CERN-based experiments and Monte-Carlo studies on focused dose delivery with very high energy electron (VHEE) beams for radiotherapy applications

Very High Energy Electron (VHEE) beams are a promising alternative to conventional radiotherapy due to their highly penetrating nature and their applicability as a modality for FLASH (ultra-high dose-rate) radiotherapy. The dose distributions due to VHEE need to be optimised; one option is through the use of quadrupole magnets to focus the beam, reducing the dose to healthy tissue and allowing for targeted dose delivery at conventional or FLASH dose-rates. This paper presents an in depth exploration of the focusing achievable at the current CLEAR (CERN Linear Electron Accelerator for Research) facility, for beam energies >200 MeV. A shorter, more optimal quadrupole setup was also investigated using the TOPAS code in Monte Carlo simulations, with dimensions and beam parameters more appropriate to a clinical situation. This work provides insight into how a focused VHEE radiotherapy beam delivery system might be achieved.

Characterisation of the UK high energy proton research beamline for high and ultra-high dose rate (FLASH) irradiation

Objective. This work sets out the capabilities of the high energy proton research beamline developed in the Christie proton therapy centre for Ultra-High Dose Rate (UHDR) irradiation and FLASH experiments. It also characterises the lower limits of UHDR operation for this Pencil Beam Scanning (PBS) proton hardware.Approach. Energy dependent nozzle transmission was measured using a Faraday Cup beam collector. Spot size was measured at the reference plane using a 2D scintillation detector. Integrated depth doses (IDDs) were measured. EBT3 Gafchromic film was used to compare UHDR and conventional dose rate spots. Our beam monitor calibration methodolgy for UHDR is described. A microDiamond detector was used to determine dose rates at zref. Instantaneous depth dose rates were calculated for 70-245 MeV. PBS dose rate distributions were calculated using Folkerts and Van der Water definitions.Main results. Transmission of 7.05 ± 0.1% is achieveable corresponding to a peak instantaneous dose rate of 112.7 Gy s-1. Beam parameters are comparable in conventional and UHDR mode with a spot size ofσx= 4.6 mm,σy= 6.6 mm. Dead time in the beam monitoring electonics warrants a beam current dependent MU correction in the present configuration. Fast beam scanning of 26.4 m s-1(X) and 12.1 m s-1(Y) allows PBS dose rates of the order tens of Grays per second.Significance. UHDR delivery is possible for small field sizes and high energies enabling research into the FLASH effect with PBS protons at our facility. To our knowledge this is also the first thorough characterisation of UHDR irradiation using the hardware of this clinical accelerator at energies less than 250 MeV. The data set out in this publication can be used for designing experiments at this UK research facility and inform the possible future clinical translation of UHDR PBS proton therapy.

Focused VHEE (very high energy electron) beams and dose delivery for radiotherapy applications

This paper presents the first demonstration of deeply penetrating dose delivery using focused very high energy electron (VHEE) beams using quadrupole magnets in Monte Carlo simulations. We show that the focal point is readily modified by linearly changing the quadrupole magnet strength only. We also present a weighted sum of focused electron beams to form a spread-out electron peak (SOEP) over a target region. This has a significantly reduced entrance dose compared to a proton-based spread-out Bragg peak (SOBP). Very high energy electron (VHEE) beams are an exciting prospect in external beam radiotherapy. VHEEs are less sensitive to inhomogeneities than proton and photon beams, have a deep dose reach and could potentially be used to deliver FLASH radiotherapy. The dose distributions of unfocused VHEE produce high entrance and exit doses compared to other radiotherapy modalities unless focusing is employed, and in this case the entrance dose is considerably improved over existing radiations. We have investigated both symmetric and asymmetric focusing as well as focusing with a range of beam energies.

In Silico Models of DNA Damage and Repair in Proton Treatment Planning: A Proof of Concept

There is strong in vitro cell survival evidence that the relative biological effectiveness (RBE) of protons is variable, with dependence on factors such as linear energy transfer (LET) and dose. This is coupled with the growing in vivo evidence, from post-treatment image change analysis, of a variable RBE. Despite this, a constant RBE of 1.1 is still applied as a standard in proton therapy. However, there is a building clinical interest in incorporating a variable RBE. Recently, correlations summarising Monte Carlo-based mechanistic models of DNA damage and repair with absorbed dose and LET have been published as the Manchester mechanistic (MM) model. These correlations offer an alternative path to variable RBE compared to the more standard phenomenological models. In this proof of concept work, these correlations have been extended to acquire RBE-weighted dose distributions and calculated, along with other RBE models, on a treatment plan. The phenomenological and mechanistic models for RBE have been shown to produce comparable results with some differences in magnitude and relative distribution. The mechanistic model found a large RBE for misrepair, which phenomenological models are unable to do. The potential of the MM model to predict multiple endpoints presents a clear advantage over phenomenological models.

Clinically relevant nanodosimetric simulation of DNA damage complexity from photons and protons

Relative Biological Effectiveness (RBE), the ratio of doses between radiation modalities to produce the same biological endpoint, is a controversial and important topic in proton therapy. A number of phenomenological models incorporate variable RBE as a function of Linear Energy Transfer (LET), though a lack of mechanistic description limits their applicability. In this work we take a different approach, using a track structure model employing fundamental physics and chemistry to make predictions of proton and photon induced DNA damage, the first step in the mechanism of radiation-induced cell death. We apply this model to a proton therapy clinical case showing, for the first time, predictions of DNA damage on a patient treatment plan. Our model predictions are for an idealised cell and are applied to an ependymoma case, at this stage without any cell specific parameters. By comparing to similar predictions for photons, we present a voxel-wise RBE of DNA damage complexity. This RBE of damage complexity shows similar trends to the expected RBE for cell kill, implying that damage complexity is an important factor in DNA repair and therefore biological effect.

Relative Biological Effectiveness (RBE) is a controversial and important topic in proton therapy. This work uses Monte Carlo simulations of DNA damage for protons and photons to probe this phenomenon, providing a plausible mechanistic understanding.

Nanodosimetric Simulation of Direct Ion-Induced DNA Damage Using Different Chromatin Geometry Models

Monte Carlo based simulation has proven useful in investigating the effect of proton-induced DNA damage and the processes through which this damage occurs. Clustering of ionizations within a small volume can be related to DNA damage through the principles of nanodosimetry. For simulation, it is standard to construct a small volume of water and determine spatial clusters. More recently, realistic DNA geometries have been used, tracking energy depositions within DNA backbone volumes. Traditionally a chromatin fiber is built within the simulation and identically replicated throughout a cell nucleus, representing the cell in interphase. However, the in vivo geometry of the chromatin fiber is still unknown within the literature, with many proposed models. In this work, the Geant4-DNA toolkit was used to build three chromatin models: the solenoid, zig-zag and cross-linked geometries. All fibers were built to the same chromatin density of 4.2 nucleosomes/11 nm. The fibers were then irradiated with protons (LET 5-80 keV/μm) or alpha particles (LET 63-226 keV/μm). Nanodosimetric parameters were scored for each fiber after each LET and used as a comparator among the models. Statistically significant differences were observed in the double-strand break backbone size distributions among the models, although nonsignificant differences were noted among the nanodosimetric parameters. From the data presented in this article, we conclude that selection of the solenoid, zig-zag or cross-linked chromatin model does not significantly affect the calculated nanodosimetric parameters. This allows for a simulation-based cell model to make use of any of these chromatin models for the scoring of direct ion-induced DNA damage.

Modelling the throughput capacity of a single-accelerator multitreatment room proton therapy centre

OBJECTIVE

We describe a model for evaluating the throughput capacity of a single-accelerator multitreatment room proton therapy centre with the aims of (1) providing quantitative estimates of the throughput and waiting times and (2) providing insight into the sensitivity of the system to various physical parameters.

METHODS

A Monte Carlo approach was used to compute various statistics about the modelled centre, including the throughput capacity, fraction times for different groups of patients and beam waiting times. A method of quantifying the saturation level is also demonstrated.

RESULTS

Benchmarking against the MD Anderson Cancer Center showed good agreement between the modelled (140 ± 4 fractions per day) and reported (133 ± 35 fractions per day) throughputs. A sensitivity analysis of that system studied the impact of beam switch time, the number of treatment rooms, patient set-up times and the potential benefit of having a second accelerator. Finally, scenarios relevant to a potential UK facility were studied, finding that a centre with the same four-room, single-accelerator configuration as the MD Anderson Cancer Center but handling a more complex UK-type caseload would have a throughput reduced by approximately 19%, but still be capable of treating in excess of 100 fractions per 16-h treatment day.

CONCLUSIONS

The model provides a useful tool to aid in understanding the operating dynamics of a proton therapy facility, and for investigating potential scenarios for prospective centres.

ADVANCES IN KNOWLEDGE

The model helps to identify which technical specifications should be targeted for future improvements.

Radiotherapy physics research in the UK: challenges and proposed solutions

In 2011, the Clinical and Translational Radiotherapy Research Working Group (CTRad) of the National Cancer Research Institute brought together UK radiotherapy physics leaders for a think tank meeting. Following a format that CTRad had previously and successfully used with clinical oncologists, 23 departments were asked to complete a pre-meeting evaluation of their radiotherapy physics research infrastructure and the strengths, weaknesses, opportunities and threats within their own centre. These departments were brought together with the CTRad Executive Group and research funders to discuss the current state of radiotherapy physics research, perceived barriers and possible solutions. In this Commentary, we summarise the submitted materials, presentations and discussions from the meeting and propose an action plan. It is clear that there are challenges in both funding and staffing of radiotherapy physics research. Programme and project funding streams sometimes struggle to cater for physics-led work, and increased representation on research funding bodies would be valuable. Career paths for academic radiotherapy physicists need to be examined and an academic training route identified within Modernising Scientific Careers; the introduction of formal job plans may allow greater protection of research time, and should be considered. Improved access to research facilities, including research linear accelerators, would enhance research activity and pass on developments to patients more quickly; research infrastructure could be benchmarked against centres in the UK and abroad. UK National Health Service departments wishing to undertake radiotherapy research, with its attendant added value for patients, need to develop a strategy with their partner higher education institution, and collaboration between departments may provide enhanced opportunities for funded research.

Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to test its suitability as a daily check device for linac output and to provide daily monitoring of beam profile parameters such as flatness, symmetry, field size and wedge factor. Open and wedged 6 and 8 MV photon beams were collected on a daily basis for a period of just over a year and analysed in software to determine daily values of these parameters. Daily output results gave agreement between EPID measured dose and ion chamber measurements with a standard deviation of 0.65%. Step changes in flatness, symmetry and field size were readily detected by the EPID and could be correlated with adjustments made on service days and QC sessions. The results could also be used to assess the long term beam stability. Recalibration of the EPID required new baseline values of the parameters to be set. Wedge factors measured at one collimator angle proved stable but sensitive to changes in beam steering. The EPID proved to be a useful daily check device for linac output which can simultaneously be used for daily monitoring of beam profiles and field sizes.

Improving IMRT quality control efficiency using an amorphous silicon electronic portal imager

An amorphous silicon electronic portal imaging device (EPID) has been investigated to determine its usefulness and efficiency for performing linear accelerator quality control checks specific to step and shoot intensity modulated radiation therapy (IMRT). Several dosimetric parameters were measured using the EPID: dose linearity and segment to segment reproducibility of low dose segments, and delivery accuracy of fractions of monitor units. Results were compared to ion chamber measurements. Low dose beam flatness and symmetry were tested by overlaying low dose beam profiles onto the profile from a stable high-dose exposure and visually checking for differences. Beam flatness and symmetry were also calculated and plotted against dose. Start-up reproducibility was tested by overlaying profiles from twenty successive two monitor unit segments. A method for checking the MLC leaf calibration was also tested, designed to be used on a daily or weekly basis, which consisted of summing the images from a series of matched fields. Daily images were coregistered with, then subtracted from, a reference image. A threshold image showing dose differences corresponding to > 0.5 mm positional errors was generated and the number of pixels with such dose differences used as numerical parameter to which a tolerance can be applied. The EPID was found to be a sensitive relative dosemeter, able to resolve dose differences of 0.01 cGy. However, at low absolute doses a reproducible dosimetric nonlinearity of up to 7% due to image lag/ghosting effects was measured. It was concluded that although the EPID is suitable to measure segment to segment reproducibility and fractional monitor unit delivery accuracy, it is still less useful than an ion chamber as a tool for dosimetric checks. The symmetry/flatness test proved to be an efficient method of checking low dose profiles, much faster than any of the alternative methods. The MLC test was found to be extremely sensitive to sudden changes in MLC calibration but works best with a composite reference image consisting of an average of five successive days' images. When used in this way it proved an effective and efficient daily check of MLC calibration. Overall, the amorphous silicon EPID was found to be a suitable device for IMRT QC although it is not recommended for dosimetric tests. Automatic procedures for low monitor unit profile analysis and MLC leaf positioning yield considerable time-savings over traditional film techniques.

Intensity-modulated Radiotherapy Planning from Limited Anatomical Information: Is Sim-CT Sufficient for Planning Women with Breast Cancer Receiving Intensity-modulated Radiotherapy?

AIMS

To investigate intensity-modulated radiotherapy (IMRT) plans for women with carcinoma of the breast, using a small number of Sim-CT slices, thus avoiding changing the patient's position and potential problems with CT capacity.

MATERIALS AND METHODS

Ten CT scans of women with breast cancer were obtained for use in the study. IMRT plans based on an open tangent pair and additional top-up segment fields were created using the full CT scan, and represented the gold standard treatment plan for comparison purposes. Five-slice CT simulator scans were artificially created by omitting intermediate slices from the full CT scans. Additionally, the intermediate CT slices were recreated via interpolation of the five slices using a standard interpolation algorithm. IMRT plans were created in the same way as for the full CT scans. To allow a suitable plan comparison to be made, the beam segments and monitor units were transferred to the full CT scans, and the dose distribution calculated.

RESULTS

The interpolated five-slice plans showed no significant difference in the volume of tissue receiving dose outside the range 95-105%, compared with the IMRT plans created using the full CT data set (1.3 +/- 2.2%, P = 0.092). In contrast, the discrete slice CT simulator plans increased by 6.3 +/- 5.4%, P = 0.0054, showing a statistically significant difference in the dose distribution produced and a clinically inferior plan.

CONCLUSIONS

Plans created using five discrete slice CT scans were inferior to full CT-derived IMRT treatment plans, and are therefore not acceptable for IMRT. However, interpolating five CT simulator slices provides adequate anatomical information to produce comparable IMRT plans to those created by full CT scans of the patient. This allows the introduction of IMRT for this patient group without the need to change treatment position to accommodate CT scanning.

Quantitative analysis of patient-specific dosimetric IMRT verification

Patient-specific dosimetric verification methods for IMRT treatments are variable, time-consuming and frequently qualitative, preventing evidence-based reduction in the amount of verification performed. This paper addresses some of these issues by applying a quantitative analysis parameter to the dosimetric verification procedure. Film measurements in different planes were acquired for a series of ten IMRT prostate patients, analysed using the quantitative parameter, and compared to determine the most suitable verification plane. Film and ion chamber verification results for 61 patients were analysed to determine long-term accuracy, reproducibility and stability of the planning and delivery system. The reproducibility of the measurement and analysis system was also studied. The results show that verification results are strongly dependent on the plane chosen, with the coronal plane particularly insensitive to delivery error. Unexpectedly, no correlation could be found between the levels of error in different verification planes. Longer term verification results showed consistent patterns which suggest that the amount of patient-specific verification can be safely reduced, provided proper caution is exercised: an evidence-based model for such reduction is proposed. It is concluded that dose/distance to agreement (e.g., 3%/3 mm) should be used as a criterion of acceptability. Quantitative parameters calculated for a given criterion of acceptability should be adopted in conjunction with displays that show where discrepancies occur. Planning and delivery systems which cannot meet the required standards of accuracy, reproducibility and stability to reduce verification will not be accepted by the radiotherapy community.

The required number of treatment imaging days for an effective off-line correction of systematic errors in conformal radiotherapy of prostate cancer--a radiobiological analysis

BACKGROUND AND PURPOSE

To use radiobiological modelling to estimate the number of initial days of treatment imaging required to gain most of the benefit from off-line correction of systematic errors in the conformal radiation therapy of prostate cancer.

MATERIALS AND METHODS

Treatment plans based on the anatomical information of a representative patient were generated assuming that the patient is treated with a multi leaf collimator (MLC) four-field technique and a total isocentre dose of 72 Gy delivered in 36 daily fractions. Target position variations between fractions were simulated from standard deviations of measured data found in the literature. Off-line correction of systematic errors was assumed to be performed only once based on the measured errors during the initial days of treatment. The tumour control probability (TCP) was calculated using the Webb and Nahum model.

RESULTS

Simulation of daily variations in the target position predicted a marked reduction in TCP if the planning target volume (PTV) margin was smaller than 4 mm (TCP decreased by 3.4% for 2 mm margin). The systematic components of target position variations had greater effect on the TCP than the random components. Off-line correction of estimated systematic errors reduced the decrease in TCP due to target daily displacements, nevertheless, the resulting TCP levels for small margins were still less than the TCP level obtained with the use of an adequate PTV margin of approximately 10 mm. The magnitude of gain in TCP expected from the correction depended on the number of treatment imaging days used for the correction and the PTV margin applied. Gains of 2.5% in TCP were estimated from correction of systematic errors performed after 6 initial days of treatment imaging for a 2 mm PTV margin. The effect of various possible magnitudes of systematic and random components on the gain in TCP expected from correction and on the number of imaging days required was also investigated.

CONCLUSIONS

Daily variations of target position markedly reduced the TCP if small margins were used. Off-line correction of systematic errors can only partly compensate for these TCP reductions. The adequate number of treatment imaging days required for systematic error correction depends on the magnitude of the random component compared with the systematic component, and on the size of PTV margin used. For random components equal to or smaller than the systematic component, 3 consecutive treatment imaging days are estimated to be sufficient to gain most of the benefit from correction for current clinically used margins (6-10 mm); otherwise more days are required.

The modelled benefits of individualizing radiotherapy patients' dose using cellular radiosensitivity assays with inherent variability

OBJECTIVE

To model the increases in local tumour control that may be achieved, without increasing normal tissue complications, by prescribing a patient's dose based on cellular radiosensitivity measured using an assay possessing inherent variability.

METHOD

Patient populations with varying radiosensitivity were simulated, based on measured distributions among cancer patients of the surviving fraction of their fibroblasts given a dose of 2 Gy in vitro (SF2). The dose-response curve for complications in the population was assessed using a formula relating SF2 to normal tissue complication probability (NTCP), by summing the data for the individuals. This curve was similar to clinically-derived dose-response curves. The effect of individualizing the patients' doses was explored, based on individual radiosensitivities measured by SF2, so that every patient had the same low (5%) value of NTCP.

RESULTS

It was found that a significant gain (up to around 30%) in tumour control probability (TCP) was predicted for the population when the doses were individualized using a predictive assay result strongly correlated with NTCP. A greater gain in TCP was predicted when each of the individuals were assumed to have a higher sensitivity and the distribution of radiosensitivity in the population was widened to compensate. The gain in TCP was less (around 20%) when considering less-sensitive patients and a narrower distribution of radiosensitivities. The effect of assay variability and other factors that could affect the predictive power of the assay was simulated. Assay variability and an imperfect correlation between in vitro cell survival and tissue complications, rapidly increased the NTCP for the population when treated with individualized doses. However the individualized doses could be reduced so that NTCP declined to an acceptable level, but in this case the TCP for the population also declined. For example, when the assay variability was half the true variability in SF2, the gain in TCP was reduced to around 6%. Also, the predicted gains in population TCP were higher if tumour and normal tissue radiosensitivity were assumed to be correlated. In this case, and in the absence of assay variability, increases in population TCP of about 50% and 30% were predicted, depending on the assumed relative sensitivities of the individual patients compared with that of the population average. For practical application, the division of the patient population simply into three groups of high, average and low radiosensitivity was also examined. The three groups were treated with different doses and the NTCP for the population was kept below 5%. Although the gain in population TCP was less than that predicted with the full individualization, considerable gains of up to 20% were still predicted. This method of dividing the population was more resilient to assay variability and other factors that may affect complications in patients. The modelling suggests that small improvements in TCP (5-10%) may still be achievable even if the correlation between SF2 and late complications is lower at around - 0.4 to - 0.6, as reported in some clinical series.

CONCLUSION

Modelling based on measured distributions of fibroblast radiosensitivity shows that improvements in tumour control rates may be achievable through the individualization of radiotherapy dose prescriptions of cancer patients, when assay variability is less than about 50% of the true variability in radiosensitivity, and with greater benefits if tumour and normal tissue radiosensitivity are correlated. Tripartite stratification of the population proved to be less sensitive to assay uncertainty, and can provide most of the benefits of the full individualization.

The influence of age, size, pubertal status and renal factors on urinary growth hormone excretion in normal children and adolescents

Overnight urinary growth hormone (GH) excretion was measured in 528 schoolchildren (aged 4-16 years) whose heights and weights were between the third and 97th centiles. Urinary GH increased significantly with age, reaching a maximum in boys at 15-years-old and in girls at 13-years-old. Peak levels were five and three times higher in boys and girls respectively than in 4-year-olds. Maximum urinary GH excretion was seen at breast stages 3 and 4 in girls and at genital stage 4 in boys followed by a decline in both sexes at stage 5. Boys excreted more GH than girls during prepubertal and pubertal years. During prepubertal years there were fluctuations of urinary excretion of GH with age. Height, weight and pubertal status predicted 31% of the variability of urinary excretion of GH, and urinary excretion of creatinine, albumin and N-acetylglucosaminidase (NAG) predicted 52% of the variability. The importance of establishing sound age and sex-related reference ranges for urinary growth hormone is stressed before application of this test to children with growth disorders.

The epidemiology of severe mental handicap

Prevalence studies of severe mental handicap in an industrial area in the North West area of the United Kingdom have shown a marked increase between 1961 and 1977, with the hint of a reduction in the most recent years. Further analysis of causes of severe mental handicap shows that the prevalence of Down Syndrome is decreasing, certain causes remain constant in prevalence, and perinatal metabolic causes are shown to be increasing through a period of 15 yr ending in 1975. While improved survival of children influences the total prevalence, high mortality in some conditions makes the analysis of prevalence at birth more significant in considering changes in the pattern of aetiology. These studies are fundamental to the formulation of policies of prevention of severe mental handicap, whether these be social or dependent solely on health care services.

Down syndrome: prevalence at birth, mortality and survival. A 17-year study

Analysis of comprehensive records over a period of 15 yr permits calculation of the total prevalence of Down Syndrome in an industrial city. The prevalence at birth is shown to have fallen from 1.70 per 1000 births in the period 1961--65 to 0.84 per 1000 births in 1971--75. The fall in prevalence can be shown to be largely due to a change in distribution of maternal age. An earlier increase in the number of Down Syndrome children is shown to reflect a greatly improved survival in the first 5 yr of life to a continuing level of 81%. The number of Down Syndrome children is unlikely to increase much even with increase of birth rate unless older mothers have further pregnancies. For these reasons the policies of amniocentesis of older women and termination of affected pregnancies will have progressively less effect on the number of affected children if this demographic trend continues. A further effect is that special schools will have a proportionately greater burden as the younger children being admitted have less prospect of development and may be more demanding of staff than Down Syndrome children.

Pertussis agglutinins in vaccinated children: better response with adjuvant

Children were immunized with a single batch of pertussis vaccine, either adsorbed on aluminium hydroxide or plain. With a primary course of three injections, adsorbed vaccine produced higher titres of pertussis agglutinins in the serum than did plain vaccine. There was no obvious difference in response between those who received the three doses at intervals of 1-2 months, starting at 3-4 months of age, and those in whom the third dose was delayed until about 6 months after the second, but the number of children in each group was small.

Agglutinin response to pertussis vaccination in the child

Children were immunized with plain pertussis vaccine made by three manufacturers in 1967. After a primary course of three injections at monthly intervals, starting at 3-4 months of age, the agglutinin response was poor. Even after a "booster" dose, given five months later, not all of the vaccines had stimulated a response to all three pertussis agglutinogens. A further investigation with current vaccines of different kinds administered according to more than one schedule is recommended.

Lead absorption in children

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Down's syndrome with a familial D/D reciprocal translocation and a G/G chromosome

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