom, and off-axis distance. Simulations showed that because of the low neutron absorption cross section of the linear accelerator head materials, the contribution to overall patient dose from neutrons can be up to 1000 times the photon dose out of the treatment field and is also dependent on field size and depth. Beyond 45cm off-axis, the dependence of the neutron dose on field size is minimal. Neutron capture gamma ray dose is also field size dependent and is at a maximum at a depth of about 7cm. It is important to remember that when treating with high energy photons, the dose from contamination neutrons must be considered as it is much greater than the photon dose.

Optimisation of output factor measurements using the Magic Plate 512 silicon dosimeter array in small megavoltage photon fields

We evaluate the impact of an air gap and optimization of this air gap for the MP512 silicon detector array when operated in dosimetry mode for small photon field measurements in solid water. We present output factor measurements for 6MV and 10 MV photon beams with the square field sizes ranging from 0.5 to 10 cm2. The size of the air gap above the MP512 detector was changed from 0.5, 1.0, 1.2, 2.0 and 2.6 mm. We compare the output factors measurements of the MP512 with EBT3 film and the MOSkin dosimeter. For the two photon energies investigated, we find that the output factor measured by the MP512 reduce with increasing air gap and reducing of field size. The reduction in output factor is most pronounced for the 0.5 and 1 cm2 field sizes. The air gap of 0.5 mm and 1.2 mm showed good agreement with the EBT3 film and MOSkin output factor for 6 and 10 MV photon fields, respectively. The negligible effect on dosimetry for the field sizes larger than 4x4 cm2 demonstrates that the electronic disequilibrium caused by small air gap only influences the dosimetry measurements for small fields. The study shows that the output factor reduction is enhanced by increasing of air gap and demonstrates that the optimal air gap for the MP512 at 6 and 10 MV photon fields is 0.5mm.

SU‐F‐T‐426: Measurement of Dose Enhancement Due to Backscatter From Modern Dental Materials

Purpose:High‐density materials used in dental restoration can cause significant localized dose enhancement due to electron backscatter in head‐and‐neck radiotherapy, increasing the risk of mucositis. The materials used in prosthetic dentistry have evolved in the last decades from metal alloys to ceramics. We aim to determine the dose enhancement caused by backscatter from currently‐used dental materials.Methods:Measurements were performed for three different dental materials: lithium disilicate (Li2Si2O5), zirconium dioxide (ZrO2), and gold alloy. Small thin squares (2×2×0.15 cm3) of the material were fabricated, and placed into a phantom composed of tissue‐equivalent material. The phantom was irradiated with a single 6 MV photon field. A thin‐window parallel‐plate ion chamber was used to measure the dose at varying distances from the proximal interface between the material and the plastic.Results:The dose enhancement at the interface between the high‐density and tissue‐equivalent materials, relative to a homogeneous phantom, was 54% for the gold alloy, 31% for ZrO2, and 9% for Li2Si2O5. This enhancement decreased rapidly with distance from the interface, falling to 11%, 5%, and 0.5%, respectively, 2 mm from the interface. Comparisons with the modeling of this effect in treatment planning systems are performed.Conclusion:While dose enhancement due to dental restoration is smaller with ceramic materials than with metal alloys, it can still be significant. A spacer of about 2–3 mm would be effective in reducing this enhancement, even for metal alloys.

SU‐F‐T‐261: Reconstruction of Initial Photon Fluence Based On EPID Images

Purpose:Verifying an algorithm to reconstruct relative initial photon fluence for clinical use. Clinical EPID and CT images were acquired to reconstruct an external photon radiation treatment field. The reconstructed initial photon fluence could be used to verify the treatment or calculate the applied dose to the patient.Methods:The acquired EPID images were corrected for scatter caused by the patient and the EPID with an iterative reconstruction algorithm. The transmitted photon fluence behind the patient was calculated subsequently. Based on the transmitted fluence the initial photon fluence was calculated using a back‐projection algorithm which takes the patient geometry and its energy dependent linear attenuation into account. This attenuation was gained from the acquired cone‐beam CT or the planning CT by calculating a water‐equivalent radiological thickness for each irradiation direction. To verify the algorithm an inhomogeneous phantom consisting of three inhomogeneities was irradiated by a static 6 MV photon field and compared to a reference flood field image.Results:The mean deviation between the reconstructed relative photon fluence for the inhomogeneous phantom and the flood field EPID image was 3% rising up to 7% for off‐axis fluence. This was probably caused by the used clinical EPID calibration, which flattens the inhomogeneous fluence profile of the beam.Conclusion:In this clinical experiment the algorithm achieved good results in the center of the field while it showed high deviation of the lateral fluence. This could be reduced by optimizing the EPID calibration, considering the off‐axis differential energy response. In further progress this and other aspects of the EPID, eg. field size dependency, CT and dose calibration have to be studied to realize a clinical acceptable accuracy of 2%.

SU‐F‐J‐50: Study On the Magnetic Field Effect On the Exradin W1 Plastic Scintillation Detector

Purpose:To study the response of the Exradin W1 plastic scintillator detector to a 6 MV photon field in the presence of a strong magnetic field (B).Methods:An Exradin W1 scintillator detector coupled to a SuperMax two‐channel electrometer, both manufactured by Standard Imaging, Inc., was first calibrated in a Co‐60 beam. The Cerenkov Light Ratio (CLR) was obtained following the procedure recommended by the manufacturer. Subtracting signal in channel 2 multiplied by CLR from the signal in channel 1 should lead to a Cerenkov‐free signal. The W1 scintillator was placed in a plastic phantom inside a dipole electromagnet (GMW Associates) that could produce a strong B field, and irradiated using a 6 MV beam from an Elekta Versa HD LINAC. Signals from both channels of the W1 scintillator were acquired as a function of B (0 ‐ 1.5 T).Results:The signals from both channels increased as a function of the B field strength. At 1.5 T, channel 1 increased by 11% compared to the baseline (B=0 T), while channel 2 increased by 22%. Applying the recommended Cerenkov correction led to a 2% difference between dose measurement with and without a magnetic field. The values between B=0.3 T and B=1.5 T remained constant.Conclusion:Signals from the Exradin W1 plastic scintillation detector increased as the B field increased. This increase mainly comes from a change in the amount of Cerenkov light coupled within the optical fiber. Removing the Cerenkov component following the procedure recommended by the manufacturer showed to be an effective way to measure dose accurately in strong magnetic fields. The cause for the residual 2% difference is currently under investigation.We acknowledge research support from Elekta AB.

An evaluation of a novel synthetic diamond probe for dosimetric applications

Abstract A study is presented that characterises the dosimetric performances of two synthetic diamond sensors (HP1 and HP2) when either one or both detectors are subjected to clinical beams of various types under large as well as small-field conditions. Detector performances were evaluated using a prototype probe housing constructed of tissue-equivalent materials. The probe can accommodate diamond sensors of various sizes and is configured for radiation detection in different exposure orientations without having first to re-orient the sensor plate within its body. Also, the diamond sensor is aligned in the same configuration as its rectangular housing and the probe is designed to be compatible with commercially available electrometer systems. Dosimetric measurements were conducted using mammography X-rays (25–32 kVp) and megavoltage electron (6–21 MeV) and photon (60Co γ-ray, 6–18 MV X-ray) beams. Whereas HP1 was evaluated using all beam types under large-flied conditions and small-photon-beam fields down to 0.7×0.7 cm2, HP2 was evaluated using small-electron and photon-beam conditions down to 0.3×0.3 cm2 6 MV photon field. Using HP1 sensor, the synthetic diamond probe was found not to require daily pre-irradiation as long as it is properly shielded from ambient light and its response stabilised. Furthermore, the diamond probe exhibited linear response characteristics with absorbed dose and on exposure parameters to various beam types, negligible energy dependence and almost no variation in angular response. Exposing the sensor HP2 under a 0.4×0.4 cm2 6 MV photon radiation field, a sensitivity value of 197.3 nC Gy−1 mm−3 was established compared to a value of 136.1 nC Gy−1 mm−3 obtained with a small-field diode detector. Also, a figure of 5.5×103 for the SNR was established for the sensor in the same radiation field. Relative beam data measured with the diamond sensors were found to agree within 1–2% with data obtained with reference detectors. The presentation illustrates that once a suitable diamond sensor is incorporated into the probe’s housing, then near-tissue equivalent synthetic diamond probe could be utilized as a multi-purpose radiation detector for the clinical dosimetry of various beam types under large as well as small radiation fields.

Effects and consequences of Digisonic SP cochlear implant on radiotherapy planning

The aim of this study was to assess dose attenuation by a Digisonic SP cochlear implant (CI) and evaluate its impact on treatment planning. The Digisonic CI was irradiated with 6 MV photons. Overall dose attenuation was assessed with MOSFET dosimeters and Gafchromic films. In addition, we evaluated the attenuation of separate CI components. Dose attenuation was also calculated using different radiation treatment planning systems (TPS) softwares and dose calculation algorithms. The CI was placed on a head phantom. Single-beam and multiple-beam plans were evaluated for dose attenuation using two radiation techniques (Conformal and Stereotactic radiotherapy) and four different algorithms (Clarkson, Point Kernel-Superposition, Ray Tracing and Monte Carlo). MOSFET and Gafchromics film showed maximal 6-7.5% radiation dose attenuation, at the center of the CI. Computerized TPS-based dose attenuation by the implant was 4-8.1%, using a single ipsilateral field. No clinically meaningful dose attenuation was found in multiple field plans owing to the contribution of various beam paths with only a couple going through the implant using either conventional conformal or stereotactic treatment plans. Dose attenuation induced by a Digisonic SP CI is about 6%, for single 6 MV photon field. This dose reduction is unlikely to be clinically significant, as single-field radiotherapy plans to this anatomic region are uncommon.