Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

PURPOSE: To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac. METHOD: Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm2 fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%). RESULTS: OSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements. CONCLUSIONS: These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac.

PURPOSE: The aim of this study is to investigate off-axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field. METHODS: Dosimetric measurements were performed using a microDiamond detector, Gafchromic® EBT3 film, and MOSkinTM . Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 . Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-to-surface distance (SSD) of 1.8 m for fields collimated centrally and off-axis. PDD measurements were also acquired at isocenter for each off-axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination. RESULTS: Off-axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkinTM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero. CONCLUSIONS: Experimental and simulation data were acquired for a range of field sizes to investigate off-axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off-axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

The use of collimator angle optimization and jaw tracking for VMAT-based single-isocenter multiple-target stereotactic radiosurgery for up to six targets in the Varian Eclipse treatment planning system.

PURPOSE: Island blocking occurs in single-isocenter multiple-target (SIMT) stereotactic radiotherapy (SRS) whenever targets share multi-leaf collimator (MLC) leaf pairs. This study investigated the effect on plan quality and delivery, of reducing island blocking through collimator angle optimization (CAO). In addition, the effect of jaw tracking in this context was also investigated. METHODS: For CAO, an algorithm was created that selects the collimator angle resulting in the lowest level of island blocking, for each beam in any given plan. Then, four volume-modulated arc therapy (VMAT) SIMT SRS plans each were generated for 10 retrospective patients: two CAO plans, with and without jaw tracking, and two plans with manually selected collimator angles, with and without jaw tracking. Plans were then assessed and compared using typical quality assurance procedures. RESULTS: There were no substantial differences between plans with and without CAO. Jaw tracking produced statistically significant reduction in low-dose level parameters; healthy brain V10% and mean dose were reduced by 9.66% and 15.58%, respectively. However, quantitative values (108 cc for V10% and 0.35 Gy for mean dose) were relatively small in relation to clinical relevance. Though there were no statistically significant changes in plan deliverability, there was a notable trend of plans with jaw tracking having lower gamma analysis pass rates. CONCLUSION: These findings suggest that CAO has limited benefit in VMAT SIMT SRS of 2-6 targets when using a low-dose penalty to the healthy brain during plan optimization in Eclipse. As clinical benefits of jaw tracking were found to be minimal and plan deliverability was potentially reduced, a cautious approach would be to exclude jaw tracking in SIMT SRS plans.

HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study.

PURPOSE: This study aims to assess the accuracy of source position verification during high-dose rate (HDR) prostate brachytherapy using a novel, in-house developed two-dimensional (2D) diode array (the Magic Plate), embedded exactly below the patient within a carbon fiber couch. The effect of tissue inhomogeneities on source localization accuracy is examined. METHOD: Monte Carlo (MC) simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 toolkit. An Ir-192 Flexisource (Isodose Control, Veenendaal, the Netherlands) was simulated inside a voxelized patient geometry, and the dose deposited in each detector of the Magic Plate evaluated. The dose deposited in each detector was then used to localize the source position using a proprietary reconstruction algorithm. RESULTS: The accuracy of source position verification using the Magic Plate embedded in the patient couch was found to be affected by the tissue inhomogeneities within the patient, with an average difference of 2.1 ± 0.8 mm (k = 1) between the Magic Plate predicted and known source positions. Recalculation of the simulations with all voxels assigned a density of water improved this verification accuracy to within 1 mm. CONCLUSION: Source position verification using the Magic Plate during a HDR prostate brachytherapy treatment was examined using MC simulations. In a homogenous geometry (water), the Magic Plate was able to localize the source to within 1 mm, however, the verification accuracy was negatively affected by inhomogeneities; this can be corrected for by using density information obtained from CT, making the proposed tool attractive for use as a real-time in vivo quality assurance (QA) device in HDR brachytherapy for prostate cancer.

Comparison of phantom materials for use in quality assurance of microbeam radiation therapy.

Microbeam radiation therapy (MRT) is a promising radiotherapy modality that uses arrays of spatially fractionated micrometre-sized beams of synchrotron radiation to irradiate tumours. Routine dosimetry quality assurance (QA) prior to treatment is necessary to identify any changes in beam condition from the treatment plan, and is undertaken using solid homogeneous phantoms. Solid phantoms are designed for, and routinely used in, megavoltage X-ray beam radiation therapy. These solid phantoms are not necessarily designed to be water-equivalent at low X-ray energies, and therefore may not be suitable for MRT QA. This work quantitatively determines the most appropriate solid phantom to use in dosimetric MRT QA. Simulated dose profiles of various phantom materials were compared with those calculated in water under the same conditions. The phantoms under consideration were RMI457 Solid Water (Gammex-RMI, Middleton, WI, USA), Plastic Water (CIRS, Norfolk, VA, USA), Plastic Water DT (CIRS, Norfolk, VA, USA), PAGAT (CIRS, Norfolk, VA, USA), RW3 Solid Phantom (PTW Freiburg, Freiburg, Germany), PMMA, Virtual Water (Med-Cal, Verona, WI, USA) and Perspex. RMI457 Solid Water and Virtual Water were found to be the best approximations for water in MRT dosimetry (within ±3% deviation in peak and 6% in valley). RW3 and Plastic Water DT approximate the relative dose distribution in water (within ±3% deviation in the peak and 5% in the valley). PAGAT, PMMA, Perspex and Plastic Water are not recommended to be used as phantoms for MRT QA, due to dosimetric discrepancies greater than 5%.

In vivo endorectal dosimetry of prostate tomotherapy using dual MOSkin detectors.

Verification of dose to the anterior rectal wall in helical tomotherapy to the prostate is important due to the close proximity of the rectal wall to the treatment field. The steep dose gradient makes these measurements challenging. A phantom-based study was completed, aimed at developing a system for measurement of anterior rectal wall doses during hypofractionated prostate stereotactic body radiotherapy (SBRT) utilizing tomotherapy delivery. An array of four dual MOSkinTM dosimeters, spaced 1 cm apart, was placed on a replica Rectafix® immobilization spacer device. This Perspex probe is a more rigid alternative to rectal balloons, to improve geometric reproducibility. The doses at each point were measured in real time and compared to doses calculated by the treatment planning system (TPS). Additionally, distance-to-agreement (DTA) measurements were acquired to assist in the comparison of measured and predicted doses. All dual MOSkin detectors measured dose to within ± 5% of the TPS at the anterior rectal wall. Whilst several points were outside of experimental error, the largest deviation from the TPS predicted dose represented a DTA of only 1.3 mm, within the acceptable DTA tolerance of 3 mm. Larger deviations of up to -11.9% were observed for the posterior and side walls; however, if acceptable DTA measurements are accounted for, then an agreement of 75% was observed. Although larger differences were observed at the other rectal wall locations, the overall effect of dose at these points was not as significant, given the lower doses. Despite the very high-dose gradient region, real-time measurements of the anterior rectal wall doses were within acceptable limits of TPS-predicted doses. The differences between measured and planned data were due to difficulties in precisely locating each detector on the TPS dose grid, which presented large variations in dose between CT voxels in regions of steep dose gradients. The dual MOSkin system would, therefore, be a useful device for detecting errors in real time, such as patient shifts or incorrect setup, during tomotherapy of the prostate.

Characterization of MOSkin detector for in vivo skin dose measurement during megavoltage radiotherapy.

In vivo dosimetry is important during radiotherapy to ensure the accuracy of the dose delivered to the treatment volume. A dosimeter should be characterized based on its application before it is used for in vivo dosimetry. In this study, we characterize a new MOSFET-based detector, the MOSkin detector, on surface for in vivo skin dosimetry. The advantages of the MOSkin detector are its water equivalent depth of measurement of 0.07 mm, small physical size with submicron dosimetric volume, and the ability to provide real-time readout. A MOSkin detector was calibrated and the reproducibility, linearity, and response over a large dose range to different threshold voltages were determined. Surface dose on solid water phantom was measured using MOSkin detector and compared with Markus ionization chamber and GAFCHROMIC EBT2 film measurements. Dependence in the response of the MOSkin detector on the surface of solid water phantom was also tested for different (i) source to surface distances (SSDs); (ii) field sizes; (iii) surface dose; (iv) radiation incident angles; and (v) wedges. The MOSkin detector showed excellent reproducibility and linearity for dose range of 50 cGy to 300 cGy. The MOSkin detector showed reliable response to different SSDs, field sizes, surface, radiation incident angles, and wedges. The MOSkin detector is suitable for in vivo skin dosimetry.

HDR brachytherapy afterloader quality assurance optimization using monolithic silicon strip detectors.

BACKGROUND: There currently exists no widespread high dose-rate (HDR) brachytherapy afterloader quality assurance (QA) tool for simultaneously assessing the afterloader's positional, temporal, transit velocity and air kerma strength accuracy. PURPOSE: The purpose of this study was to develop a precise and rigorous technique for performing daily QA of HDR brachytherapy afterloaders, incorporating QA of: dwell position accuracy, dwell time accuracy, transit velocity consistency and relative air kerma strength (AKS) of an Ir-192 source. METHOD: A Sharp ProGuide 240 mm catheter (Elekta Brachytherapy, Veenendaal, The Netherlands) was fixed 5 mm above a 256 channel epitaxial diode array 'dose magnifying glass' (DMG256) (Centre for Medical and Radiation Physics, University of Wollongong). Three dwell positions, each of 5.0 s dwell times, were spaced 13.0 mm apart along the array with the Flexitron HDR afterloader (Elekta Brachytherapy, Veenendaal, The Netherlands). The DMG256 was connected to a data acquisition system (DAQ) and a computer via USB2.0 link for live readout and post-processing. The outputted data files were analyzed using a Python script to provide positional and temporal localization of the Ir-192 source by tracking the centroid of the detected response. Measurements were repeated on a weekly basis, for a period of 5 weeks to determine the consistency of the measured parameters over an extended period. RESULTS: Using the DMG256 for relative AKS measurements resulted in measured values within 0.6%-3.0% of the expected activity over a 7-week period. The sub-millisecond temporal accuracy of the device allowed for measurements of the transit velocity with an average of (10.88 ± 1.01) cm/s for 13 mm steps. The dwell position localization for 1, 2, 3, 5, and 10 mm steps had an accuracy between 0.1 and 0.3 mm (3σ), with a fixed temporal accuracy of 10 ms. CONCLUSION: The DMG256 silicon strip detector allows for clinics to perform rigorous daily QA of HDR afterloader dwell position and dwell time accuracy with greater precision than the current standard methodology using closed circuit television and a stopwatch. Additionally, DMG256 unlocks the ability to perform measurements of transit velocity/time and relative AKS, which are not possible using current standard techniques.

Development of patient and catheter specific error thresholds for high dose rate prostate brachytherapy.

BACKGROUND: In-vivo source tracking has been an active topic of research in the field of high-dose rate brachytherapy in recent years to verify accuracy in treatment delivery. Although detection systems for source tracking are being developed, the allowable threshold of treatment error is still unknown and is likely patient-specific due to anatomy and planning variation. PURPOSE: The purpose of this study was to determine patient and catheter-specific shift error thresholds for in-vivo source tracking during high-dose-rate prostate brachytherapy (HDRPBT). METHODS: A module was developed in the previously described graphical processor unit multi-criteria optimization (gMCO) algorithm. The module generates systematic catheter shift errors retrospectively into HDRPBT treatment plans, performed on 50 patients. The catheter shift model iterates through the number of catheters shifted in the plan (from 1 to all catheters), the direction of shift (superior, inferior, medial, lateral, cranial, and caudal), and the magnitude of catheter shift (1-6 mm). For each combination of these parameters, 200 error plans were generated, randomly selecting the catheters in the plan to shift. After shifts were applied, dose volume histogram (DVH) parameters were re-calculated. Catheter shift thresholds were then derived based on plans where DVH parameters were clinically unacceptable (prostate V100 < 95%, urethra D0.1cc > 118%, and rectum Dmax > 80%). Catheter thresholds were also Pearson correlated to catheter robustness values. RESULTS: Patient-specific thresholds varied between 1 to 6 mm for all organs, in all shift directions. Overall, patient-specific thresholds typically decrease with an increasing number of catheters shifted. Anterior and inferior directions were less sensitive than other directions. Pearson's correlation test showed a strong correlation between catheter robustness and catheter thresholds for the rectum and urethra, with correlation values of -0.81 and -0.74, respectively (p < 0.01), but no correlation was found for the prostate. CONCLUSIONS: It was possible to determine thresholds for each patient, with thresholds showing dependence on shift direction, and number of catheters shifted. Not every catheter combination is explorable, however, this study shows the feasibility to determine patient-specific thresholds for clinical application. The correlation of patient-specific thresholds with the equivalent robustness value indicated the need for robustness consideration during plan optimization and treatment planning.

Investigation of in vivo source tracking error thresholds for interstitial and intra-cavitary high-dose-rate cervical brachytherapy.

Purpose: The purpose of this study was to determine a comprehensive in vivo source tracking error thresholds in high-dose-rate (HDR) brachytherapy for cervical cancer. Achieving this enables the definition of an action level for imminent in vivo source tracking technologies and treatment monitoring devices, preventing clinically relevant changes to the applied dose. Material and methods: Retrospective HDR interstitial (n = 10) and intra-cavitary (n = 20) cervical brachytherapy patients were randomly selected to determine the feasibility of implementing in vivo source tracking error thresholds. A script was developed to displace all dwell positions in each treatment plan, along all major axes from their original position. Dose-volume histogram (DVH) indices were calculated without re-optimization of modified plans to determine the appropriate in vivo source tracking error thresholds in each direction. Results: In vivo source tracking error thresholds were directionally dependent; the smallest were found to be 2 mm in the anterior and posterior directions for both interstitial and intra-cavitary treatments. High-risk clinical treatment volume (HR-CTV) coverage was significantly impacted by displacements of 4 to 5 mm along each axis. Critically, there was a large variation in DVH metrics with displacement due to change in dwell weightings and patient anatomy. Conclusions: Determining the dosimetric impact of dwell position displacement provides a clinical benchmark for the development of pre-treatment verification devices and an action level for real-time treatment monitoring. It was established that an in vivo source tracking error threshold needs to be patient-specific. In vivo source tracking error thresholds should be determined for each patient, and can be conducted with extension of the method established in this work.

Feasibility of online adaptive HDR prostate brachytherapy: A novel treatment concept.

PURPOSE: The purpose of this study was to determine the feasibility of online adaptive transrectal ultrasound (TRUS)-based high-dose-rate prostate brachytherapy (HDRPBT) through retrospective simulation of source positioning and catheter swap errors on patient treatment plans. METHOD: Source positioning errors (catheter shifts in 1 mm increments in the cranial/caudal, anterior/posterior, and medial/lateral directions up to ±6 mm) and catheter swap errors (between the most and least heavily weighted) were introduced retrospectively into DICOM treatment plans of 20 patients that previously received TRUS HDRPBT. Dose volume histogram (DVH) indices were monitored as errors were introduced sequentially into individual catheters, simulating potential errors throughout treatment. Whenever DVH indices were outside institution thresholds: prostate V100% <95%, urethra D0.1cc >118% and rectum Dmax >80%, the plan was adapted using remaining catheters (i.e., simulating previous catheters as previously delivered). The final DVH indices were recorded. RESULTS: Prostate coverage (V100% >95%) could be maintained for source position errors up to 6 mm through online plan adaptation. The source position error at which the urethra D0.1cc and rectum Dmax was able to return to clinically acceptable levels using online adaptation varied between 6 mm to 1 mm, depending on the direction of the source position error and patient anatomy. After introduction of catheter swap errors to patient plans, prostate V100% was recoverable using online adaptation to near original plan characteristics. Urethra D0.1cc and rectum Dmax showed less recoverability. CONCLUSION: Online adaptive HDRPBT maintains the prostate V100% to clinically acceptable values for majority of directional shifts. However, the current online adaptive method may not correct for source position errors near organs at risk.

HDR prostate brachytherapy plan robustness and its effect on in-vivo source tracking error thresholds: A multi-institutional study.

PURPOSE: The purpose of this study was to examine the effect of departmental planning techniques on appropriate in-vivo source tracking error thresholds for high dose rate (HDR) prostate brachytherapy (BT) treatments, and to determine if a single in-vivo source tracking error threshold would be appropriate for the same patient anatomy. METHODS: The prostate, rectum, and urethra were contoured on a single patient transrectal ultrasound (TRUS) dataset. Anonymized DICOM files were disseminated to 16 departments who created an HDR prostate BT treatment plan on the dataset with a prescription dose of 15 Gy in a single fraction. Departments were asked to follow their own local treatment planning guidelines. Source positioning errors were then simulated in the 16 treatment plans and the effect on dose-volume histogram (DVH) indices calculated. Change in DVH indices were used to determine appropriate in-vivo source tracking error thresholds. Plans were considered to require intervention if the following DVH conditions occurred: prostate V100% < 90%, urethra D0.1cc > 118%, and rectumtt Dmax > 80%. RESULTS: There was wide variation in appropriate in-vivo source tracking error thresholds among the 16 participating departments, ranging from 1 to 6 mm. Appropriate in-vivo source tracking error thresholds were also found to depend on the direction of the source positioning error and the endpoint. A robustness parameter was derived, and found to correlate with the sensitivity of plans to source positioning errors. CONCLUSIONS: A single HDR prostate BT in-vivo source tracking error threshold cannot be applied across multiple departments, even for the same patient anatomy. The burden on in-vivo source tracking devices may be eased through improving HDR prostate BT plan robustness during the plan optimisation phase.

Megavoltage cone beam CT near surface dose measurements: potential implications for breast radiotherapy.

PURPOSE: Cone beam computed tomography (CBCT) is fast becoming standard on modern linear accelerators. CBCT increases the dose to regions within and outside the treatment field, potentially increasing secondary cancer induction and toxicity. This study quantified megavoltage (MV) CBCT skin dose and compared it to skin dose delivered during standard tangential breast radiotherapy. METHOD: Dosimetry was performed both in- and out-of-field using thermoluminescent dosimeters (TLDs) and a metal-oxide-semiconductor-field-effect-transistor (MOSFET) detector specifically designed for skin dosimetry; these were placed superficially on a female anthropomorphic phantom. RESULTS: The skin dose from a single treatment fraction ranged from 0.5 to 1.4 Gy on the ipsilateral breast, 0.031-0.18 Gy on the contralateral breast, and 0-0.02 Gy in the head and pelvic region. An 8 MU MV CBCT delivered a skin dose that ranged from 0.02 to 0.05 Gy in the chest region and was less than 0.01 Gy in the head and pelvis regions. One MV CBCT per fraction was found to increase the out-of-field skin dose from both the CBCT and the treatment fields by approximately 20%. The imaging dose as a percentage of treatment doses in the ipsilateral breast region was 3% for both dosimeters. CONCLUSION: Imaging increases the skin dose to regions outside the treatment field particularly regions immediately adjacent the target volume. This small extra dose to the breasts should be considered when developing clinical protocols and assessing dose for clinical trials.

Towards real time in-vivo rectal dosimetry during trans-rectal ultrasound based high dose rate prostate brachytherapy using MOSkin dosimeters.

PURPOSE: To compare the dose measured by MOSkin dosimeters coupled to a trans-rectal ultrasound (TRUS) probe to the dose predicted by the brachytherapy treatment planning system (BTPS) during high dose rate (HDR) prostate brachytherapy (pBT), and to examine the feasibility of performing real-time catheter-by-catheter analysis of in-vivo rectal dosimetry during TRUS based HDR pBT. METHOD: Four MOSkin dosimeters were coupled to a TRUS probe during 20 TRUS-based HDR pBT treatment fractions. The measured MOSkin doses were retrospectively compared to those predicted by the BTPS for the total treatment fraction, as well as on a per catheter basis. RESULTS: The average relative percentage difference between MOSkin measured and BTPS predicted doses for a total treatment fraction was 0.3% ± 11.6% (k = 1), with a maximum of 23.2% and a minimum of -29.0%. The average relative percentage difference per catheter was +2.5% ± 16.9% (k = 1). The majority (64%) of per catheter MOSkin measured doses agreed with the treatment planning system within the calculated uncertainty budget of 12.3%. CONCLUSION: The results of the study agreed well with previously published data, despite differences in clinical workflows. To improve the redundancy to potential dosimeter errors, a minimum of 4 MOSkin dosimeters should be used when performing real-time in-vivo rectal dosimetry for HDR pBT, and error thresholds should be based off the total combined uncertainty estimate of measurement. 'Real time' error thresholds can be more confidently applied in the future through enhanced integration between IVD systems with both the imaging device and the BTPS/afterloader.

eXaSkin: A novel high-density bolus for 6MV X-rays radiotherapy.

PURPOSE: To evaluate eXaSkin, a novel high-density bolus alternative to commercial tissue-equivalent Superflab, for 6MV photon-beam radiotherapy. MATERIALS AND METHODS: We delivered a 10 × 10 cm2 open field at 90° and head-and-neck clinical plan, generated with the volumetric modulated arc therapy (VMAT) technique, to an anthropomorphic phantom in three scenarios: with no bolus on the phantom's surface, with Superflab, and with eXaSkin. In each scenario, we measured dose to a central planning target volume (PTV) in the nasopharynx region with an ionization chamber, and we measured dose to the skin, at three different positions within the vicinity of a neck lymph node PTV, with MOSkin™, a semiconductor dosimeter. Measurements were compared against calculations with the treatment planning system (TPS). RESULTS: For the static field, MOSkin results underneath the eXaSkin were in agreement with calculations to within 1.22%; for VMAT, to within 5.68%. Underneath Superflab, those values were 3.36% and 11.66%. The inferior agreement can be explained by suboptimal adherence of Superflab to the phantom's surface as well as difficulties in accurately reproducing its placement between imaging and treatment session. In all scenarios, dose measured at the central target agreed to within 1% with calculations. CONCLUSIONS: eXaSkin was shown to have superior adaptation to the phantom's surface, producing minimal air gaps between the skin surface and bolus, allowing for accurate positioning and reproducibility of set-up conditions. eXaSkin with its high density material provides sufficient build-up to achieve full skin dose with less material thickness than Superflab.

In vivo verification of superficial dose for head and neck treatments using intensity-modulated techniques.

Skin dose is one of the key issues for clinical dosimetry in radiation therapy. Currently planning computer systems are unable to accurately predict dose in the buildup region, leaving ambiguity as to the dose levels actually received by the patient's skin during radiotherapy. This is one of the prime reasons why in vivo measurements are necessary to estimate the dose in the buildup region. A newly developed metal-oxide-semiconductor-field-effect-transistor (MOSFET) detector designed specifically for dose measurements in rapidly changing dose gradients was introduced for accurate in vivo skin dosimetry. The feasibility of this detector for skin dose measurements was verified in comparison with plane parallel ionization chamber and radiochromic films. The accuracy of a commercial treatment planning system (TPS) in skin dose calculations for intensity-modulated radiation therapy treatment of nasopharyngeal carcinoma was evaluated using MOSFET detectors in an anthropomorphic phantom as well as on the patients. Results show that this newly developed MOSFET detector can provide a minimal but highly reproducible intrinsic buildup of 7 mg cm(-2) corresponding to the requirements of personal surface dose equivalent Hp (0.07). The reproducibility of the MOSFET response, in high sensitivity mode, is found to be better than 2% at the phantom surface for the doses normally delivered to the patients. The MOSFET detector agrees well with the Attix chamber and the EBT Gafchromic film in terms of surface and buildup region dose measurements, even for oblique incident beams. While the dose difference between MOSFET measurements and TPS calculations is within measurement uncertainty for the depths equal to or greater than 0.5 cm, an overestimation of up to 8.5% was found for the surface dose calculations in the anthropomorphic phantom study. In vivo skin dose measurements reveal that the dose difference between the MOSFET results and the TPS calculations was on average -7.2%, ranging from -4.3% to -9.2%. The newly designed MOSFET detector encapsulated into a thin water protective film has a minimal reproducible intrinsic buildup recommended for skin dosimetry. This feature makes it very suitable for routine IMRT QA and accurate in vivo skin dosimetry.

A novel quality assurance system for eye plaque brachytherapy.

Eye Plaque brachytherapy pre-treatment quality assurance (QA) conducted clinically involves an activity verification of individual seeds via well chamber and does not include a physical measurement of dose-rate of the final assembly. A novel spectroscopic, dose-rate detection system, was evaluated for pre-treatment QA of eye plaque brachytherapy. The system includes a water phantom with sterility management. The system was calibrated using a known-activity I-125 seed, measured at 1 cm in water along the radial axis, compared to TG-43 U1 calculations and verified over a number of distances. A depth dose curve was acquired for a clinical, mixed activity eye plaque and two 'error' plaques. The probe was stepped from a water equivalent source to a detector distance (SDD) of 2.5 to 12 mm along the plaque central axis. The latter measurements aimed to characterise the sensitivity of the system. The calculated and measured single-seed dose-rates agreed to within 0.5 cGy/h from a SDD of 3 mm and above. The clinical plaque showed agreement between measured and treatment planning system (TPS) calculated dose-rates within 2%. Sensitivity testing resulted in a maximum deviation from TPS data of 18%, therefore was able to detect the presence of packing errors. The dose-rate detection system was successfully evaluated for verification of I-125 based eye plaques without compromising sterility, allowing for quick pre-treatment, dose-rate verification of patient-ready plaques. Its agreement with TPS data for the unmodified plaque and its deviation when introducing errors confirms the approach suggested is a viable QA tool.

Dose verification for liver target volumes undergoing respiratory motion.

Respiratory motion has a significant impact on dose delivered to abdominal targets during radiotherapy treatment. Accurate treatment of liver tumours adjacent to the diaphragm is complicated by large respiratory movement, as well as differing tissue densities at the lung-liver interface. This study aims to evaluate the accuracy of dose delivered to superior liver tumours using passive respiratory monitoring, in the absence of gating technology, for a range of treatment techniques. An in-house respiratory phantom was designed and constructed to simulate the lung and liver anatomy. The phantom consisted of adjacent slabs of lung and liver equivalent materials and a cam drive system to emulate respiratory motion. A CC04 ionisation chamber and Gafchromic EBT3 film were used to perform point dose and dose plane measurements respectively. Plans were calculated using an Elekta Monaco treatment planning system (TPS) on exhale phase study sets for conformal, volume modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) techniques, with breathing rates of 8, 14 and 23 bpm. Analysis confirmed the conformal delivery protocol currently used for this site within the department is suitable. The experiments also determined that VMAT is a viable alternative technique for treatment of superior liver lesions undergoing respiratory motion and was superior to IMRT. Furthermore, the measurements highlighted the need for respiratory management in these cases. Displacements due to respiration exceeding planned margins could result in reduced coverage of the clinical target volume and much higher doses to the lung than anticipated.

Characterisation of out-of-field dose at shallow depths for external beam radiotherapy: implications for eye lens dose.

Re-evaluation of the eye lens radio-sensitivity by the ICRP in 2011 resulted in a significant reduction of the threshold for lens opacities from 8 Gy to 0.5 Gy. This has led to an increase in concern for eye lens doses from treatment sites further from the eye than previously considered. The aim of this study was to examine the out-of-field dose far from the field edge and develop an effective method to accurately characterise the constituent components of this dose at varying depths. Dose profile scans using a 0.6 cm3 cylindrical ionisation chamber in a motorised water tank were compared with previous studies and displayed good agreement. At points more than 20 cm from the field edge patient scatter becomes insignificant, and the dose is dominated by head leakage and collimator scatter. Point depth-dose measurements made with a Roos parallel plate chamber in solid water at distances of 52 cm and 76 cm from central axis showed that the highest dose is at the surface. Since the sensitive region of the eye can be as shallow as 3 mm, in vivo measurements carried out with a detector with buildup more than 3 mm water equivalent thickness may be underestimating the dose to the lens. It is therefore recommended that for in vivo measurements for the eye lens further than 20 cm from the field edge the detector should have only 3 mm build-up material over the effective point of measurement.

Derivation of in vivo source tracking error thresholds for TRUS-based HDR prostate brachytherapy through simulation of source positioning errors.

PURPOSE: The purpose of this study was to simulate treatment planning source positioning errors in transrectal ultrasound-based real-time high-dose-rate prostate brachytherapy treatments and determine appropriate in vivo source tracking error thresholds. METHODS AND MATERIALS: Treatment planning source positioning errors were simulated for 20 patient plans in the brachytherapy treatment planning system by manually adjusting the dwell position coordinates within selected catheters without plan reoptimization. The change in dose-volume histogram (DVH) indices was calculated as a function of the source positioning error. The magnitude of the change in the DVH indices was then used to derive appropriate in vivo source tracking error thresholds. RESULTS: Source positioning error thresholds to prevent potentially significant changes in prostate (target) DVH metrics ranged from 2 to 5 mm, dependent on the direction of the source positioning error, as well as the relative weight of the dwell position within the plan, and its position relative to the patient anatomy. Source positioning error thresholds to prevent potentially clinically significant changes in organ at risk DVH metrics were found to be complex and patient-dependent. CONCLUSIONS: In vivo source tracking error thresholds for transrectal ultrasound-based real-time high-dose-rate prostate brachytherapy were investigated via the simulation of treatment planning source positioning errors. These error thresholds were found to be dependent not only on the direction of the error, but also on the endpoint. There is still the potential for larger changes in DVH indices to occur for catheter shifts smaller than the proposed threshold levels in this study.