BACKGROUND:Diffusion-weighted imaging (DWI), a quantitative magnetic resonance imaging (qMRI) technique, has the potential to aid in disease characterization and treatment response monitoring. MR-Linacs (MRLs) enable simultaneous DWI acquisitions during radiotherapy, uniquely aiding in the collection of large-scale datasets for imaging biomarkers, such as the DWI-derived apparent diffusion coefficient (ADC), without additional patient burden. However, the limited data reporting on variability in MRL scanner performance characteristics, and a lack of established clinical trial quality assurance (QA) procedures, are barriers to this route for biomarker validation. PURPOSE:This study aims to quantify the accuracy, intra-scanner repeatability, and inter-scanner reproducibility of ADC measurements across three MRLs in Australia in both a phantom and in vivo. These measurements will inform the feasibility of carrying out prospective multi-center studies in Australia investigating ADC as a biomarker and form a core set of QA procedures and baselines to assess biomarker and sequence suitability. METHODS:An isotropic diffusion phantom (at 0°C) and one healthy volunteer were scanned on three Unity MRLs (Elekta AB, Stockholm, Sweden). Standardized (QIBA Diffusion Profile) and anatomy-specific DWI sequences, including sequences recommended by the MR-Linac Consortium Imaging Biomarker Working Group, were used to image the phantom and volunteer. ADC maps generated using the MRL scanner software (inline ADC) and diffusion-weighted (b-value) images were exported from the scanner console. The latter was used to generate ADC maps using commercial software (offline ADC) for a separate comparative analysis. Performance metrics were computed for each sequence, including a coefficient of variation to assess between-session intra-scanner repeatability (CVBS) and inter-scanner reproducibility (CV), for each phantom vial and contoured organ. Additionally, using the phantoms' known ADC vial values, a percentage bias (bias) was calculated to determine ADC accuracy. RESULTS:Phantom-based measurements for the standardized QIBA sequence had intra- and inter-scanner CV and bias well within recommended guideline (QIBA Diffusion Profile) tolerance limits of 2.2% and ±3.6%, respectively. All anatomy-specific phantom DWI sequences were also within these tolerances, except for the cervix sequence at one site which showed an average intra-scanner bias of +4.5%. Both accuracy and reproducibility for all sequences were worse for lower diffusivity vials measured in the phantom. Additionally, inline and offline ADC maps had high similarity with average percent differences of +0.2%. Volunteer-based results had worse reproducibility, with the average inter-scanner CV for the brain and pancreas sequences within 9.0%, however, reaching up to 27.1% for pelvis and abdomen sequences. CONCLUSIONS:This study demonstrated accuracy, intra-scanner repeatability, and inter-scanner reproducibility comparable to metrics reported in the literature, using both the phantom and volunteer datasets. The cervix sequence had the largest variability in both phantom and volunteer results and was recommended for further investigation. This study suggests that qMRI techniques utilizing DWI could be a viable option for future multi-centered patient-based studies utilizing Australian MRLs, with phantom-based quality assurance recommended alongside patient imaging.

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.

Objective.Dose due to the electron streaming effect (ESE) is a significant contribution to out-of-field dose on the Elekta Unity MR-Linac. The aim of this work is to provide a systematic comparison of calculated and measured streaming dose for this system.Approach.Beams 1.0 × 1.0 cm2to 5.0 × 5.0 cm2, gantry 90.0°, 1000 MU, were incident on an in-house phantom. At the beam entrance and exit surfaces of the phantom, ESE was generated in theY-direction (IEC 61217). EBT3 film, orientated within theX-Zplane and at 14.0 mm depth in a solid water block, was used to determine ESE dose 5.0 cm beyond the phantom. The experimental arrangement was simulated in the Monaco v5.4 treatment planning system (TPS), utilising a CT phantom dataset with differing relative electron densities (RED) for the surrounding air. Horizontal (Xdirection) and vertical (Zdirection) film dose profiles were compared to the corresponding TPS profiles.Main results. For each field, the maximum ESE dose was observed at the beam exit, the magnitude of which decreases with decreasing field size. For the 5.0 × 5.0 cm2field, the exit and entry ESE doses were 19.6% and 7.0% of theDmaxdose to water, respectively. Across horizontal profiles, differences (simulated-measured) were reduced with smaller fields and lower RED. The maximum absolute profile difference was 1.7% of theDmaxdose to water for optimal RED and isocentre location. In vertical profiles an offset consistent with the Lorentz force was observed relative to theX-Yisoplane.Significance. For the fields investigated, maximum absolute differences (simulated-measured) ≤ 5.2% occurred in peak regions of ESE, at the beam entrance and exit from the phantom. Generally, there is good agreement between Monaco simulated and measured ESE. Simulated out-of-field dose is sensitive to the RED assigned to air structures and unforced RED optimises out-of-field dose calculation accuracy.

INTRODUCTION:The magnetic resonance linear accelerator (MRL) combines both magnetic resonance imaging and a linear accelerator, allowing for daily treatment adaptation. This study aimed to assess the impact of radiologist-delivered training in magnetic resonance (MR) contouring of relevant structures within the male pelvis.METHODS:Two radiation oncologists, two radiation oncology registrars and seven radiation therapists completed contouring on 10 male pelvis MR datasets both pre- and post-training. A 2-hour MR anatomy training session was delivered by a radiologist, who also provided the 'gold standard' contours. The pre- and post-training contours were compared against the gold standard with Dice similarity coefficient (DSC) and Hausdorff distances calculated; and the pre- and post-confidence scores and timing were compared.RESULTS:The improvement in DSC were significant in prostate, rectum and seminal vesicles, with a post-training median DSC of 0.87 ± 0.06, 0.92 ± 0.04 and 0.80 ± 0.14, respectively. The median Hausdorff improved with a median of 1.46 ± 0.78 mm, 0.52 ± 0.32 mm and 1.11 ± 0.86 mm for prostate, rectum and seminal vesicles, respectively. Bladder concordance was high both pre- and post-training. Urethra contours improved post-training, however, remained difficult to contour with a median post-DSC of 0.51 ± 0.24. Overall, confidence scoring improved (P < 0.001) and timing decreased by an average of 4.4 ± 16.4 min post-training.CONCLUSION:Radiologist-delivered training improved concordance of male pelvis contouring on MR datasets. Further work is required in the identification of urethra on MRs. These findings are of importance in the MRL adaptive workflow.

Magnetic resonance-guided radiotherapy technology is relatively new and commissioning publications, quality assurance (QA) protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptations of vendor supplied phantoms facilitated determination of gantry angle accuracy and linac isocentre, whereas in-house developed phantoms were used for end-to-end testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during treatment plan commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was approximately 0.6%. Beam quality as determined by TPR20,10 was 0.705 ± 0.001, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0% (2.0%, 2.0 mm). Machine output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During treatment plan commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0% (2.0%, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The plan QA techniques used in this work can highlight issues with machine behaviour when appropriate gamma criteria are set.

The last decade has shown a growing interest in image-guided radiotherapy. One such promising avenue of clinical growth is the use of hybrid MRI-linear accelerators which offer soft-tissue contrast of target volumes and organs at risk at the time of treatment for each fraction. This chapter describes the technical aspects associated with their development and subsequent use. It focuses on both the two currently available commercial systems and the two works-in-progress systems that exist.

The presence of the static field in magnetic resonance guided radiation therapy (MRgRT) systems, such as the Elekta Unity MR-Linac (MRL), influences charged particle motion due to the Lorentz force in and around patients. Consequently, for the Unity, changes to out-of-field dose (OFD) relative to conventional linacs occur due to the electron streaming effect (ESE) and spiraling contaminant electrons (SCE). This work investigates OFD associated with irradiation of the anterior MR imaging coil, which is in situ for all treatments on the Unity. Film measurements and Monaco simulations were performed to quantify the magnitude of OFD at the superior and inferior ends of the coil as a function of coil tilt relative to the beam direction. The dependence of OFD on field size and the relative electron density (RED) assigned to the coil and surrounding air are reported. The doses at both coil ends were clinically significant, with nearly 23.0% of the Dmax dose to water being recorded for the largest field (8.0 × 22.0 cm2) and 6.8% for the smallest field (3.0 × 3.0 cm2). Monaco simulations of OFD agreed with film within 5.0%, when appropriate calculation conditions were set. OFD decreases as coil tilt is reduced, and there is no evidence of ESE when the coil is horizontal. Clinically, the potential magnitude of cranio–caudal streaming dose from a tilted coil necessitates the use of appropriate shielding. A clinical case involving coil-induced ESE during treatment of a lesion in the right angle is presented. The planning-based investigation revealed that ESE doses associated with the coil and an immobilization vacuum bag can be clinically significant.

We investigate the properties of a light emitting diode (LED) flatbed scanner for use with EBT3 and EBT-XD film types in a clinical radiochromic film (RCF) dosimetry program with modern treatment techniques. The flatbed scanner was characterised in terms of lateral and longitudinal response, X–Y scaling integrity, scanning reproducibility, scanner warm up dependence and film orientation dependence. The preferred lateral response artefact (LRA) corrections are investigated for the LED light source. Supporting evidence is provided regarding the dose independent nature of the corrections while also providing results suggesting a potential film type independence. Results from 2D gamma analysis of four patient treatments were compared between the new 12000XL and existing 10000XL model. Lastly, a dose uncertainty analysis was performed for the film–scanner system combination. It may be concluded that the lateral response variation requires correction while the longitudinal response variation is insignificant. The linear scaling in the lateral and longitudinal directions are within 0.5% and the scanner reproducibility is stable. Scanner warm up dependence no longer exists, and effort should be made to maintain all film orientation in a study set within 15°. The LRA corrections are as reported substantially dose independent and there is evidence to support film type independence. Comparative gamma analysis of patient specific dose maps between the EPSON 10000XL (xenon fluorescent lamp) and 12000XL (LED) scanners showed that results are indistinguishable for both film types across the two scanner models when the necessary corrections are applied. Dose uncertainty is in agreement with the literature and can be kept below 3% with necessary corrections applied.

During the adaptive workflow associated with MRgRT, a secondary dose calculation is required and MU2net (DOSIsoft, France) is one commercial option. The suitability of MU2net to be used in conjunction with the online Monaco treatment planning system of the Elekta Unity (Elekta AB, Stockholm, Sweden), is evaluated in this work. Monaco and MU2net point doses are compared for various fields on and off axis and at different SSDs. To investigate the comparative effects of attenuation due to the cryostat, couch and posterior coil, measured, MU2net and Monaco dose outputs at the isocentre, as a function of gantry angle, were compared. Point doses for the beams of nine step and shoot IMRT (SSIMRT) test plans (courtesy Elekta) were calculated with Monaco v5.4 and compared to corresponding doses computed with MU2net. In addition, Monaco v5.4 and MU2net point doses were compared for 1552 beams treated on the Unity at our facility. For the on-axis fields investigated the agreement between MU2net and measured data is acceptable. MU2net and Monaco point doses for the Elekta SSIMRT test plans were within ± 5.0% and ± 6.4% for beams delivered from gantry zero and at planned beam angles, respectively. For the 1552 beams delivered approximately 80.0% of MU2net and Monaco point doses agree within ± 5.0%, therefore it is recommended to correlate MU2net Dose Reference Points (DRPs) with pre and post treatment dosimetry verification. Computational accuracy of MU2net could be enhanced with improved modelling of attenuation due to the couch, cryostat and posterior MR imaging coil.

Lead shielding is commonly used in the delivery of superficial radiotherapy albeit that the toxicity of this substance is of concern. The feasibility of using a non-toxic alternative, AttenuFlex™, is assessed using Xstrahl and Sensus treatment units. A series of lead and AttenuFlex™ circular cut outs and applicators were used with superficial beams (1.0–8.5 mm Al HVL) to measure percentage depth dose (PDD), output factors (OF) and surface dose correction factors (DCF). X-ray transmission for each material was determined for each beam quality. For these measurements an Advanced Markus chamber either embedded within a virtual water phantom (PDD, OF, transmission) or placed on the surface of the phantom with entrance window downstream (DCF), was used. The depth of the phantom is 10 cm for PDD and surface OF measurements. DCF(t) measurements were obtained with underlying lead or AttenuFlex™ at depth t = 0.1–10 cm. Additionally, using EBT3 film fluorescent surface doses, to non-target tissue, due to underlying lead or AttenuFlex™ were compared. PDDs and OFs for both materials were within ± 1%. Lead and AttenuFlex™ transmission differences were clinically acceptable, all transmission values were < 5% and non-target doses were comparable. The variation of DCF(t) for lead and AttenuFlex™ exhibit a minima for all beams. In the minima region energy and applicator dependent differences between DCF(lead) and DCF(AttenuFlex™) are observed. These differences do not preclude the use of AttenuFlex™ as an alternative to lead in superficial therapy.

This issue of Journal of Medical Radiation Sciences includes two papers presenting different uses of magnetic resonance (MR) in radiation therapy (RT). With the advancement of MR-simulators and Magnetic resonance linear accelerators (MRL), in addition to the use of diagnostic MR becoming more common place in the radiotherapy setting, there are a number of challenges to be considered. In this article, we present the perspectives of radiation therapists and medical physicists involved in the commissioning of an MRL in our centre. Image shows in-house 3D printed supports mounted on the vendor-supplied QA platform. The supports locate an array so that it is centred in the radiation field.

With the clinical introduction of MR-linacs, out-of-field dose (OFD) associated with head leakage/scatter (HLS), spiralling contaminant electrons (SCE) and the electron streaming effect (ESE) is of interest. To investigate HLS and SCE, EBT3 film on solid water 5.0 cm beyond each edge of a 10.0 × 10.0 cm2 field was used to determine depth-dose for 0 T and 1.5 T, in the isocentric plane. Additionally, ESE induced by the anterior imaging coil was quantified and the experimental arrangements to measure SCE and ESE were modelled using Monaco. For a clinical treatment of supraclavicular nodal disease, Monaco OFD was compared to in vivo measurements. For 0 T, depth-dose was isotropic and surface dose was approximately 4.4% of Dmax. With 1.5 T surface doses were approximately 3.8% of Dmax at ± Y (IEC61217), compared to 2.6% and 0.6% of Dmax at − X and X, respectively. For both field strengths, the TPS depth-dose variation was consistent with experimental trends; however, near surface doses calculated at ± Y differed significantly from measurements. For the field sizes investigated, measured coil ESE dose was between 9.0 and 28.0% of Dmax and Monaco coil ESE was less than measured by up to 13.0%. OFD in 0 T and 1.5 T are comparable at ± Y, inconsistent with previous work. Anterior coil ESE should be mitigated during treatment and for the clinical case investigated, in vivo OFD was within 2σ of TPS calculations. Monaco overestimates near surface SCE and underestimates coil ESE.

Background and purpose: Synthetic diamond detectors offer real time measurement of dose in radiotherapy applications which require high spatial resolution. Additional considerations and corrections are required for measurements where the diamond detector is orientated at various angles to the incident beam. This study investigated diamond detectors for end-to-end testing of Stereotactic Body Radiotherapy (SBRT) and Stereotactic Radiosurgery (SRS) in the context of dosimetry audits. Material and methods: Seven individual diamond detectors were investigated and compared with respect to warm up stability, dose-rate dependence, linearity, detector shadowing, energy response, cross-calibration, angular dependence and positional sensitivity in SBRT and SRS. Results: Large variation in the cross calibration factors was found between the seven individual detectors. For each detector, the energy dependence in the cross calibration factor was on average <0.6% across the beam qualities investigated (Co-60 Gamma Knife, and MV beams with TPR20, 10 0.684-0.733). The angular corrections for individual fields were up to 5%, and varied with field size. However, the average angular dependence for all fields in a typical SRS treatment delivery was <1%. The overall measurement uncertainty was 3.6% and 3.1% (2 sigma) for an SRS and SBRT treatment plan respectively. Conclusion: Synthetic diamond detectors were found to be reliable and robust for end-to-end dosimetry in SBRT and SRS applications. Orientation of the detector relative to the beam axis is an important consideration, as significant corrections are required for angular dependence.

An Advanced Markus chamber on the surface of solid water phantom was used to determine surface dose reduction, with either a lead or air interface, as a function of surface-interface separation (t). The beam quality dependence of dose reduction was investigated using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 superficial X-ray unit. For each beam the dose correction factor, DCF(t), namely the ratio of surface dose (t) to surface dose (t = 100 mm), was determined. Monte Carlo simulations of DCF(t) with a lead interface were compared with corresponding measured values. Simulated spectra were calculated at the phantom surface for full backscatter (t = 100 mm) and with either a lead or air interface at 2 mm or 8 mm depth. For each depth and beam quality lead fluorescent radiation at the surface was evident. The variation of DCF(t) for each beam and field size exhibits a minima at t ≈ 5 mm and in the range 1 mm ≤ t ≤ 40 mm surface dose reduction is larger for 100 kV than 150 kV. Monte Carlo simulated DCF(t) are consistent with corresponding measured DCF(t). From simulated spectra L-series fluorescent X-rays (≈ 15 keV) emanating from lead at t = 2 mm are evident for all beams and fluorescent K-series X-rays only occur with 100 kV and 150 kV beams.

Abstract Introduction To implement the modulated arc total body irradiation (MATBI) technique within the existing infrastructure of a radiation oncology department. The technique needed to treat paediatric patients of all ages, some of whom would require general anaesthesia (GA). Methods The MATBI technique required minor modifications to be incorporated within existing departmental infrastructure. Ancillary equipment essential to the technique were identified and in some cases custom designed to meet health and safety criteria. GA equipment was also considered. To evaluate the effectiveness of the implemented technique, an audit of the cases clinically treated was conducted. Results A motorised treatment couch was designed to allow the patient to be positioned in stabilisation equipment at a height, then lowered to the floor to accommodate source‐to‐skin‐distances from 180 cm to 198 cm to treat the fixed 40 cm × 40 cm field size. Treatment couch design also facilitated positioning of the bespoke two‐part spoiler. While organ at risk dose is limited using a beam weight optimisation technique, the dose is further reduced using compensators placed close to the patient's skin on a 3D printed custom‐made support bridge. A digital radiography system is used to verify compensator position. Fifteen patients have been treated to date for various diseases using a variety of dose fractionations ranging from 2 Gy in a single fraction to 12 Gy in 6 fractions. Five patients have required GA due to age or behavioural issues. Conclusion The modified MATBI technique and the equipment required for treatment delivery has been found to be well tolerated by all patients.

PURPOSE:Measurement of surface dose reduction effects for superficial x-rays incident on tissue with underlying air or bone and comparison with Monte Carlo simulations of such effects. Further to investigate the correlation between surface dose reduction and changes in Compton backscatter spectra with tissue-bone separation.METHODS:An Advanced Markus chamber with entrance window facing downstream on the surface of a solid water phantom was used to investigate changes in surface dose with an underlying air or bone interface located at various depths below the surface. Chamber readings were obtained for interface depths ranging from 1 to 100 mm using the 50 kV, 100 kV and 150 kV beams of an Xstrahl 150 x-ray unit, with field diameters (ϕ) = 2.5 cm and 5 cm. For each beam quality and field size the dose correction factor, DCF(t), namely the ratio of measured dose (t) to dose (t = 100 mm) was determined. Monte Carlo simulations of DCF(t) for air and bone interfaces in tissue are used to validate corresponding measured data. For a given beam and field size, the difference between simulated spectra with an air or bone interface at t = 3 mm was used to determine the Compton backscatter from bone at the surface.RESULTS:For air, DCF(t < 40 mm) is less than unity and for a given t DCF(t) decreases as beam energy increases from 50 kV to 150 kV. Conversely for bone DCF(t < 40 mm) increases for a given t with increasing beam energy. In particular for t = 1 mm, ϕ = 5 cm, DCF for air(bone) are 0.90(0.92) and 0.86(0.96) for 50 kV and 150 kV, respectively. For the same tube potentials corresponding factors, ϕ = 2.5 cm, for air(bone) are 0.94(0.96) and 0.92(0.99). Calculated DCF(t) based on Monte Carlo simulations are consistent with experimental observations to within 2%. Monte Carlo simulations of x-ray spectra demonstrate the presence of Compton backscatter from underlying bone in tissue. With bone at 3 mm depth calculated backscatter spectra at the tissue surface suggest that surface dose is influenced by the proximity of bone and that this effect depends on beam quality.CONCLUSIONS:This work demonstrates the feasibility of using an Advanced Markus chamber with entrance window facing downstream to investigate surface dose reduction with underlying air or bone in tissue. As the field size decreases and beam quality increases surface dose with underlying bone tends to full backscatter values even though tissue thicknesses are below those normally associated with full backscatter. Conversely with underlying bone close to the surface dose will increasingly fall below full backscatter values as the beam energy is reduced and field size is increased.

In this work, overshoot and undershoot effects associated with step‐and‐shoot IMRT (SSIMRT) delivery on a Varian Clinac 21iX are investigated, and their impact on patient‐specific QA point dose measurements and treatment plan delivery are evaluated. Pinnacle3 SSIMRT plans consisting of 5, 10, and 15 identical MLC defined segments and MU/segment values of 5 MU, 10 MU, and 20 MU were utilized and delivered at 600/300 MU/min. Independent of the number of segments the overshoot and undershoot at 600 MU/min were approximately , and for 5 MU/segment, 10 MU/segment, and 20 MU/segment, respectively. At 300 MU/min, each of these values is approximately halved. Interfractional variation of these effects (10 fractions), as well as dosimetric variations for intermediate segments, are reduced at the lower dose rate. QA point‐dose measurements for a sample of head and neck SSIMRT beams were on average 2.9% (600 MU/min) and 1.7% (300 MU/min) higher than Pinnacle3 planned doses. In comparison for prostate beams , measured point doses were 0.8% (600 MU/min) and 0.4% (300 MU/min) higher. The reduction in planned‐measured point‐dose discrepancies at 300 MU/min can be attributed in part to the inclusion of the first segment (overshoot) in the admixture of segments that deliver measured dose. Pinnacle3 plans for 10/9 head and neck/prostate treatments were adjusted by to include the effects of overshoot and undershoot at 600 MU/min. Comparing original and adjusted plans for each site indicated that the original plan was preferred in 70% and 89% of head and neck and prostate cases, respectively. The disparity between planned and delivered treatment that this suggests can potentially be mitigated by treating SSIMRT at a dose rate below 600 MU/min.PACS number(s): 87.55.Qr, 87.56.bd, 87.56.N‐

Using a Pantak Therapax SXT 150 system HVL values for clinical beams generated with filters 4–8, were determined as a function of FCD (30–130 cm). Aluminium absorbers were placed midway between the focus and chamber with collimation to define both narrow and broad beam geometries. For filters 4–7 with broad beam geometry the HVL initially decreases as the FCD is increased from 30 cm and then increases as the FCD approaches 130 cm. In contrast filter 8 exhibits a reduction in HVL with increasing FCD attributed to the decreasing influence of absorber scatter. With narrow beam geometry the HVL of filter 4 increases as the FCD is increased. For other filters the HVL variation is similar to that for the broader beam albeit that for a given FCD the HVL is smaller, a consequence of reduced absorber scatter. Monte Carlo BEAMnrc simulations of filter 4–8 beams demonstrated a quality dependent air attenuation effect associated with an increase in HVL for lower quality beams with increasing FCD. Thus for the beams investigated in this work the variation of HVL with FCD can be interpreted in terms of the competing influences of absorber scatter, which tends to decrease the measured HVL, and a quality dependent in air attenuation that tends to increase the HVL with increasing FCD. In terms of an absorbed dose determination it is shown that changes of HVL with FCD resulted in variations of D w,z = 0 < ±0.5 %.