D.I. Thwaites

Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors

PURPOSE The goal of this work was to implement a recently proposed small field dosimetry formalism [Alfonso et al., Med. Phys. 35(12), 5179-5186 (2008)] for a comprehensive set of diode detectors and provide the required Monte Carlo generated factors to correct measurement. METHODS Jaw collimated square small field sizes of side 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 3.0 cm normalized to a reference field of 5.0 cm × 5.0 cm were used throughout this study. Initial linac modeling was performed with electron source parameters at 6.0, 6.1, and 6.2 MeV with the Gaussian FWHM decreased in steps of 0.010 cm from 0.150 to 0.100 cm. DOSRZnrc was used to develop models of the IBA stereotactic field diode (SFD) as well as the PTW T60008, T60012, T60016, and T60017 field diodes. Simulations were run and isocentric, detector specific, output ratios (OR(det)) calculated at depths of 1.5, 5.0, and 10.0 cm. This was performed using the following source parameter subset: 6.1 and 6.2 MeV with a FWHM = 0.100, 0.110, and 0.120 cm. The source parameters were finalized by comparing experimental detector specific output ratios with simulation. Simulations were then run with the active volume and surrounding materials set to water and the replacement correction factors calculated according to the newly proposed formalism. RESULTS In all cases, the experimental field size widths (at the 50% level) were found to be smaller than the nominal, and therefore, the simulated field sizes were adjusted accordingly. At a FWHM = 0.150 cm simulation produced penumbral widths that were too broad. The fit improved as the FWHM was decreased, yet for all but the smallest field size worsened again at a FWHM = 0.100 cm. The simulated OR(det) were found to be greater than, equivalent to and less than experiment for spot size FWHM = 0.100, 0.110, and 0.120 cm, respectively. This is due to the change in source occlusion as a function of FWHM and field size. The corrections required for the 0.5 cm field size were 0.95 (± 1.0%) for the SFD, T60012 and T60017 diodes and 0.90 (± 1.0%) for the T60008 and T60016 diodes-indicating measured output ratios to be 5% and 10% high, respectively. Our results also revealed the correction factors to be the same within statistical variation at all depths considered. CONCLUSIONS A number of general conclusions are evident: (1) small field OR(det) are very sensitive to the simulated source parameters, and therefore, rigorous Monte Carlo linac model commissioning, with respect to measurement, must be pursued prior to use, (2) backscattered dose to the monitor chamber should be included in simulated OR(det) calculations, (3) the corrections required for diode detectors are design dependent and therefore detailed detector modeling is required, and (4) the reported detector specific correction factors may be applied to experimental small field OR(det) consistent with those presented here.

Back to the future: the history and development of the clinical linear accelerator

The linear accelerator (linac) is the accepted workhorse in radiotherapy in 2006. The first medical linac treated its first patient, in London, in 1953, so the use of these machines in clinical practice has been almost co-existent with the lifetime of Physics in Medicine and Biology. This review is a personal selection of things the authors feel are interesting in the history, particularly the early history, and development of clinical linacs. A brief look into the future is also given. One significant theme throughout is the continuity of ideas, building on previous experience. We hope the review might re-connect younger radiotherapy physicists in particular with some of the history and emphasize the continual need, in any human activity, to remain aware of the past, in order to make best use of past experience when taking decisions in the present.

Monte Carlo modelling of diode detectors for small field MV photon dosimetry: detector model simplification and the sensitivity of correction factors to source parameterization

The goal of this work was to examine the use of simplified diode detector models within a recently proposed Monte Carlo (MC) based small field dosimetry formalism and to investigate the influence of electron source parameterization has on MC calculated correction factors. BEAMnrc was used to model Varian 6 MV jaw-collimated square field sizes down to 0.5 cm. The IBA stereotactic field diode (SFD), PTW T60016 (shielded) and PTW T60017 (un-shielded) diodes were modelled in DOSRZnrc and isocentric output ratios (OR(fclin)(detMC)) calculated at depths of d = 1.5, 5.0 and 10.0 cm. Simplified detector models were then tested by evaluating the percent difference in (OR(fclin)(detMC)) between the simplified and complete detector models. The influence of active volume dimension on simulated output ratio and response factor was also investigated. The sensitivity of each MC calculated replacement correction factor (k(fclin,fmsr)(Qclin,Qmsr)), as a function of electron FWHM between 0.100 and 0.150 cm and energy between 5.5 and 6.5 MeV, was investigated for the same set of small field sizes using the simplified detector models. The SFD diode can be approximated simply as a silicon chip in water, the T60016 shielded diode can be modelled as a chip in water plus the entire shielding geometry and the T60017 unshielded diode as a chip in water plus the filter plate located upstream. The detector-specific (k(fclin,fmsr)(Qclin,Qmsr)), required to correct measured output ratios using the SFD, T60016 and T60017 diode detectors are insensitive to incident electron energy between 5.5 and 6.5 MeV and spot size variation between FWHM = 0.100 and 0.150 cm. Three general conclusions come out of this work: (1) detector models can be simplified to produce OR(fclin)(detMC) to within 1.0% of those calculated using the complete geometry, where typically not only the silicon chip, but also any high density components close to the chip, such as scattering plates or shielding material is necessary to be included in the model, (2) diode detectors of smaller active radius require less of a correction and (3) (k(fclin,fmsr)(Qclin,Qmsr)) is insensitive to the incident the electron energy and spot size variations investigated. Therefore, simplified detector models can be used with acceptable accuracy within the recently proposed small field dosimetry formalism.

Physics and technology in ESTRO and in Radiotherapy and Oncology: Past, present and into the 4th dimension

‘Those who cannot remember the past are condemned to repeat it’: George Santayana, The Life of Reason, volume 1, 1905 ‘History is more or less bunk. It’s tradition. We dont want tradition. We want to live in the present’: Henry Ford, interview in the Chicago Tribune, May 25th 1916 ‘Prediction is difficult, especially about the future’: attributed to Neils Bohr (citing Storm P, Danish comedian and cartoonist)

The evaluation of a deformable image registration segmentation technique for semi-automating internal target volume (ITV) production from 4DCT images of lung stereotactic body radiotherapy (SBRT) patients.

PURPOSE To evaluate a deformable image registration (DIR) segmentation technique for semi-automating ITV production from 4DCT for lung patients, in terms of accuracy and efficiency. METHODS Twenty-five stereotactic body radiotherapy lung patients were selected in this retrospective study. ITVs were manually delineated by an oncologist and semi-automatically produced by propagating the GTV manually delineated on the mid-ventilation phase to all other phases using two different DIR algorithms, using commercial software. The two ITVs produced by DIR were compared to the manually delineated ITV using the dice similarity coefficient (DSC), mean distance between agreement and normalised DSC. DIR-produced ITVs were assessed for their clinical suitability and also the time savings were estimated. RESULTS Eighteen out of 25 ITVs had normalised DSC>1 indicating an agreement with the manually produced ITV within 1mm uncertainty. Four of the other seven ITVs were deemed clinically acceptable and three would require a small amount of editing. In general, ITVs produced by DIR were smoother than those produced by manual delineation. It was estimated that using this technique would save clinicians on average 28 min/patient. CONCLUSIONS ABAS was found to be a useful tool in the production of ITVs for lung patients. The ITVs produced are either immediately clinically acceptable or require minimal editing. This approach represents a significant time saving for clinicians.

Advances in kilovoltage x-ray beam dosimetry

This topical review provides an up-to-date overview of the theoretical and practical aspects of therapeutic kilovoltage x-ray beam dosimetry. Kilovoltage x-ray beams have the property that the maximum dose occurs very close to the surface and thus, they are predominantly used in the treatment of skin cancers but also have applications for the treatment of other cancers. In addition, kilovoltage x-ray beams are used in intra operative units, within animal irradiators and in on-board imagers on linear accelerators and kilovoltage dosimetry is important in these applications as well. This review covers both reference and relative dosimetry of kilovoltage x-ray beams and provides recommendations for clinical measurements based on the literature to date. In particular, practical aspects for the selection of dosimeter and phantom material are reviewed to provide suitable advice for medical physicists. An overview is also presented of dosimeters other than ionization chambers which can be used for both relative and in vivo dosimetry. Finally, issues related to the treatment planning and the use of Monte Carlo codes for solving radiation transport problems in kilovoltage x-ray beams are presented.

A methodological approach to reporting corrected small field relative outputs.

PURPOSE The goal of this work was to set out a methodology for measuring and reporting small field relative output and to assess the application of published correction factors across a population of linear accelerators. METHODS AND MATERIALS Measurements were made at 6 MV on five Varian iX accelerators using two PTW T60017 unshielded diodes. Relative output readings and profile measurements were made for nominal square field sizes of side 0.5 to 1.0 cm. The actual in-plane (A) and cross-plane (B) field widths were taken to be the FWHM at the 50% isodose level. An effective field size, defined as √FS eff=A · B, was calculated and is presented as a field size metric. FSeff was used to linearly interpolate between published Monte Carlo (MC) calculated [Formula in text] values to correct for the diode over-response in small fields. RESULTS The relative output data reported as a function of the nominal field size were different across the accelerator population by up to nearly 10%. However, using the effective field size for reporting showed that the actual output ratios were consistent across the accelerator population to within the experimental uncertainty of ± 1.0%. Correcting the measured relative output using [Formula in text] at both the nominal and effective field sizes produce output factors that were not identical but differ by much less than the reported experimental and/or MC statistical uncertainties. CONCLUSIONS In general, the proposed methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs.

Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations

BACKGROUND AND PURPOSE The goal of this work was to measure 6MV small field, detector specific, output ratios (OR(det)) using the IBA stereotactic field diode (SFD) and the PTW T60008, T60012, T60016 and T60017 field diodes on both Varian iX and Elekta Synergy accelerators, to establish estimates for the experimental uncertainty and characterize the measurement precision under various conditions. MATERIALS AND METHODS Data were acquired at depths of 1.5, 5.0 and 10.0 cm for square field sizes of 3.0, 1.0, 0.9, 0.8, 0.7, 0.6 and 0.5 cm. Three isocentric measurements comprised of five readings were made to calculate an experimental output ratio OR(det) with respect to a field size of 5.0 cm. The coefficient of variation (CV) was calculated to characterize the precision associated with each detector-linac combination. Another measurement set was made to investigate the influence of jaw position accuracy. RESULTS As expected for field sizes smaller than 3.0 cm, the measured OR(det) were not consistent across all detectors. The standard percent uncertainty in measured OR(det) was found to be nearly consistent across all detector-linac combinations: less than ±0.25% for the 3.0 cm field size, increasing to approximately ±1.25% for the smallest field sizes. As the field size was reduced to 0.5 cm the CV increased to 0.10% and 0.15% on the Varian and Elekta linacs, respectively. CONCLUSION Experimental small field OR(det) measured with the diode detectors used in this study are reproducible to within ±1.25% (standard uncertainty), with the precision of any one set of measurements can be characterized with a CV between 0.10% and 0.15%.

An evaluation of four CT-MRI co-registration techniques for radiotherapy treatment planning of prone rectal cancer patients.

OBJECTIVES MRI is the preferred staging modality for rectal carcinoma patients. This work assesses the CT-MRI co-registration accuracy of four commercial rigid-body techniques for external beam radiotherapy treatment planning for patients treated in the prone position without fiducial markers. METHODS 17 patients with biopsy-proven rectal carcinoma were scanned with CT and MRI in the prone position without the use of fiducial markers. A reference co-registration was performed by consensus of a radiologist and two physicists. This was compared with two automated and two manual techniques on two separate treatment planning systems. Accuracy and reproducibility were analysed using a measure of target registration error (TRE) that was based on the average distance of the mis-registration between vertices of the clinically relevant gross tumour volume as delineated on the CT image. RESULTS An automated technique achieved the greatest accuracy, with a TRE of 2.3 mm. Both automated techniques demonstrated perfect reproducibility and were significantly faster than their manual counterparts. There was a significant difference in TRE between registrations performed on the two planning systems, but there were no significant differences between the manual and automated techniques. CONCLUSION For patients with rectal cancer, MRI acquired in the prone treatment position without fiducial markers can be accurately registered with planning CT. An automated registration technique offered a fast and accurate solution with associated uncertainties within acceptable treatment planning limits.

Investigation of uncertainties in image registration of cone beam CT to CT on an image-guided radiotherapy system

Methods of measuring uncertainties in rigid body image registration of fan beam computed tomography (FBCT) to cone beam CT (CBCT) have been developed for automatic image registration algorithms in a commercial image guidance system (Synergy, Elekta, UK). The relationships between image registration uncertainty and both imaging dose and image resolution have been investigated with an anthropomorphic skull phantom and further measurements performed with patient images of the head. A new metric of target registration error is proposed. The metric calculates the mean distance traversed by a set of equi-spaced points on the surface of a 5 cm sphere, centred at the isocentre when transformed by the residual error of registration. Studies aimed at giving practical guidance on the use of the Synergy automated image registration, including choice of algorithm and use of the Clipbox are reported. The chamfer-matching algorithm was found to be highly robust to the increased noise induced by low-dose acquisitions. This would allow the imaging dose to be reduced from the current clinical norm of 2 mGy to 0.2 mGy without a clinically significant loss of accuracy. A study of the effect of FBCT slice thickness/spacing and CBCT voxel size showed that 2.5 mm and 1 mm, respectively, gave acceptable image registration performance. Registration failures were highly infrequent if the misalignment was typical of normal clinical set-up errors and these were easily identified. The standard deviation of translational registration errors, measured with patient images, was 0.5 mm on the surface of a 5 cm sphere centred on the treatment centre. The chamfer algorithm is suitable for routine clinical use with minimal need for close inspection of image misalignment.