Steven Sylvander

Technical Note: Relationships between gamma criteria and action levels: Results of a multicenter audit of gamma agreement index results.

PURPOSE The aim of this work was to use a multicenter audit of modulated radiotherapy quality assurance (QA) data to provide a practical examination of gamma evaluation criteria and action level selection. The use of the gamma evaluation method for patient-specific pretreatment QA is widespread, with most commercial solutions implementing the method. METHODS Gamma agreement indices were calculated using the criteria 1%/1 mm, 2%/2 mm, 2%/3 mm, 3%/2 mm, 3%/3 mm, and 5%/3 mm for 1265 pretreatment QA measurements, planned at seven treatment centers, using four different treatment planning systems, delivered using three different delivery systems (intensity-modulated radiation therapy, volumetric-modulated arc therapy, and helical tomotherapy) and measured using three different dose measurement systems. The sensitivity of each pair of gamma criteria was evaluated relative to the gamma agreement indices calculated using 3%/3 mm. RESULTS A linear relationship was observed for 2%/2 mm, 2%/3 mm, and 3%/2 mm. This result implies that most beams failing at 3%/3 mm would also fail for those criteria, if the action level was adjusted appropriately. Some borderline plans might be passed or failed depending on the relative priority (tighter tolerance) used for dose difference or distance to agreement evaluation. Dosimeter resolution and treatment modality were found to have a smaller effect on the results of QA measurements than the number of dimensions (2D or 3D) over which the gamma evaluation was calculated. CONCLUSIONS This work provides a method (and a large sample of results) for calculating equivalent action levels for different gamma evaluation criteria. This work constitutes a valuable guide for clinical decision making and a means to compare published gamma evaluation results from studies using different evaluation criteria. More generally, the data provided by this work support the recommendation that gamma criteria that specifically prioritize the property of greatest clinical importance for each treatment modality of anatomical site should be selected when using gamma evaluations for modulated radiotherapy QA. It is therefore suggested that departments using the gamma evaluation as a QA analysis tool should consider the relative importance of dose difference and distance to agreement, when selecting gamma evaluation criteria.

Effects of changing modulation and pitch parameters on tomotherapy delivery quality assurance plans

This study was aimed at investigating delivery quality assurance (DQA) discrepancies observed for helical tomotherapy plans. A selection of tomotherapy plans that initially failed the DQA process was chosen for this investigation. These plans failed the fluence analysis as assessed using gamma criteria (3%, 3 mm) with radiographic film. Each of these plans was modified (keeping the planning constraints the same), beamlets rebatched and reoptimized. By increasing and decreasing the modulation factor, the fluence in a circumferential plane as measured with a diode array was assessed. A subset of these plans was investigated using varied pitch values. Metrics for each plan that were examined were point doses, fluences, leaf opening times, planned leaf sinograms, and uniformity indices. In order to ensure that the treatment constraints remained the same, the dose‐volume histograms (DVHs) of all the modulated plans were compared to the original plan. It was observed that a large increase in the modulation factor did not significantly improve DVH uniformity, but reduced the gamma analysis pass rate. This also increased the treatment delivery time by slowing down the gantry rotation speed which then increases the maximum to mean non‐zero leaf open time ratio. Increasing and decreasing the pitch value did not substantially change treatment time, but the delivery accuracy was adversely affected. This may be due to many other factors, such as the complexity of the treatment plan and site. Patient sites included in this study were head and neck, right breast, prostate, abdomen, adrenal, and brain. The impact of leaf timing inaccuracies on plans was greater with higher modulation factors. Point‐dose measurements were seen to be less susceptible to changes in pitch and modulation factors. The initial modulation factor used by the optimizer, such that the TPS generated ‘actual’ modulation factor within the range of 1.4 to 2.5, resulted in an improved deliverable plan. PACS number: 87.55.‐x, 87.55.Qr, 87.55.D‐

Radiological properties of 3D printed materials in kilovoltage and megavoltage photon beams

PURPOSE This study evaluates the radiological properties of different 3D printing materials for a range of photon energies, including kV and MV CT imaging and MV radiotherapy beams. METHODS The CT values of a number of materials were measured on an Aquilion One CT scanner at 80kVp, 120kVp and a Tomotherapy Hi Art MVCT imaging beam. Attenuation of the materials in a 6MV radiotherapy beam was investigated. RESULTS Plastic filaments printed with various infill densities have CT values of -743±4, -580±1 and -113±3 in 120kVp CT images which approximate the CT values of low-density lung, high-density lung and soft tissue respectively. Metal-infused plastic filaments printed with a 90% infill density have CT values of 658±1 and 739±6 in MVCT images which approximate the attenuation of cortical bone. The effective relative electron density REDeff is used to describe the attenuation of a megavoltage treatment beam, taking into account effects relating to the atomic number and mass density of the material. Plastic filaments printed with a 90% infill density have REDeff values of 1.02±0.03 and 0.94±0.02 which approximate the relative electron density RED of soft tissue. Printed resins have REDeff values of 1.11±0.03 and 1.09±0.03 which approximate the RED of bone mineral. CONCLUSIONS 3D printers can model a variety of body tissues which can be used to create phantoms useful for both imaging and dosimetric studies.

Radiotherapy dose measurements using a fluorescing quinine solution

Quinine solutions fluoresce when exposed to ionising radiation, through the production and absorption of Cerenkov radiation. This study evaluated the feasibility of using ‘household’ tonic water as a radiotherapy dosimeter. Tonic water samples were irradiated with static beams for a variety of energies and dose rates: 6 and 10 MV photons at 600 MU/min; 6 MV flattening-filter-free photons at 1400 MU/min; 6, 9, 12, 15 and 18 meV electrons at 400 MU/min and 6 meV electrons at approximately 2474 MU/min (used for total skin electron irradiation). A sliding window IMRT field was delivered using a 6 MV photon beam at 600 MU/min, to assess dynamic response. Fluorescence was successfully recorded using a monochrome low light CCD camera placed on the treatment couch as well as the treatment room visual monitoring system in the linear accelerator control area. Energy dependence and dose rate independence were observed. While limitations in the bit-depth and focal length of the camera prevented precise quantitative analysis of depth dose profiles for conventional dose rates (≤600 MU/min), performance for higher dose rates (in terms of signal-to-noise in depth dose profiles) was comparable to radiochromic film. Potential use includes measurement of dose in the build-up region, efficient checks of beam energy and tomographic reconstruction of 4D dose delivery, though further optimisation of fluorescent signal acquisition is required.

Optimising a Radiotherapy Optical Surface Monitoring System to Account for the Effects of Patient Skin Contour and Skin Colour

Optical surface monitoring systems (OSMSs) are designed to assist patient setup and patient motion management during radiotherapy treatments. Systems use projected and reflected patterns of coloured light on the patients surface and therefore depend on the skin’s optical absorbance and reflectance properties, which can vary with surface shape and colour. This study aimed to identify optimal operating parameters for the Catalyst HD OSMS (C-rad, Uppsala, Sweden) when used to monitor the surfaces of 3D-printed objects with various convex and concave surfaces, one of which was painted in six different colours with various levels of red and black saturation (from light pink to dark grey). The degree of surface detection was assessed via the Catalyst HD interface, with different levels of gain (100–600%) and signal integration time (1–7 ms). The OSMS was able to detect horizontal and convex shapes more consistently than vertical or steeply angled surfaces. The OSMS was not able to detect the darkest surface at all, even with the highest gain and the longest integration times. Mid-grey surfaces were detectable only when the integration time was increased to 2 s. All pink surfaces were easily detectable at the shortest integration time, with the OSMS performing best when red saturation was highest. Further work is recommended, as the red undertone of all human skin may lead to improved results for real patients. However, these preliminary results indicate that careful commissioning and optimisation of OSMS systems may be required before they can be used in radiotherapy treatments for a broad patient cohort.

Technical Note: Dose distributions in the vicinity of high‐density implants using 3D gel dosimeters

Purpose In this work, we develop a methodology for using Fricke gel dosimeters for dose distribution measurements surrounding high‐density implants which circumvents artifact production by removing the obstruction during imaging. Methods Custom 3D printed molds were used to set cavities in Fricke gel phantoms to allow for the suspension of high‐density implants in different geometries. This allowed for the metal valve extracted from a temporary tissue expander to be suspended during irradiation, and removed during optical‐CT scanning. Results The removal of the metal implant and subsequent backfilling of the remaining cavity with optically matched fluid prior to dose evaluation enables accurate optical‐CT scanning of the gel dosimeters. Results have shown very good agreement between measured and calculated doses within 2 mm from the surface of the implant. Slight deviations are present within 1 mm of the interface. Conclusions Artifacts in the form of radial streaking, cold spots, and hot spots were all reduced using this technique, enabling the broader and more accurate use of optical‐CT for the imaging of gels containing opaque objects.

Can a commercial gel dosimetry system be used to verify stereotactic spinal radiotherapy treatment dose distributions

This study investigated the use of the TruView xylenol-orange-based gel and VISTA optical CT scanner (both by Modus Medical Inc, London, Canada), for use in verifying the accuracy of planned dose distributions for hypo-fractionated (stereotactic) vertebral treatments. Gel measurements were carried out using three stereotactic vertebral treatments and compared with planned doses calculated using the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, USA) as well as with film measurements made using Gafchromic EBT3 film (Ashland Inc, Covington, USA), to investigate the accuracy of the gel system. The gel was calibrated with reference to a moderate-dose gradient region in one of the gel samples. Generally, the gel measurements were able to approximate the close agreement between the doses calculated by the treatment planning system and the doses measured using film (which agreed with each other within 2%), despite lower resolution and bit depth. Poorer agreement was observed when the dose delivered to the gel exceeded the range of doses delivered in the calibration region. This commercial gel dosimetry system may be used to verify hypo-fractionated treatments of vertebral targets, although separate gel calibration measurements are recommended.