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PURPOSE: Medical physics staffing models require periodic review due to the rapid evolution of technology and clinical techniques in radiation oncology. We present an update to a grid-based physics staffing algorithm for radiation oncology (originally published in 2012) that has been widely used in Canada over the last decade. MATERIALS AND METHODS: The physics staffing algorithm structure was modified to improve the clarity and consistency of input data. We collected information on clinical procedures, equipment inventory, and teaching activities from 15 radiation treatment centers in the province of Ontario from April 1, 2018, to March 31, 2019. Using these data sets, the algorithm's weighting parameters were adjusted to align the prediction of full-time equivalent (FTE) personnel with actual staffing levels in Ontario. The algorithm computes FTE estimates for medical physicists, physics assistants, engineering (electrical and mechanical), and information technology (IT) support. The performance of the algorithm was also tested in eight Canadian cancer centers outside of Ontario. RESULTS: The mean difference between the algorithm and actual staffing for the 23 Canadian cancer centers did not exceed 0.5 FTE for any staffing group. The results were slightly better in Ontario than in other provinces, as expected since the algorithm was optimized using Ontario data. There was a linear correlation between the algorithm predictions and the number of annual-treated cases for physicists, and physicists plus physics assistants. For other staff categories, the algorithm weighting parameters were not significantly altered, except for a reduction in mechanical engineering staff. Comparison with other published models suggests that the updated algorithm should be considered as a minimum recommended staffing level for the clinical support of radiation oncology programs. CONCLUSIONS: We support the use of grid-based physics staffing algorithms that account for clinical workload with flexibility to adapt to local conditions with variable academic and research demands.
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Oncología por Radiación , Algoritmos , Canadá , Física Sanitaria , Humanos , Física , Recursos HumanosRESUMEN
PURPOSE: In a recent article, our group proposed a fast direct aperture optimization (DAO) algorithm for fixed-gantry intensity-modulated radiation therapy (IMRT) called fast inverse direct aperture optimization (FIDAO). When tested on fixed-gantry IMRT plans, we observed up to a 200-fold increase in the optimization speed. Compared to IMRT, rotational volumetric-modulated arc therapy (VMAT) is a much larger optimization problem and has many more delivery constraints. The purpose of this work is to extend and evaluate FIDAO for inverse planning of VMAT plans. METHODS: A prototype FIDAO algorithm for VMAT treatment planning was developed in MATLAB using the open-source treatment planning toolkit matRad (v2.2 dev_VMAT build). VMAT treatment plans using one 3600 arc were generated on the AAPM TG-119 phantom, as well as sample clinical liver and prostate cases. The plans were created by first performing fluence map optimization on 28° equispaced beams, followed by aperture sequencing and arc sequencing with a gantry angular sampling rate of 4°. After arc sequencing, a copy of the plan underwent DAO using the prototype FIDAO algorithm, while another copy of the plan underwent DAO using matRad's DAO method, which served as the conventional algorithm. RESULTS: Both algorithms achieved similar plan quality, although the FIDAO plans had considerably fewer hot spots in the unspecified normal tissue. The optimization time (number of iterations) for FIDAO and the conventional DAO algorithm, respectively, were: 65 s (245) vs 602 s (275) in the TG-119 phantom case; 25 s (85) vs 803 s (159) in the liver case; and 99 s (174) vs 754 s (149) in the prostate case. CONCLUSIONS: This study demonstrated promising speed enhancements in using FIDAO for the direct aperture optimization of VMAT plans.
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Algoritmos , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia de Intensidad Modulada , Factores de TiempoRESUMEN
PURPOSE: The goal of this work was to develop and evaluate a fast inverse direct aperture optimization (FIDAO) algorithm for IMRT treatment planning and plan adaptation. METHODS: A previously proposed fluence map optimization algorithm called fast inverse dose optimization (FIDO) was extended to optimize the aperture shapes and weights of IMRT beams. FIDO is a very fast fluence map optimization algorithm for IMRT that finds the global minimum using direct matrix inversion without unphysical negative beam weights. In this study, an equivalent second-order Taylor series expansion of the FIDO objective function was used, which allowed for the objective function value and gradient vector to be computed very efficiently during direct aperture optimization, resulting in faster optimization. To evaluate the speed gained with FIDAO, a proof-of-concept algorithm was developed in MATLAB using an interior-point optimization method to solve the reformulated aperture-based FIDO problem. The FIDAO algorithm was used to optimize four step-and-shoot IMRT cases: on the AAPM TG-119 phantom as well as a liver, prostate, and head-and-neck clinical cases. Results were compared with a conventional DAO algorithm that uses the same interior-point method but using the standard formulation of the objective function and its gradient vector. RESULTS: A substantial gain in optimization speed was obtained with the prototype FIDAO algorithm compared to the conventional DAO algorithm while producing plans of similar quality. The optimization time (number of iterations) for the prototype FIDAO algorithm vs the conventional DAO algorithm was 0.3 s (17) vs 56.7 s (50); 2.0 s (28) vs 134.1 s (57); 2.5 s (26) vs 180.6 s (107); and 6.7 s (20) vs 469.4 s (482) in the TG-119 phantom, liver, prostate, and head-and-neck examples, respectively. CONCLUSIONS: A new direct aperture optimization algorithm based on FIDO was developed. For the four IMRT test cases examined, this algorithm executed approximately 70-200 times faster without compromising the IMRT plan quality.
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Algoritmos , Neoplasias de Cabeza y Cuello/radioterapia , Neoplasias Hepáticas/radioterapia , Fantasmas de Imagen , Neoplasias de la Próstata/radioterapia , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia de Intensidad Modulada/normas , Humanos , Masculino , Dosificación Radioterapéutica , Radioterapia de Intensidad Modulada/instrumentación , Radioterapia de Intensidad Modulada/métodosRESUMEN
PURPOSE: In this work, we propose a new method of calibrating cone beam computed tomography (CBCT) data sets for radiotherapy dose calculation and plan assessment. The motivation for this patient-specific calibration (PSC) method is to develop an efficient, robust, and accurate CBCT calibration process that is less susceptible to deformable image registration (DIR) errors. METHODS: Instead of mapping the CT numbers voxel-by-voxel with traditional DIR calibration methods, the PSC methods generates correlation plots between deformably registered planning CT and CBCT voxel values, for each image slice. A linear calibration curve specific to each slice is then obtained by least-squares fitting, and applied to the CBCT slice's voxel values. This allows each CBCT slice to be corrected using DIR without altering the patient geometry through regional DIR errors. A retrospective study was performed on 15 head-and-neck cancer patients, each having routine CBCTs and a middle-of-treatment re-planning CT (reCT). The original treatment plan was re-calculated on the patient's reCT image set (serving as the gold standard) as well as the image sets produced by voxel-to-voxel DIR, density-overriding, and the new PSC calibration methods. Dose accuracy of each calibration method was compared to the reference reCT data set using common dose-volume metrics and 3D gamma analysis. A phantom study was also performed to assess the accuracy of the DIR and PSC CBCT calibration methods compared with planning CT. RESULTS: Compared with the gold standard using reCT, the average dose metric differences were ≤ 1.1% for all three methods (PSC: -0.3%; DIR: -0.7%; density-override: -1.1%). The average gamma pass rates with thresholds 3%, 3 mm were also similar among the three techniques (PSC: 95.0%; DIR: 96.1%; density-override: 94.4%). CONCLUSIONS: An automated patient-specific calibration method was developed which yielded strong dosimetric agreement with the results obtained using a re-planning CT for head-and-neck patients.
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Algoritmos , Tomografía Computarizada de Haz Cónico/normas , Neoplasias de Cabeza y Cuello/radioterapia , Fantasmas de Imagen , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia de Intensidad Modulada/métodos , Calibración , Tomografía Computarizada de Haz Cónico/métodos , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Pronóstico , Dosificación Radioterapéutica , Estudios RetrospectivosRESUMEN
PURPOSE: Iterative CT reconstruction algorithms are gaining popularity as GPU-based computation becomes more accessible. These algorithms are desirable in x-ray CT for their ability to achieve similar image quality at a fraction of the dose required for standard filtered backprojection reconstructions. In optical CT dosimetry, the noise reduction capability of such algorithms is similarly desirable because noise has a detrimental effect on the precision of dosimetric analysis, and can create misleading test results. In this study, we evaluate an iterative CT reconstruction algorithm for gel dosimetry, with special attention to the challenging dosimetry of small fields. METHODS: An existing ordered subsets convex algorithm using total variation minimization regularization (OSC-TV) was implemented. Three datasets, which represent the extreme cases of gel dosimetry, were examined: a large, 15 cm diameter uniform phantom, a 1.35 cm diameter finger phantom, and a 15 cm gel dosimeter irradiated with 3 × 3, 2 × 2, 1 × 1, and 0.6 × 0.6 cm fields. These were scanned on an in-house scanning laser system, and reconstructed with both filtered backprojection and OSC-TV with a range of regularization constants. The contrast to artifact + noise ratio (CANR) and penumbra width measurements (80% to 20% and 95% to 5% distances) were used to compare reconstructions. RESULTS: Our results showed that OSC-TV can achieve 3-5× improvement in contrast to artifact + noise ratio compared to filtered backprojection, while preserving the shape of steep dose gradients. For very small objects (≤ 0.6 × 0.6 cm fields in a 16 × 16 cm field of view), the mean value in the center of the object can be suppressed if the regularization constant is improperly set, which must be avoided. CONCLUSIONS: Overall, the results indicate that OSC-TV is a suitable reconstruction algorithm for gel dosimetry, provided care is taken in setting the regularization parameter when reconstructing objects that are small compared to the scanner field of view.
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Algoritmos , Procesamiento de Imagen Asistido por Computador/métodos , Radiometría/instrumentación , Tomografía Computarizada por Rayos X , Fantasmas de ImagenRESUMEN
During radiation therapy of head and neck cancer, the decision to consider replanning a treatment because of anatomical changes has significant resource implications. We developed an algorithm that compares cone-beam computed tomography (CBCT) image pairs and provides an automatic alert as to when remedial action may be required. Retrospective CBCT data from ten head and neck cancer patients that were replanned during their treatment was used to train the algorithm on when to recommend a repeat CT simulation (re-CT). An additional 20 patients (replanned and not replanned) were used to validate the predictive power of the algorithm. CBCT images were compared in 3D using the gamma index, combining Hounsfield Unit (HU) difference with distance-to-agreement (DTA), where the CBCT study acquired on the first fraction is used as the reference. We defined the match quality parameter (MQPx ) as a difference between the xth percentiles of the failed-pixel histograms calculated from the reference gamma comparison and subsequent comparisons, where the reference gamma comparison is taken from the first two CBCT images acquired during treatment. The decision to consider re-CT was based on three consecutive MQP values being less than or equal to a threshold value, such that re-CT recommendations were within ±3 fractions of the actual re-CT order date for the training cases. Receiver-operator characteristic analysis showed that the best trade-off in sensitivity and specificity was achieved using gamma criteria of 3 mm DTA and 30 HU difference, and the 80th percentile of the failed-pixel histogram. A sensitivity of 82% and 100% was achieved in the training and validation cases, respectively, with a false positive rate of ~30%. We have demonstrated that gamma analysis of CBCT-acquired anatomy can be used to flag patients for possible replanning in a manner consistent with local clinical practice guidelines.
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Tomografía Computarizada de Haz Cónico/métodos , Rayos gamma , Neoplasias de Cabeza y Cuello/patología , Radioterapia Guiada por Imagen/métodos , Algoritmos , Neoplasias de Cabeza y Cuello/diagnóstico por imagen , Neoplasias de Cabeza y Cuello/radioterapia , Humanos , Pronóstico , Dosificación Radioterapéutica , Radioterapia de Intensidad Modulada/métodos , Estudios RetrospectivosRESUMEN
Stray light causes artifacts in optical computed tomography (CT) that negatively affect the accuracy of radiation dosimetry in gels or solids. Scatter effects are exacerbated by a large dosimeter volume, which is desirable for direct verification of modern radiotherapy treatment plans such as multiple-isocenter radiosurgery. The goal in this study was to design and characterize an optical CT system that achieves high accuracy primary transmission measurements through effective stray light rejection, while maintaining sufficient scan speed for practical application. We present an optical imaging platform that uses a galvanometer mirror for horizontal scanning, and a translation stage for vertical movement of a laser beam and small area detector for minimal stray light production and acceptance. This is coupled with a custom lens-shaped optical CT aquarium for parallel ray sampling of projections. The scanner images 15 cm diameter, 12 cm height cylindrical volumes at 0.33 mm resolution in approximately 30 min. Attenuation coefficients reconstructed from CT scans agreed with independent cuvette measurements within 2% for both absorbing and scattering solutions as well as small 1.25 cm diameter absorbing phantoms placed within a large, scattering medium that mimics gel. Excellent linearity between the optical CT scanner and the independent measurement was observed for solutions with between 90% and 2% transmission. These results indicate that the scanner should achieve highly accurate dosimetry of large volume dosimeters in a reasonable timeframe for clinical application to radiotherapy dose verification procedures.
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Rayos Láser , Fantasmas de Imagen , Tomografía Óptica/métodos , Tomografía Computarizada por Rayos X/instrumentación , Artefactos , Humanos , Radiometría/métodos , Radioterapia de Intensidad Modulada , Tomografía Computarizada por Rayos X/métodosRESUMEN
PURPOSE: The practical use of the PRESAGE® solid plastic dosimeter is limited by the inconvenience of immersing it in high-viscosity oils to achieve refractive index matching for optical computed tomography (CT) scanning. The oils are slow to mix and difficult to clean from surfaces, and the dosimeter rotation can generate dynamic Schlieren inhomogeneity patterns in the reference liquid, limiting the rotational and overall scan speed. Therefore, it would be beneficial if lower-viscosity, water-based solutions with slightly unmatched refractive index could be used instead. The purpose of this work is to demonstrate the feasibility of allowing mismatched conditions when using a scanning laser system with a large acceptance angle detector. A fiducial-based ray path measurement technique is combined with an iterative CT reconstruction algorithm to reconstruct images. METHODS: A water based surrounding liquid with a low viscosity was selected for imaging PRESAGE® solid dosimeters. Liquid selection was optimized to achieve as high a refractive index as possible while avoiding rotation-induced Schlieren effects. This led to a refractive index mismatch of 6% between liquid and dosimeters. Optical CT scans were performed with a fan-beam scanning-laser optical CT system with a large area detector to capture most of the refracted rays. A fiducial marker placed on the wall of a cylindrical sample occludes a given light ray twice. With knowledge of the rotation angle and the radius of the cylindrical object, the actual internal path of each ray through the dosimeter can be calculated. Scans were performed with 1024 projections of 512 data samples each, and rays were rebinned to form 512 parallel-beam projections. Reconstructions were performed on a 512 × 512 grid using 100 iterations of the SIRT iterative CT algorithm. Proof of concept was demonstrated with a uniformly attenuating solution phantom. PRESAGE® dosimeters (11 cm diameter) were irradiated with Cobalt-60 irradiator to achieve either a uniform dose or a 2-level "step-dose" pattern. RESULTS: With 6% refractive index mismatching, a circular field of view of 85% of the diameter of a cylindrical sample can be reconstructed accurately. Reconstructed images of the test solution phantom were uniform (within 3%) inside this radius. However, the dose responses of the PRESAGE® samples were not spatially uniform, with variations of at least 5% in sensitivity. The variation appears as a "cupping" artifact with less sensitivity in the middle than at the periphery of the PRESAGE® cylinder. Polarization effects were also detected for these samples. CONCLUSIONS: The fiducial-based ray path measurement scheme, coupled with an iterative reconstruction algorithm, enabled optical CT scanning of PRESAGE® dosimeters immersed in mismatched refractive index solutions. However, improvements to PRESAGE® dose response uniformity are required.
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Radiometría/instrumentación , Radiometría/métodos , Tomografía Óptica/métodos , Algoritmos , Artefactos , Radioisótopos de Cobalto , Diseño de Equipo , Láseres de Gas , Fantasmas de Imagen , Refractometría , Tomografía Óptica/instrumentación , Viscosidad , AguaRESUMEN
PURPOSE: Recently our group developed a unified intensity-modulated arc therapy (UIMAT) technique which allows for the simultaneous inverse-optimization and the combined delivery of volume-modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT). The aim of this study was to evaluate the dosimetric benefits of UIMAT plans for radiation treatment of complex head-and-neck cancer cases. METHODS AND MATERIALS: A retrospective treatment planning study was performed on 30 head-and-neck cases, 15 of which were treated clinically with VMAT while the other 15 were treated with step-and-shoot IMRT. These cases were re-planned using our UIMAT technique and the results were compared with the clinically delivered plans. Plans were assessed in terms of clinically relevant metrics describing target volume coverage, dose conformity, and the sparing of organs at risk. RESULTS: When compared to stand-alone VMAT or IMRT, UIMAT plans offered slightly better tumor volume coverage (Median D95: 98.1% vs. 97.5%, p=0.01) and similar dose conformity (Median CI: 0.69 vs. 0.69, p=0.09). More significantly, UIMAT plans had substantially lower doses to all organs at risk, including the spinal cord (Median D2%: 29.9Gy vs. 35.6Gy, p<0.01), brainstem (Median D2%: 21.2Gy vs. 25.6Gy, p<0.01), left parotid (Median DMean: 26.1Gy vs. 28.0Gy, p<0.01), and right parotid (Median DMean: 23.6Gy vs. 27.2Gy, p<0.01). The reduction in OAR doses did not result from the redistribution of dose to unspecified tissue. Furthermore, UIMAT plans can be delivered with comparable delivery times to VMAT (Median time: 135s vs. 168s, p=0.394) but with fewer monitor units (Median MU: 486 vs. 635, p<0.01). CONCLUSIONS: Compared to stand-alone IMRT or VMAT, UIMAT was demonstrated to have a dosimetric advantage for the radiation treatment of head-and-neck cancer.
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Neoplasias de Cabeza y Cuello/radioterapia , Radioterapia de Intensidad Modulada/métodos , Neoplasias de Cabeza y Cuello/patología , Humanos , Órganos en Riesgo , Glándula Parótida/efectos de la radiación , Dosificación Radioterapéutica , Planificación de la Radioterapia Asistida por Computador/métodos , Estudios Retrospectivos , Carga TumoralRESUMEN
Optical cone-beam computed tomographic (CBCT) scanning of 3D radiochromic dosimeters may provide a practical method for 3D dose verification in radiation therapy. However, in cone-beam geometry stray light contaminates the projection images, degrading the accuracy of reconstructed linear attenuation coefficients. Stray light was measured using a beam pass aperture array (BPA) and structured illumination methods. The stray-to-primary ray ratio (SPR) along the central axis was found to be 0.24 for a 5% gelatin hydrogel, representative of radiochromic hydrogels. The scanner was modified by moving the spectral filter from the detector to the source, changing the light's spatial fluence pattern and lowering the acceptance angle by extending distance between the source and object. These modifications reduced the SPR significantly from 0.24 to 0.06. The accuracy of the reconstructed linear attenuation coefficients for uniform carbon black liquids was compared to independent spectrometer measurements. Reducing the stray light increased the range of accurate transmission readings. In order to evaluate scanner performance for the more challenging application to small field dosimetry, a carbon black finger gel phantom was prepared. Reconstructions of the phantom from CBCT and fan-beam CT scans were compared. The modified source resulted in improved agreement. Subtraction of residual stray light, measured with BPA or structured illumination from each projection further improved agreement. Structured illumination was superior to BPA for measuring stray light for the smaller 1.2 and 0.5 cm diameter phantom fingers. At the costs of doubling the scanner size and tripling the number of scans, CBCT reconstructions of low-scattering hydrogel dosimeters agreed with those of fan-beam CT scans.
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Tomografía Computarizada de Haz Cónico/métodos , Tomografía Computarizada de Haz Cónico/instrumentación , Luz , Fantasmas de Imagen , Radiometría/instrumentación , Radiometría/métodos , Sensibilidad y EspecificidadRESUMEN
Optical cone beam computed tomography (CBCT) using a broad beam and CCD camera is a fast method for densitometry of 3D optical gel dosimeters. However, diffuse light sources introduce considerable stray light into the imaging system, leading to underestimation of attenuation coefficients and non-uniformities in CT images unless corrections are applied to each projection image. In this study, the light source of a commercial optical CT scanner is replaced with a convergent cone beam source consisting of almost exclusively image forming primary rays. The convergent source is achieved using a small isotropic source and a Fresnel lens. To characterize stray light effects, full-field cone beam CT imaging is compared to fan beam CT (FBCT) using a 1 cm high fan beam aperture centered on the optic axis of the system. Attenuating liquids are scanned within a large 96 mm diameter uniform phantom and in a small 13.5 mm diameter finger phantom. For the uniform phantom, cone and fan beam CT attenuation coefficients agree within a maximum deviation of (1 ± 2)% between mean values over a wide range from 0.036 to 0.43 cm(-1). For the finger phantom, agreement is found with a maximum deviation of (4 ± 2)% between mean values over a range of 0.1-0.47 cm(-1). With the convergent source, artifacts associated with refractive index mismatch and vessel optical features are more pronounced. Further optimization of the source size to achieve a balance between quantitative accuracy and artifact reduction should enable practical, accurate 3D dosimetry, avoiding time consuming 3D scatter measurements.
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Tomografía Computarizada de Haz Cónico/métodos , Luz , Tomografía Computarizada de Haz Cónico/instrumentación , Fantasmas de Imagen , Radiometría/métodos , RefractometríaRESUMEN
PURPOSE: To study the feasibility of unified intensity-modulated arc therapy (UIMAT) which combines intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) optimization and delivery to produce superior radiation treatment plans, both in terms of dose distribution and efficiency of beam delivery when compared with either VMAT or IMRT alone. METHODS: An inverse planning algorithm for UIMAT was prototyped within the pinnacle treatment planning system (Philips Healthcare). The IMRT and VMAT deliveries are unified within the same arc, with IMRT being delivered at specific gantry angles within the arc. Optimized gantry angles for the IMRT and VMAT phases are assigned automatically by the inverse optimization algorithm. Optimization of the IMRT and VMAT phases is done simultaneously using a direct aperture optimization algorithm. Five treatment plans each for prostate, head and neck, and lung were generated using a unified optimization technique and compared with clinical IMRT or VMAT plans. Delivery verification was performed with an ArcCheck phantom (Sun Nuclear) on a Varian TrueBeam linear accelerator (Varian Medical Systems). RESULTS: In this prototype implementation, the UIMAT plans offered the same target dose coverage while reducing mean doses to organs at risk by 8.4% for head-and-neck cases, 5.7% for lung cases, and 3.5% for prostate cases, compared with the VMAT or IMRT plans. In addition, UIMAT can be delivered with similar efficiency as VMAT. CONCLUSIONS: In this proof-of-concept work, a novel radiation therapy optimization and delivery technique that interlaces VMAT or IMRT delivery within the same arc has been demonstrated. Initial results show that unified VMAT/IMRT has the potential to be superior to either standard IMRT or VMAT.
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Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia de Intensidad Modulada/métodos , Estudios de Factibilidad , Humanos , Masculino , Neoplasias/radioterapiaRESUMEN
Stereotactic body radiation therapy (SBRT) has quickly become a preferred treatment option for early-stage lung cancer patients who are ineligible for surgery. This technique uses tightly conformed megavoltage (MV) x-ray beams to irradiate a tumour with ablative doses in only a few treatment fractions. Small high energy x-ray fields can cause lateral electron disequilibrium (LED) to occur within low density media, which can reduce tumour dose. These dose effects may be challenging to predict using analytic dose calculation algorithms, especially at higher beam energies. As a result, previous authors have suggested using low energy photons (<10 MV) and larger fields (>5 × 5 cm(2)) for lung cancer patients to avoid the negative dosimetric effects of LED. In this work, we propose a new form of SBRT, described as LED-optimized SBRT (LED-SBRT), which utilizes radiotherapy (RT) parameters designed to cause LED to advantage. It will be shown that LED-SBRT creates enhanced dose gradients at the tumour/lung interface, which can be used to manipulate tumour dose, and/or normal lung dose. To demonstrate the potential benefits of LED-SBRT, the DOSXYZnrc (National Research Council of Canada, Ottawa, ON) Monte Carlo (MC) software was used to calculate dose within a cylindrical phantom and a typical lung patient. 6 MV or 18 MV x-ray fields were focused onto a small tumour volume (diameter â¼1 cm). For the phantom, square fields of 1 × 1 cm(2), 3 × 3 cm(2), or 5 × 5 cm(2) were applied. However, in the patient, 3 × 1 cm(2), 3 × 2 cm(2), 3 × 2.5 cm(2), or 3 × 3 cm(2) field sizes were used in simulations to assure target coverage in the superior-inferior direction. To mimic a 180° SBRT arc in the (symmetric) phantom, a single beam profile was calculated, rotated, and beams were summed at 1° segments to accumulate an arc dose distribution. For the patient, a 360° arc was modelled with 36 equally weighted (and spaced) fields focused on the tumour centre. A planning target volume (PTV) was generated by considering the extent of tumour motion over the patient's breathing cycle and set-up uncertainties. All patient dose results were normalized such that at least 95% of the PTV received at least 54 Gy (i.e. D95 = 54 Gy). Further, we introduce 'LED maps' as a novel clinical tool to compare the magnitude of LED resulting from the various SBRT arc plans. Results from the phantom simulation suggest that the best lung sparing occurred for RT parameters that cause severe LED. For equal tumour dose coverage, normal lung dose (2 cm outside the target region) was reduced from 92% to 23%, comparing results between the 18 MV (5 × 5 cm(2)) and 18 MV (1 × 1 cm(2)) arc simulations. In addition to reduced lung dose for the 18 MV (1 × 1 cm(2)) arc, maximal tumour dose increased beyond 125%. Thus, LED can create steep dose gradients to spare normal lung, while increasing tumour dose levels (if desired). In the patient simulation, a LED-optimized arc plan was designed using either 18 MV (3 × 1 cm(2)) or 6 MV (3 × 3cm(2)) beams. Both plans met the D95 dose coverage requirement for the target. However, the LED-optimized plan increased the maximum, mean, and minimum dose within the PTV by as much as 80 Gy, 11 Gy, and 3 Gy, respectively. Despite increased tumour dose levels, the 18 MV (3 × 1 cm(2)) arc plan improved or maintained the V20, V5, and mean lung dose metrics compared to the 6 MV (3 × 3 cm(2)) simulation. We conclude that LED-SBRT has the potential to increase dose gradients, and dose levels within a small lung tumour. The magnitude of tumour dose increase or lung sparing can be optimized through manipulation of RT parameters (e.g. beam energy and field size).
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Electrones , Neoplasias Pulmonares/cirugía , Pulmón/efectos de la radiación , Tratamientos Conservadores del Órgano/métodos , Radiocirugia/métodos , Tomografía Computarizada Cuatridimensional , Humanos , Pulmón/citología , Pulmón/diagnóstico por imagen , Neoplasias Pulmonares/diagnóstico por imagen , Neoplasias Pulmonares/patología , Método de Montecarlo , Fantasmas de Imagen , Radiocirugia/efectos adversosRESUMEN
PURPOSE: To examine a range of scenarios for image-guided adaptive radiation therapy of prostate cancer, including different schedules for megavoltage CT imaging, patient repositioning, and dose replanning. METHODS AND MATERIALS: We simulated multifraction dose distributions with deformable registration using 35 sets of megavoltage CT scans of 13 patients. We computed cumulative dose-volume histograms, from which tumor control probabilities and normal tissue complication probabilities (NTCPs) for rectum were calculated. Five-field intensity modulated radiation therapy (IMRT) with 18-MV x-rays was planned to achieve an isocentric dose of 76 Gy to the clinical target volume (CTV). The differences between D95, tumor control probability, V70Gy, and NTCP for rectum, for accumulated versus planned dose distributions, were compared for different target volume sizes, margins, and adaptive strategies. RESULTS: The CTV D95 for IMRT treatment plans, averaged over 13 patients, was 75.2 Gy. Using the largest CTV margins (10/7 mm), the D95 values accumulated over 35 fractions were within 2% of the planned value, regardless of the adaptive strategy used. For tighter margins (5 mm), the average D95 values dropped to approximately 73.0 Gy even with frequent repositioning, and daily replanning was necessary to correct this deficit. When personalized margins were applied to an adaptive CTV derived from the first 6 treatment fractions using the STAPLE (Simultaneous Truth and Performance Level Estimation) algorithm, target coverage could be maintained using a single replan 1 week into therapy. For all approaches, normal tissue parameters (rectum V(70Gy) and NTCP) remained within acceptable limits. CONCLUSIONS: The frequency of adaptive interventions depends on the size of the CTV combined with target margins used during IMRT optimization. The application of adaptive target margins (<5 mm) to an adaptive CTV determined 1 week into therapy minimizes the need for subsequent dose replanning.
Asunto(s)
Órganos en Riesgo/efectos de la radiación , Neoplasias de la Próstata/radioterapia , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia Guiada por Imagen/métodos , Radioterapia de Intensidad Modulada/métodos , Recto/efectos de la radiación , Fraccionamiento de la Dosis de Radiación , Fémur/diagnóstico por imagen , Fémur/efectos de la radiación , Humanos , Masculino , Tratamientos Conservadores del Órgano/métodos , Órganos en Riesgo/diagnóstico por imagen , Posicionamiento del Paciente , Próstata/diagnóstico por imagen , Próstata/efectos de la radiación , Neoplasias de la Próstata/diagnóstico por imagen , Neoplasias de la Próstata/patología , Traumatismos por Radiación/prevención & control , Radioterapia Guiada por Imagen/efectos adversos , Radioterapia de Intensidad Modulada/efectos adversos , Recto/diagnóstico por imagen , Factores de Tiempo , Tomografía Computarizada por Rayos X/métodos , Carga Tumoral , Vejiga Urinaria/diagnóstico por imagen , Vejiga Urinaria/efectos de la radiaciónRESUMEN
Cone-beam computed tomography (CBCT) has rapidly become a clinically useful imaging modality for image-guided radiation therapy. Unfortunately, CBCT images of the thorax are susceptible to artefacts due to scattered photons, beam hardening, lag in data acquisition, and respiratory motion during a slow scan. These limitations cause dose errors when CBCT image data are used directly in dose computations for on-line, dose adaptive radiation therapy (DART). The purpose of this work is to assess the magnitude of errors in CBCT numbers (HU), and determine the resultant effects on derived tissue density and computed dose accuracy for stereotactic body radiation therapy (SBRT) of lung cancer. Planning CT (PCT) images of three lung patients were acquired using a Philips multi-slice helical CT simulator, while CBCT images were obtained with a Varian On-Board Imaging system. To account for erroneous CBCT data, three practical correction techniques were tested: (1) conversion of CBCT numbers to electron density using phantoms, (2) replacement of individual CBCT pixel values with bulk CT numbers, averaged from PCT images for tissue regions, and (3) limited replacement of CBCT lung pixels values (LCT) likely to produce artificial lateral electron disequilibrium. For each corrected CBCT data set, lung SBRT dose distributions were computed for a 6 MV volume modulated arc therapy (VMAT) technique within the Philips Pinnacle treatment planning system. The reference prescription dose was set such that 95% of the planning target volume (PTV) received at least 54 Gy (i.e. D95). Further, we used the relative depth dose factor as an a priori index to predict the effects of incorrect low tissue density on computed lung dose in regions of severe electron disequilibrium. CT number profiles from co-registered CBCT and PCT patient lung images revealed many reduced lung pixel values in CBCT data, with some pixels corresponding to vacuum (-1000 HU). Similarly, CBCT data in a plastic lung phantom were reduced by 200 HU compared with known CT number values. For the three patients, dose results using the CBCT number data registered with PCT showed a prescription dose reduction ranging from 4 to 13% (D95 = 47 Gy). Therefore, accurate determination of lung density, especially for very low lung density (<0.2 g cm(-3)) is essential, but difficult to achieve using the CBCT data. Applying corrective techniques (1) and (2) to CBCT patient data produced unacceptable dose differences. For one typical VMAT SBRT patient, the D95 for the corrected CBCT and BCT image-based plans differed by -4% (D95 = 52 Gy) and 9% (D95 = 59 Gy) compared to the co-registered PCT image-based plan. However, corrective technique (3) produced negligible dose differences comparing LCT and PCT image-based plans. With regard to implementing on-line DART, dose errors must be minimized because they affect re-optimization decisions, and prevent accurate accumulation of the dose distribution.
Asunto(s)
Artefactos , Tomografía Computarizada de Haz Cónico , Electrones , Pulmón/diagnóstico por imagen , Pulmón/cirugía , Radiocirugia/métodos , Humanos , Neoplasias Pulmonares/diagnóstico por imagen , Neoplasias Pulmonares/radioterapia , Masculino , Fantasmas de Imagen , Radiografía Torácica , Dosificación Radioterapéutica , Planificación de la Radioterapia Asistida por ComputadorRESUMEN
Modern radiation therapy techniques such as intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) use tightly conformed megavoltage x-ray fields to irradiate a tumour within lung tissue. For these conditions, lateral electron disequilibrium (LED) may occur, which systematically perturbs the dose distribution within tumour and nearby lung tissues. The goal of this work is to determine the combination of beam and lung density parameters that cause significant LED within and near the tumour. The Monte Carlo code DOSXYZnrc (National Research Council of Canada, Ottawa, ON) was used to simulate four 20 × 20 × 25 cm(3) water-lung-water slab phantoms, which contained lung tissue only, or one of three different centrally located small tumours (sizes: 1 × 1 × 1, 3 × 3 × 3, 5 × 5 × 5 cm(3)). Dose calculations were performed using combinations of six beam energies (Co-60 up to 18 MV), five field sizes (1 × 1 cm(2) up to 15 × 15 cm(2)), and 12 lung densities (0.001 g cm(-3) up to 1 g cm(-3)) for a total of 1440 simulations. We developed the relative depth-dose factor (RDDF), which can be used to characterize the extent of LED (RDDF <1.0). For RDDF <0.7 severe LED occurred, and both lung and tumour dose were drastically reduced. For example, a 6 MV (3 × 3 cm(2)) field was used to irradiate a 1 cm(3) tumour embedded in lung with ultra-low density of 0.001 g cm(-3) (RDDF = 0.2). Dose in up-stream lung and tumour centre were reduced by as much as 80% with respect to the water density calculation. These reductions were worse for smaller tumours irradiated with high energy beams, small field sizes, and low lung density. In conclusion, SBRT trials based on dose calculations in homogeneous tissue are misleading as they do not reflect the actual dosimetric effects due to LED. Future clinical trials should only use dose calculation engines that can account for electron scatter, with special attention given to patients with low lung density (i.e. emphysema). In cases where tissue inhomogeneity corrections are applied, the nature of the correction used may be inadequate in predicting the correct level of LED. In either case, the dose to the tumour is not the prescribed dose and clinical response data are uncertain. The new information from this study can be used by radiation oncologists who wish to perform advanced radiation therapy techniques while avoiding the deleterious predictable dosimetric effects of LED.
Asunto(s)
Neoplasias Pulmonares/radioterapia , Planificación de la Radioterapia Asistida por Computador/métodos , Algoritmos , Simulación por Computador , Electrones , Enfisema/complicaciones , Enfisema/diagnóstico por imagen , Humanos , Pulmón/diagnóstico por imagen , Pulmón/efectos de la radiación , Neoplasias Pulmonares/complicaciones , Neoplasias Pulmonares/diagnóstico por imagen , Método de Montecarlo , Fantasmas de Imagen , Radiografía , Radiocirugia/estadística & datos numéricos , Dosificación Radioterapéutica , Radioterapia de Intensidad Modulada/estadística & datos numéricosRESUMEN
The January 2010 articles in The New York Times generated intense focus on patient safety in radiation treatment, with physics staffing identified frequently as a critical factor for consistent quality assurance. The purpose of this work is to review our experience with medical physics staffing, and to propose a transparent and flexible staffing algorithm for general use. Guided by documented times required per routine procedure, we have developed a robust algorithm to estimate physics staffing needs according to center-specific workload for medical physicists and associated support staff, in a manner we believe is adaptable to an evolving radiotherapy practice. We calculate requirements for each staffing type based on caseload, equipment inventory, quality assurance, educational programs, and administration. Average per-case staffing ratios were also determined for larger-scale human resource planning and used to model staffing needs for Ontario, Canada over the next 10 years. The workload specific algorithm was tested through a survey of Canadian cancer centers. For center-specific human resource planning, we propose a grid of coefficients addressing specific workload factors for each staff group. For larger scale forecasting of human resource requirements, values of 260, 700, 300, 600, 1200, and 2000 treated cases per full-time equivalent (FTE) were determined for medical physicists, physics assistants, dosimetrists, electronics technologists, mechanical technologists, and information technology specialists, respectively.
Asunto(s)
Algoritmos , Física Sanitaria/estadística & datos numéricos , Selección de Personal/estadística & datos numéricos , Admisión y Programación de Personal/estadística & datos numéricos , Oncología por Radiación/estadística & datos numéricos , Ontario , Selección de Personal/tendencias , Admisión y Programación de Personal/tendencias , Oncología por Radiación/tendencias , Recursos HumanosRESUMEN
Ion chamber dosimetry requires a high degree of precision, at all steps within the dosimetric process, in order to ensure accurate dose measurements. This work presents a novel technique for ion chamber volume determination and quality assurance, using micro-computed tomography (micro-CT). Four nominally identical Exradin A1SL chambers (0.056 cm(3)) (Standard Imaging, WI, USA) were imaged using a micro-CT system (GE Locus, GE Healthcare, London, Ontario) and irradiated in a 6 MV x-ray reference field. Air volumes were calculated from the CT datasets using 3D analysis software (Microview 2.1.1, General Electric Healthcare, London, Ontario). Differences in the volumes of each chamber determined using micro-CT images agreed with differences in the ionization response within 1% for each chamber. Calibration coefficients were then compared through cross-calibration with a calibrated ion chamber and from the CT-measured volumes. The average ratio of these values was found to be 0.958 +/- 0.009 indicating good correlation. The results demonstrate the promise of using micro-CT imaging for the absolute volumetric characterization of ion chambers. The images have the potential to be an important clinical tool for quality assurance of ion chamber construction and integrity after routine clinical usage.