B Disher

An in-depth Monte Carlo study of lateral electron disequilibrium for small fields in ultra-low density lung: implications for modern radiation therapy

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.

Rapid block matching based nonlinear registration on GPU for image guided radiation therapy

To compensate for non-uniform deformation due to patient motion within and between fractions in image guided radiation therapy, a block matching technique was adapted and implemented on a standard graphics processing unit (GPU) to determine the displacement vector field that maps the nonlinear transformation between successive CT images. Normalized cross correlation (NCC) was chosen as the similarity metric for the matching step, with regularization of the displacement vector field being performed by Gaussian smoothing. A multi-resolution framework was adopted to further improve the performance of the algorithm. The nonlinear registration algorithm was first applied to estimate the intrafractional motion from 4D lung CT images. It was also used to calculate the inter-fractional organ deformation between planning CT (PCT) and Daily Cone Beam CT (CBCT) images of thorax. For both experiments, manual landmark-based evaluation was performed to quantify the registration performance. In 4D CT registration, the mean TRE of 5 cases was 1.75 mm. In PCT-CBCT registration, the TRE of one case was 2.26mm. Compared to the CPU-based AtamaiWarp program, our GPU-based implementation achieves comparable registration accuracy and is ~25 times faster. The results highlight the potential utility of our algorithm for online adaptive radiation treatment.

Sci—Sat AM(1): Planning — 11: Use of a Graphics Processor (GPU) for High-Performance Deformable Registration of Cone Beam (kV) and Megavoltage (MV) CT Images

To compensate for the inter-fraction deformation in fractionated radiotherapy, it is essential that “images of the day” used for treatment guidance be co-registered with the 3D images used initially for treatment planning and dose prescription. We implemented a high performance deformable image registration algorithm on the standard graphics process unit (GPU) to accomplish this very efficiently with the ultimate goal of enabling adaptive dose computations at the treatment console. Normalized cross correlation (NCC) was employed as the similarity metric in a block-matching algorithm. Regularization of the resulting displacement vector field was performed by Gaussian smoothing. A multi-resolution strategy was adopted to further improve the performance of the algorithm. To evaluate performance, we compared results with two popular deformable registration algorithms (Diffeomorphic Demons and B-spline) based from the Insight Toolkit (ITK). All three algorithms were first applied to register thoracic planning CT (PCT) to cone-beam CT (CBCT) scans of three lung cancer patients. Next, they were used to align the pelvic PCT to megavoltage CT (MVCT) scans from a tomotherapy unit of a prostate cancer patient. For both types of anatomy and image features (contrast, noise), manual landmark-based evaluation was performed to quantify the registration accuracy. In PCT-CBCT registration experiment, mean registration error (MRE) was 2.53mm. In PCT-MVCT registration, MRE was 2.15 mm. Compared to Diffeomorphic Demons and B-spline-based algorithms, our GPU-based implementation achieves comparable registration accuracy and is ∼20 times faster (completes registration in 15 seconds). The results highlight the potential utility of our algorithm for on-line adaptive radiation treatment.

Poster — Thur Eve — 62: A Retrospective Assessment of the Prevalence and Dosimetric Effect of Lateral Electron Disequilibrium in a Population of Lung Cancer Patients Treated by Stereotactic Body Radiation Therapy

Stereotactic Body Radiation Therapy (SBRT) is a treatment option for early stage non-small cell lung cancer (NSCLC). SBRT uses tightly conformed megavoltage (MV) x-ray beams to ablate the tumour. However, small MV x-ray fields may produce lateral electron disequilibrium (LED) within lung tissue, which can reduce the dose to tumour. The goal of this work is to estimate the prevalence of LED in NSCLC patients treated with SBRT, and determine dose effects for patients prone or averse to LED. Thirty NSCLC patients were randomly selected for analysis. 4-dimensional CT lung images were segmented into the right and left upper and lower lobes (RUL, RLL, LUL, LLL), and the right middle lobe. Dose calculations were performed using volume-modulated arc therapy in the Pinnacle3 TPS. Most tumours were located in the upper lobes (RUL 53%, LUL 27%) where density was significantly lower (RUL −808±46 HU vs. RLL −743±71 HU; LUL −808 ±56 HU vs. LLL −746±70 HU; p<0.001). In general, the prevalence of LED increased with higher beam energy. Using 6MV photons, patients with a RUL tumour experienced moderate (81 %), and mild (19%) levels of LED. At 18MV, LED became more prominent with severe (50%) and moderate (50%) LED exhibited. Dosimetrically, for patients prone to LED, poorer target coverage (i.e. increased R100 by 20%) and improved lung sparing (i.e. reduced V20 by −46%) was observed. The common location of lung cancers in the upper lobes, coupled with lower lung density, results in the potential occurrence of LED, which may underdose the tumour.

Sci—Thur PM: YIS — 04: Forcing lateral electron disequilibrium to spare lung tissue: A novel technique for SBRT of small lung tumours

Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x-ray fields, improves local control of lung cancer. However, small MV x-ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT: LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lung tissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantoms diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm3 ) shells surrounding a small water tumor (volume = 0.8 cm3 ). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm2 (RDDF∼0.5) and 3×3cm2 (RDDF∼1). Analyzing dose results, the 1×1cm2 arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm2 arc. Although, central tumor dose was also reduced by 15% using the 1×1cm2 arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, its possible to design a SBRT treatment plan that reduces lung dose while maintaining relatively high tumor dose levels. Clinical application requires an accurate dose algorithm and may lower SBRT dose-induced toxicity levels in patients.

Poster — Thur Eve — 08: A1SL ion chamber charged particle disequilibrium corrections for lung dose measurements using Monte Carlo

An in house inhomogeneous insert for use with ArcCHECK ™ was developed for dose calculation verification of Stereotactic Body Radiation Therapy (SBRT) lung plans. The inhomogeneous insert has various ion chamber inserts for different geometrical configurations (lung, soft tissue, bone, air). However, the insertion of an ion chamber in a low density medium perturbs the dose to that region by creating Charged Particle Disequilibrium (CPD), limiting the accuracy of ion chamber measurements. By simulating the ion chamber and phantom using Monte Carlo, a correction factor could be calculated and measured to verify the dose difference caused by CPD. BEAMnrc was used to generate a phase space input file for DOSXYZnrc with beam characteristics that matched clinical commissioning data. A model of the A1SL ion chamber geometry (shell, collector, stem, guard) was simulated in a simple water-lung-water slab phantom. Dose to the active area of the ion chamber was measured in several locations throughout the phantom. The active area of the ion chamber was replaced by the surrounding medium; i.e., water or lung within the phantom, and the dose to the same voxels was calculated. The dose was measured on a Linac and the results agreed within 3% and confirmed that the presence of the ion chamber in low density lung perturbs the dose measured in the field by over 31%.

SU‐E‐T‐882: Electron Disequilibrium Pitfalls for Small Megavoltage Photon Fields Incident on Lung Tumors

Purpose: Stereotactic body radiation therapy(SBRT) of lung uses sub‐centimeter MV x‐ray fields. Under these conditions, lateral electron disequilibrium (LED) can occur in lungtissue, which causes perturbations of the dose distribution near the tumor. This purpose of this work is to characterize the LED effect in lung for clinically relevant ranges of beam energies, field sizes, and lung densities. Methods: The MC code DOSXYZnrc (National Research Council of Canada, Ottawa, ON) was employed to simulate two 20×20×25cm3 water‐lung‐water slab phantoms. The two phantoms were identical in composition except that the second phantom also included a 3×3×3cm3 centrally located water cube to mimic a small lungtumor. To characterize LED,dose calculations were performed using combinations of beam energy (Co‐60 up to 18MV), field sizes (1×1cm2 up to 15×15cm2), and lung densities (0.001g/cm3 up to 1g/cm3) for both phantoms. Results: MClung slab phantom simulations revealed that for each combination of beam energy and field size, a critical lung density (CLD) could be defined to establish LED. For example, a 6MV 5×5cm2 photon field was subject to LED for lung densities of 0.2g/cm3 or lower. On the contrary, employing an 18MV 5×5cm2 photon field increased the CLD to 0.5g/cm3. With regard to the second lungtumor phantom, the LED effect caused major reductions in the calculated dose near to the tumor. For instance, dose reductions of 24% and 16% were found within the distal and proximal tumor surfaces, respectively. Conclusion: We have fully characterized the LED effect and shown that it causes dose reductions in both lung and tumortissues. To avoid these dose perturbations, SBRT of lungcancer patients should be optimized to select radiation therapy parameters carefully in accordance with patient lung density. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Institutes of Health Research (CIHR) are gratefully acknowledged.

Poster — Thur Eve — 23: Artificial Electron Disequilibrium Due to Inaccurate Cone‐Beam CT Data for Adaptive Lung Radiation Therapy

Cone-beam computed tomography (CBCT) is becoming a clinically useful imaging modality for image-guided adaptive radiation therapy. The Varian On-Board Imaging system (Varian Medical Systems Inc., Palo Alto California) uses a 125 kVp source, emitting a conical x-ray beam, and a flat-panel amorphous silicon detector. Unfortunately, CBCT images are prone to artifacts such as those caused by acceptance of x-ray scatter from the patient at the detector plane, intra-fraction motion, and x-ray spectral “beam-hardening”. Previous studies indicate that large dose inaccuracies occur when using CBCT lung images for adaptive dose computations, compared to other treatment sites with less tissue heterogeneity. We have compared dose distributions calculated using CBCT and 4-dimensional (4D) time-averaged CT (Philips Inc., Cleveland, OH) lung images of the same patient. Using 6MV fields, an under-dosage of 55Gy was predicted for the CBCT planned target volume, compared to 60Gy predicted by the 4DCT based plan. CT number profiles from CBCT and 4DCT lung images revealed many undervalued pixels in the CBCT data, some corresponding to vacuum (−1000HU)! Monte Carlo simulations of dose deposition, using a water and lung slab geometry, were used to study the effects of ultra-low density on the 3D dose distribution. It was found that a specific transition-density induces lateral electron disequilibrium, and causes an undervaluation of dose in mid-lung, along the central axis of the beam. Thus, CBCT images containing depressed CT number values in lung caused an artificial electron disequilibrium problem, which can be misinterpreted in adaptive treatment re-planning.

Poster — Wed Eve—40: Comparison of CBCT CT Numbers to CT‐Simulator CT Numbers using Mutual Information and Joint Probability Distributions

The Varian On‐Board Imaging (OBI) system is an imaging tool used by the London Regional Cancer Program for image‐guidedradiation therapy. The system consists of a kilovoltage source (120 kVp), emitting a conical X‐ray beam, and a flat panel amorphous silicon detector. Both source and detector are mounted on the linear accelerator gantry, rotating 360° about the patient creating 3‐dimensional volumetric computed tomographyimages. Unfortunately, cone‐beam computed tomography(CBCT)images are susceptible to many forms of artifacts resulting primarily from: increased amounts of scatter due to CBCT geometry, spectral X‐ray attenuation, and intra‐scanning organ motion. In order to assess the extent of CBCTimage artifacts, we propose a new technique that compares CT Numbers in corresponding tissue regions obtained by the CBCT and Phillips Brilliance CT‐simulator systems. Our approach registers CTimages from two imaging modalities using a mutual information (MI) algorithm. Once aligned, quantitative CT Number analysis is performed on the CTimages using a 2‐dimensional histogram (or joint probability distribution) of pixel counts for geographically‐matched pairs of CT Numbers. Phantom studies have shown similarity between CBCT and CT‐simulation CT Numbers when the imaged medium is head‐like in diameter (20 cm) and homogeneous (Catphan 504 phantom). Greater discrepancies were observed (400 HU) when CTimages of larger phantoms (30 cm diameter, Gammex RMI Tomo phantom) and increased heterogeneity were compared. Presently, quantitative CBCTCT Number inaccuracy precludes the use of CBCTimages for whole body treatment planning, and adaptive dose computations.

Poster — Thurs Eve‐18: Performance evaluation of MV CT imaging on the HI ART II tomotherapy unit

The HI-ART II unit (TomoTherapy Inc., Madison WI) is a modality used by the London Regional Cancer Program (LRCP) for radiation therapy. This machine uses the same source of Megavoltage energy radiation to image (3.5 MV) and to treat (6MV) patients, combining the functionality of a traditional linear accelerator and CT simulator into one unit. Thus, it is possible to assess patient positioning and adjust for anatomy changes just prior to radiation therapy. Unfortunately, at MV energy levels, the physics of radiation interaction limits image quality, and gives rise to an inherent dose limitation concern that enhances noise levels. Therefore, we propose to quantify the image quality produced by the HI-ART II unit using techniques established for kVCT scanner technology. Our study involved the use of three standard phantoms to test image resolution, noise, uniformity, and linearity for a 512 × 512 reconstruction matrix and three scan pitch settings (0.8, 1.6, and 2.4). Results follow: linearity between MV CT number versus relative electron density was observed, noise calculations ranged from 2.15-2.51%, and a distinct central artifact was revealed during uniformity testing. The linearity between MV CT number versus relative electron density implies that MV CT images are highly suitable for dose calculations. MV CT image quality of uniform phantoms were acceptable and demonstrated noise levels higher than those produced by kVCT simulators. Further study is necessary to correct for the central artifact in MV CT images.