Determining tolerance levels for quality assurance of 3D printed bolus for modulated arc radiotherapy of the nose.

Given the existing literature on the subject, there is obviously a need for specific advice on quality assurance (QA) tolerances for departments using or implementing 3D printed bolus for radiotherapy treatments. With a view to providing initial suggested QA tolerances for 3D printed bolus, this study evaluated the dosimetric effects of changes in bolus geometry and density, for a particularly common and challenging clinical situation: specifically, volumetric modulated arc therapy (VMAT) treatment of the nose. Film-based dose verification measurements demonstrated that both the AAA and the AXB algorithms used by the Varian Eclipse treatment planning system (Varian Medical Systems, Palo Alto, USA) were capable of providing sufficiently accurate dose calculations to allow this planning system to be used to evaluate the effects of bolus errors on dose distributions from VMAT treatments of the nose. Thereafter, the AAA and AXB algorithms were used to calculate the dosimetric effects of applying a range of simulated errors to the design of a virtual bolus, to identify QA tolerances that could be used to avoid clinically significant effects from common printing errors. Results were generally consistent, whether the treatment target was superficial and treated with counter-rotating coplanar arcs or more-penetrating and treated with noncoplanar arcs, and whether the dose was calculated using the AAA algorithm or the AXB algorithm. The results of this study suggest the following QA tolerances are advisable, when 3D printed bolus is fabricated for use in photon VMAT treatments of the nose: bolus relative electron density variation within [Formula: see text] (although an action level at [Formula: see text] may be permissible); bolus thickness variation within [Formula: see text] mm (or 0.5 mm variation on opposite sides); and air gap between bolus and skin [Formula: see text] mm. These tolerances should be investigated for validity with respect to other treatment modalities and anatomical sites. This study provides a set of baselines for future comparisons and a useful method for identifying additional or alternative 3D printed bolus QA tolerances.

ASSESSMENT OF INTEGRITY AND LEAD-EQUIVALENCE OF SHIELDED GARMENTS USING TWO-DIMENSIONAL X-RAY IMAGES FROM A COMPUTED TOMOGRAPHY SCANNER.

Shielded garments are widely recommended for occupational radiation protection in diagnostic and interventional radiology. This study investigated a novel method for efficiently verifying shielded garment integrity while simultaneously acquiring data for lead-equivalence measurements, using two-dimensional topogram images from computed tomography (CT) scanners. This method was tested against more-conventional measurements with superficial and orthovoltage radiotherapy treatment beams, for 12 shielded garments containing 3 different lead-free shielding materials. Despite some energy-dependent results, all shielded garments approximately achieved their specified lead-equivalence for the energy range expected during clinical use for fluoroscopy procedures, except for three shielded skirts that required two layers of material to be overlapped at the front. All lead-equivalence measurements from CT topograms agreed with or conservatively underestimated the kV narrow-beam results. This method is potentially useful for independently assessing the shielding properties of new shielded garments and performing annual checks for damage or degradation of existing shielded garments.

Patient-specific quality assurance on a Varian Halcyon linear accelerator using a PTW Octavius 4D device.

This study investigates and validates the use of the Octavius 4D system for patient specific quality assurance on Halcyon, which is capable of rotating at 4 revolutions per minute (RPM). A commercially available PTW Octavius 4D system was used for this study which had a maximum rotation speed of 3 RPM. Initial validation included testing the accuracy of the inclinometer, percent depth doses (PDD), output factors, and dose profiles for selected static square fields. The same static fields were also subject to a gamma comparison with the TPS. This was followed by an evaluation of twelve clinical treatment plans and seven non-clinical plans with varying gantry rotation speeds. All testing was completed using detector array measurement times of 200 ms and 100 ms. Inclinometer accuracy was within 0.3° of actual gantry angle. Output factors varied less than 0.6%, PDD differences were no greater than 1.4%, and dose profile differences were less than 2.2%. Gamma pass rates for the static fields were 96.7% (2%/2mm) and 99.7% (3%/3mm). A prototype control unit, which had a maximum rotation speed of 4 RPM was also used to test the clinical and non-clinical plans. For the clinical plans, the mean gamma pass rates (2%/2mm) were 86.1% and 88.1% for the commercial unit and prototype unit respectively. Results using a measurement time of 200 ms were superior to those using 100 ms. For Halcyon deliveries greater than 3 RPM, worst case gamma results for the commercial unit were 28.6% compared to 98.5% using the prototype unit. Accurate patient specific quality assurance results can be obtained using the Octavius 4D system with a Halcyon linac, provided that the system measurement time is kept at 200 ms and the rotation speed of Halycon does not exceed 3 RPM. For higher RPM deliveries, an Octavius 4D unit with 4 RPM rotation capability is recommended.

Development of a method for treating lower-eyelid carcinomas using superficial high dose rate brachytherapy.

In this study, a method was developed for delivering high dose rate (HDR) brachytherapy treatments to basal cell carcinomas (BCCs) as well as squamous cell carcinomas (SCCs) of the lower eyelid via superficial catheters. Clinically-realistic BCC/SCC treatment areas were marked in the lower-eyelid region on a head phantom and several arrangements of catheters and bolus were trialled for treating those areas. The use of one or two catheters of different types was evaluated, and sources of dosimetric uncertainty (including air gaps) were evaluated and mitigated. Test treatments were planned for delivery with an iridium-192 source, using the Oncentra Brachy treatment planning system (Elekta AB, Stockholm, Sweden). Dose distributions were evaluated using radiochromic film. The proposed method was shown to be clinically viable, for using superficial HDR brachytherapy to overcome anatomical difficulties and create non-surgical treatments for BCC and SCC of the lower eyelid.

Clinical implementation of a Monte Carlo based independent TPS dose checking system.

The increase in complexity of treatment plans over time through modalities such as intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) has often not been met with an increase in capability of the secondary dose calculation checking systems typically used to verify the treatment planning system. Monte Carlo (MC) codes such as EGSnrc have become easily available and are capable of performing calculations of highly complex radiotherapy treatments. This educational note demonstrates a method for implementing and using a fully automated system for performing and analysing full MC calculations of conformal, IMRT and VMAT radiotherapy plans. Example calculations were based on BEAMnrc/DOSXYZnrc and are performed automatically after either uploading exported plan DICOM data through a Python-based web interface, or exporting DICOM data to a monitored network location. This note demonstrates how completed MC calculations can then be analysed using an automatically generated dose point comparison report, or easily re-imported back into the treatment planning system. Agreement between the TPS and MC calculation was an improvement on agreement between RadCalc and the TPS, with differences ranging from 1.2 to 5.5% between RadCalc and the treatment planning system (TPS), and 0.1-1.7% between MC and TPS. Comparison of the dose-volume histogram (DVH) parameters [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] for the example VMAT plans showed agreement for the mean planning target volume dose within [Formula: see text], [Formula: see text] and [Formula: see text] generally within [Formula: see text] with the exception of a brain case, and [Formula: see text] within [Formula: see text]. Overall, this note provides a demonstration of a system that has been integrated well into existing clinical workflow, and has been shown to be a valuable additional tool in the secondary checking of treatment plan calculations.

Assessing the fit of 3D printed bolus from CT, optical scanner and photogrammetry methods.

Bolus plays an important role in the radiation therapy of superficial lesions and the application of 3D printing to its design can improve fit and dosimetry. This study quantitatively compares the fits of boluses designed from different imaging modalities. A head phantom was imaged using three systems: a CT simulator, a 3D optical scanner, and an interchangeable lens camera. Nose boluses were designed and 3D printed from each modality. A 3D printed phantom with air gaps of known thicknesses was used to calibrate mean HU to measure air gaps of unknown thickness and assess the fit of each bolus on the head phantom. The bolus created from the optical scanner data resulted in the best fit, with a mean air gap of 0.16 mm. Smoothing of the CT bolus resulted in a more clinically suitable model, comparable to that from the optical scanner method. The bolus produced from the photogrammetry method resulted in air gaps larger than 1 mm in thickness. The use of optical scanner and photogrammetry models have many advantages over the conventional bolus-from-CT method, however workflow should be refined to ensure accuracy if implemented clinically.

Quasi-simultaneous 3D printing of muscle-, lung- and bone-equivalent media: a proof-of-concept study.

3D printing is a promising solution for the production of bespoke phantoms and phantom components, for radiotherapy dosimetry and quality assurance (QA) purposes. This proof-of-concept study investigated the use of a dual-head printer to deposit two different filaments (polylactic acid (PLA) and StoneFil PLA-concrete (Formfutura BV, Nijmegen, Netherlands)) at several different in-fill densities, to achieve quasi-simultaneous 3D printing of muscle-, lung- and bone-equivalent media. A Raise 3D Pro 3D printer (Raise 3D Technologies Inc, Irvine, USA) was used to print one thoracic and one cranial phantom slab. Analysis using in-house 3D print QA software showed that the two humanoid phantom slabs geometrically matched the stereolithography (STL) files on which they were based, within 0.3 mm, except in one area of the thoracic slab that was affected by thermal warping by up to 3.4 mm. The 3D printed muscle, lung and bone materials in the two humanoid phantom slabs were approximately radiologically-equivalent to human muscle, lung and bone. In particular, the use of StoneFil with a nominally constant in-fill density of 100% resulted in regions that were approximately inner-bone-equivalent, at kV and MV energies. These regions were bounded by walls that were substantially denser than inner bone, although generally not dense enough to be truly cortical-bone-equivalent. This proof-of-concept study demonstrated a method by which multiple tissue-equivalent materials (eg. muscle-, lung- and bone-equivalent media) can be deposited within one 3D print, allowing complex phantom components to be fabricated efficiently in a clinical setting.

A very low diffusion Fricke gel dosimeter with functionalised xylenol orange-PVA (XOPVA).

A gel dosimeter has been developed utilising a recently reported system for reducing Fe3+ diffusion in a Fricke gel dosimeter which chelates xylenol orange to the gelling agent poly(vinyl alcohol) (PVA). Formulations were investigated using both gelatin and PVA as the gelling agent, along with the inclusion of glyoxal. The resulting gel had an optical density dose response of 0.0031 Gy-1, an auto-oxidation rate of 0.000 23 h-1, and a diffusion rate of 0.132 mm2 h-1 which is a significant improvement over previously reported gelatin based Fricke gel dosimeters. The gel was also shown to be energy and dose-rate independent and could be reused after irradiation. Thus, this gel dosimeter has the potential to provide a safe and practical solution to three dimensional radiation dosimetry in the medical environment.

Application of retrospective data analysis to clinical protocol design: can the potential benefits of breath-hold techniques for breast radiotherapy be assessed without testing on patients?

The advantages, in terms of heart dose sparing, resulting from using a breath-hold technique when treating supine left breast radiotherapy patients are widely accepted, and increasing numbers of radiotherapy departments are implementing breath-hold techniques. However, due to differences in patient setup and treatment planning protocols between radiotherapy departments, it is important to assess the benefits of using a breath-hold technique within each department, before or during implementation. This study investigated the use of retrospective analysis of past patient treatment plans, as a means to identify the potential for breath-hold techniques to benefit patients. In-house "Treatment and Dose Assessor" code was used to complete a bulk retrospective evaluation of dose-volume metrics for 708 supine and 13 prone breast and chest wall radiotherapy treatments, that were planned using the same clinical protocols, which did not utilise a breath hold technique. For supine patients, results showed statistically significant differences between heart doses from left and right breast treatment plans, in the absence of significant differences between lung doses from left and right breast treatment plans, confirming the potential benefit of using a breath-hold technique for supine left breast radiotherapy patients. Fewer than 1% of the right breast treatment plans showed heart doses high enough to suggest a possible benefit from using a breath-hold technique. Approximately 50% of the prone left breast treatment plans included very low heart doses without intervention, and may therefore have shown no noticeable dosimetric benefit from the use of a breath hold. This study demonstrated the extent of information that can be obtained using retrospective data analysis, before or instead of obtaining multiple CT images of patients and completing a process of dual planning and prospective dose evaluation.

Accuracy and efficiency of published film dosimetry techniques using a flat-bed scanner and EBT3 film.

Gafchromic EBT3 film is widely used for patient specific quality assurance of complex treatment plans. Film dosimetry techniques commonly involve the use of transmission scanning to produce TIFF files, which are analysed using a non-linear calibration relationship between the dose and red channel net optical density (netOD). Numerous film calibration techniques featured in the literature have not been independently verified or evaluated. A range of previously published film dosimetry techniques were re-evaluated, to identify whether these methods produce better results than the commonly-used non-linear, netOD method. EBT3 film was irradiated at calibration doses between 0 and 4000 cGy and 25 pieces of film were irradiated at 200 cGy to evaluate uniformity. The film was scanned using two different scanners: The Epson Perfection V800 and the Epson Expression 10000XL. Calibration curves, uncertainty in the fit of the curve, overall uncertainty and uniformity were calculated following the methods described by the different calibration techniques. It was found that protocols based on a conventional film dosimetry technique produced results that were accurate and uniform to within 1%, while some of the unconventional techniques produced much higher uncertainties (> 25% for some techniques). Some of the uncommon methods produced reliable results when irradiated to the standard treatment doses (< 400 cGy), however none could be recommended as an efficient or accurate replacement for a common film analysis technique which uses transmission scanning, red colour channel analysis, netOD and a non-linear calibration curve for measuring doses up to 4000 cGy when using EBT3 film.

Improving the quality of reconstructed X-ray CT images of polymer gel dosimeters: zero-scan coupled with adaptive mean filtering.

This study evaluated the feasibility of combining the 'zero-scan' (ZS) X-ray computed tomography (CT) based polymer gel dosimeter (PGD) readout with adaptive mean (AM) filtering for improving the signal to noise ratio (SNR), and to compare these results with available average scan (AS) X-ray CT readout techniques. NIPAM PGD were manufactured, irradiated with 6 MV photons, CT imaged and processed in Matlab. AM filter for two iterations, with 3 × 3 and 5 × 5 pixels (kernel size), was used in two scenarios (a) the CT images were subjected to AM filtering (pre-processing) and these were further employed to generate AS and ZS gel images, and (b) the AS and ZS images were first reconstructed from the CT images and then AM filtering was carried out (post-processing). SNR was computed in an ROI of 30 × 30 for different pre and post processing cases. Results showed that the ZS technique combined with AM filtering resulted in improved SNR. Using the previously-recommended 25 images for reconstruction the ZS pre-processed protocol can give an increase of 44% and 80% in SNR for 3 × 3 and 5 × 5 kernel sizes respectively. However, post processing using both techniques and filter sizes introduced blur and a reduction in the spatial resolution. Based on this work, it is possible to recommend that the ZS method may be combined with pre-processed AM filtering using appropriate kernel size, to produce a large increase in the SNR of the reconstructed PGD images.

Effects of inaccurate small field dose measurements on calculated treatment doses.

Given the difficulty and potential time- or financial-costs associated with accurate small field dosimetry, this study aimed to establish the clinical necessity of obtaining accurate small field output factor measurements and to evaluate the effects on planned doses that could arise if accurate measurements are not used in treatment planning dose calculations. Isocentre doses, in heterogeneous patient anatomy, were calculated and compared for 571 beams from 48 clinical radiotherapy treatments, using a clinical radiotherapy treatment planning system, with reference to two different sets of beam configuration data. One set of beam configuration data included field output factors (total scatter factors) from precisely positioned and response-corrected diode measurements and the other included field output factors measured using a conventional technique that would have been better suited to larger field measurements. Differences between the field output factor measurements made with the two different techniques equated to 14.2 % for the 6 [Formula: see text] 6 mm[Formula: see text] field, 1.8 % for the 12 [Formula: see text] 12 mm[Formula: see text] field, and less than 0.5 % for the larger fields. This led to isocentre dose differences of up to 3.3 % in routine clinical fields smaller than 9 mm across and and up to 11 % in convoluted fields smaller than 15 mm across. If field widths smaller than 15 mm are used clinically, then accurate measurement (or-remeasurement) of small field output factors in the treatment planning system's beam data is required in order to achieve dose calculation accuracy within 3 %. If such measurements are not completed, then errors in excess of 10 % may occur if very small, narrow, concave or convoluted treatment fields are used.

Bulk evaluation and comparison of radiotherapy treatment plans for breast cancer.

This study provides a bulk, retrospective analysis of 151 breast and chest wall radiotherapy treatment plans, as a small-scale demonstration of the potential breadth and value of the information that may be obtained from clinical data mining. The treatments were planned at three centres belonging to one organisation over a period of 3 months. All 151 plans were used to evaluate inter-centre consistency and compliance with a local planning protocol. A subset of 79 plans, from one centre, were used in a more detailed evaluation of the effects of anatomical asymmetry on heart and lung dose, the effects of a metallic temporary tissue expander port on dose homogeneity and the overall conformity and homogeneity achieved in routine breast treatment planning. Differences in anatomical structure contouring and nomenclature were identified between the three centres, with all centres showing some non-compliance with the local planning protocol. When evaluated against standard conformity indices, these breast plans performed relatively poorly. However, when evaluated against recommended organ-at-risk tolerances, all evaluated plans performed sufficiently well that tighter planning tolerances could be recommended for future planning. Heart doses calculated in left breast and chest wall treatments were significantly higher than heart doses calculated in right sided breast and chest wall treatments (p < 0.001). In the treatment involving a temporary tissue expander, the inflated implant effectively pushed the targeted breast tissue away from the healthy tissues, leading to a dose distribution that was relatively conformal, although attenuation through the tissue expander's metallic port may have been underestimated by the treatment planning system. The results of this study exemplify the use of bulk treatment planning data to evaluate clinical workloads and inform ongoing treatment planning.

Women in medical physics: a preliminary analysis of workforce and research participation in Australia and New Zealand.

Although the participation of women within the science, technology, engineering and mathematics workforces has been widely discussed over recent decades, the recording and analysis of data pertaining to the gender balance of medical physicists in Australia and New Zealand remains rare. This study aimed to provide a baseline for evaluating future changes in workforce demographics by quantifying the current level of representation of women in the Australasian medical physics workforce and providing an indication of the relative contribution made by those women to the local research environment. The 2015 Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) member directory and list of chief physicists at ACPSEM-accredited radiation oncology and diagnostic imaging training centres were interrogated to identify the gender balance of medical physicists working in Australia and New Zealand. A specific investigation of the employment levels of all medical physicists in Queensland was undertaken to provide an example of the gender balance at different levels of seniority in one large Australian state. Lists of authors of medical physics presentations at ACPSEM annual conferences and authors of publications in the ACPSEM's official journal, were used to provide an indication of the gender balance in published research within Australia and New Zealand. The results of this study showed that women currently constitute approximately 28 % of the medical physics workforce in Australia and New Zealand, distributed disproportionally in junior roles; there is a decrease in female participation in the field with increasing levels of seniority, which is particularly apparent in the stratified data obtained for the Queensland workforce. Comparisons with older data suggest that this situation has changed little since 2008. Examination of ACPSEM conference presentations suggested that there are similar disparities between the gender-balance of proffered and invited or keynote speakers (28 % and 13 % from female authors) and the gender balance of certified and chief physicists (28 % and 21 % female). The representation of women in the ACPSEM journal does not differ substantially between authorship of proffered versus invited work (22 % and 19 % from female authors). While this work was limited to evaluating the membership, annual conference and official journal of the ACPSEM (rather than evaluating the entire medical physics workforce and the contributions of male and female physicists to international conferences and publications), this study nonetheless led to the following recommendations: that a longitudinal study analysing correlations between age, period of service, seniority and gender should be undertaken and that future ACPSEM workforce surveys should include analyses of gender representation.

Use of electronic portal imaging devices for electron treatment verification.

This study aims to help broaden the use of electronic portal imaging devices (EPIDs) for pre-treatment patient positioning verification, from photon-beam radiotherapy to photon- and electron-beam radiotherapy, by proposing and testing a method for acquiring clinically-useful EPID images of patient anatomy using electron beams, with a view to enabling and encouraging further research in this area. EPID images used in this study were acquired using all available beams from a linac configured to deliver electron beams with nominal energies of 6, 9, 12, 16 and 20 MeV, as well as photon beams with nominal energies of 6 and 10 MV. A widely-available heterogeneous, approximately-humanoid, thorax phantom was used, to provide an indication of the contrast and noise produced when imaging different types of tissue with comparatively realistic thicknesses. The acquired images were automatically calibrated, corrected for the effects of variations in the sensitivity of individual photodiodes, using a flood field image. For electron beam imaging, flood field EPID calibration images were acquired with and without the placement of blocks of water-equivalent plastic (with thicknesses approximately equal to the practical range of electrons in the plastic) placed upstream of the EPID, to filter out the primary electron beam, leaving only the bremsstrahlung photon signal. While the electron beam images acquired using a standard (unfiltered) flood field calibration were observed to be noisy and difficult to interpret, the electron beam images acquired using the filtered flood field calibration showed tissues and bony anatomy with levels of contrast and noise that were similar to the contrast and noise levels seen in the clinically acceptable photon beam EPID images. The best electron beam imaging results (highest contrast, signal-to-noise and contrast-to-noise ratios) were achieved when the images were acquired using the higher energy electron beams (16 and 20 MeV) when the EPID was calibrated using an intermediate (12 MeV) electron beam energy. These results demonstrate the feasibility of acquiring clinically-useful EPID images of patient anatomy using electron beams and suggest important avenues for future investigation, thus enabling and encouraging further research in this area. There is manifest potential for the EPID imaging method proposed in this work to lead to the clinical use of electron beam imaging for geometric verification of electron treatments in the future.

Technical Note: Preliminary investigations into the use of a functionalised polymer to reduce diffusion in Fricke gel dosimeters.

PURPOSE: A modification of the existing PVA-FX hydrogel has been made to investigate the use of a functionalised polymer in a Fricke gel dosimetry system to decrease Fe(3+) diffusion. METHODS: The chelating agent, xylenol orange, was chemically bonded to the gelling agent, polyvinyl alcohol (PVA) to create xylenol orange functionalised PVA (XO-PVA). A gel was created from the XO-PVA (20% w/v) with ferrous sulfate (0.4 mM) and sulfuric acid (50 mM). RESULTS: This resulted in an optical density dose sensitivity of 0.014 Gy(-1), an auto-oxidation rate of 0.0005 h(-1), and a diffusion rate of 0.129 mm(2) h(-1); an 8% reduction compared to the original PVA-FX gel, which in practical terms adds approximately 1 h to the time span between irradiation and accurate read-out. CONCLUSIONS: Because this initial method of chemically bonding xylenol orange to polyvinyl alcohol has inherently low conversion, the improvement on existing gel systems is minimal when compared to the drawbacks. More efficient methods of functionalising polyvinyl alcohol with xylenol orange must be developed for this system to gain clinical relevance.

A comparison between direct TMR measurements and TMRs calculated from PDDs using BJR Supplement 25 data for flattened and unflattened photon beams.

This study assessed the validity of the conversion from percentage depth dose (PDD) to tissue maximum ratio (TMR) using BJR Supplement 25 data for flattened and flattening filter free (FFF) beams. PDD and TMR scans for a variety of field sizes were measured in water using a Sun Nuclear Corporation 3D SCANNER™ on a Varian TrueBeam linear accelerator in 6 MV, 10 MV and 6 MV FFF beams. The BJR Supplement 25 data was used to convert the measured PDDs to TMRs and these were compared with the directly measured TMR data. The TMR plots calculated from PDD were within 1% for the 10 MV and 6 MV flattened beams, for field sizes 3 cm × 3 cm to 40 cm × 40 cm inclusive, at depths measured beyond the depth of maximum dose. The disagreement between the measured and calculated TMR plots for the 6 MV FFF beam increased with depth and field size to a maximum of 1.7% for a 40 cm × 40 cm field. The results found in this study indicate that the BJR Supplement 25 data should not be used for field sizes larger than 20 cm × 20 cm at depths greater than 15 cm for the 6 MV FFF beam. It is advised that PDD to TMR conversion for FFF beams should be done with phantom scatter ratios appropriate to FFF beams, or the TMR should be directly measured if required.

Field size consistency of nominally matched linacs.

Given that there is increasing recognition of the effect that sub-millimetre changes in collimator position can have on radiotherapy beam dosimetry, this study aimed to evaluate the potential variability in small field collimation that may exist between otherwise matched linacs. Field sizes and field output factors were measured using radiochromic film and an electron diode, for jaw- and MLC-collimated fields produced by eight dosimetrically matched Varian iX linacs (Varian Medical Systems, Palo Alto, USA). This study used nominal sizes from 0.6 × 0.6 to 10 × 10 cm(2), for jaw-collimated fields, and from 1 × 1 to 10 × 10 cm(2) for MLC-collimated fields, delivered from a zero (head up, beam directed vertically downward) gantry angle. Differences between the field sizes measured for the eight linacs exceeded the uncertainty of the film measurements and the repositioning uncertainty of the jaws and MLCs on one linac. The dimensions of fields defined by MLC leaves were more consistent between linacs, while also differing more from their nominal values than fields defined by orthogonal jaws. The field output factors measured for the different linacs generally increased with increasing measured field size for the nominal 0.6 × 0.6 to 1 × 1 cm(2) fields, and became consistent between linacs for nominal field sizes of 2 × 2 cm(2) and larger. The inclusion in radiotherapy treatment planning system beam data of small field output factors acquired in fields collimated by jaws (rather than the more-reproducible MLCs), associated with either the nominal or the measured field sizes, should be viewed with caution. The size and reproducibility of the fields (especially the small fields) used to acquire treatment planning data should be investigated thoroughly as part of the linac or planning system commissioning process. Further investigation of these issues, using different linac models, collimation systems and beam orientations, is recommended.

Clinical use of diodes and micro-chambers to obtain accurate small field output factor measurements.

There have been substantial advances in small field dosimetry techniques and technologies, over the last decade, which have dramatically improved the achievable accuracy of small field dose measurements. This educational note aims to help radiation oncology medical physicists to apply some of these advances in clinical practice. The evaluation of a set of small field output factors (total scatter factors) is used to exemplify a detailed measurement and simulation procedure and as a basis for discussing the possible effects of simplifying that procedure. Field output factors were measured with an unshielded diode and a micro-ionisation chamber, at the centre of a set of square fields defined by a micro-multileaf collimator. Nominal field sizes investigated ranged from 6 × 6 to 98 × 98 mm(2). Diode measurements in fields smaller than 30 mm across were corrected using response factors calculated using Monte Carlo simulations of the diode geometry and daisy-chained to match micro-chamber measurements at intermediate field sizes. Diode measurements in fields smaller than 15 mm across were repeated twelve times over three separate measurement sessions, to evaluate the reproducibility of the radiation field size and its correspondence with the nominal field size. The five readings that contributed to each measurement on each day varied by up to 0.26  %, for the "very small" fields smaller than 15 mm, and 0.18 % for the fields larger than 15 mm. The diode response factors calculated for the unshielded diode agreed with previously published results, within uncertainties. The measured dimensions of the very small fields differed by up to 0.3 mm, across the different measurement sessions, contributing an uncertainty of up to 1.2 % to the very small field output factors. The overall uncertainties in the field output factors were 1.8 % for the very small fields and 1.1 % for the fields larger than 15 mm across. Recommended steps for acquiring small field output factor measurements for use in radiotherapy treatment planning system beam configuration data are provided.

Examination of the properties of IMRT and VMAT beams and evaluation against pre-treatment quality assurance results.

This study aimed to provide a detailed evaluation and comparison of a range of modulated beam evaluation metrics, in terms of their correlation with QA testing results and their variation between treatment sites, for a large number of treatments. Ten metrics including the modulation index (MI), fluence map complexity, modulation complexity score (MCS), mean aperture displacement (MAD) and small aperture score (SAS) were evaluated for 546 beams from 122 intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans targeting the anus, rectum, endometrium, brain, head and neck and prostate. The calculated sets of metrics were evaluated in terms of their relationships to each other and their correlation with the results of electronic portal imaging based quality assurance (QA) evaluations of the treatment beams. Evaluation of the MI, MAD and SAS suggested that beams used in treatments of the anus, rectum, head and neck were more complex than the prostate and brain treatment beams. Seven of the ten beam complexity metrics were found to be strongly correlated with the results from QA testing of the IMRT beams (p < 0.00008). For example, values of SAS (with multileaf collimator apertures narrower than 10 mm defined as 'small') less than 0.2 also identified QA passing IMRT beams with 100% specificity. However, few of the metrics are correlated with the results from QA testing of the VMAT beams, whether they were evaluated as whole 360° arcs or as 60° sub-arcs. Select evaluation of beam complexity metrics (at least MI, MCS and SAS) is therefore recommended, as an intermediate step in the IMRT QA chain. Such evaluation may also be useful as a means of periodically reviewing VMAT planning or optimiser performance.