A Kerr

The subtleties of ultrasound images of an ensemble of cells: simulation from regular and more random distributions of scatterers

Significant differences in the backscatter amplitudes which are correlated with different tissue morphology have been observed in ultrasound images of tissue. While many factors could be linked to subtle changes in the images, the purpose of this paper is to explore the possibility that backscatter signals are linked to the organization of the spatial distribution of individual cells that produce an ensemble of scattering sources. Simple one- and two-dimensional simulations of backscatter signals produced by weak scatters separated by << lambda to < lambda in regular, random, and pseudo-random distributions in a sample are performed. Both regular and pseudo-random distributions produce large boundary signals, and in the central regions of the sample, the square root of the backscatter power is directly related to the amount of randomization, R, over a large range. Large changes in backscattering intensities are predicted for the same density of scatterers with differing R in different regions of the same sample. Thus, the subtle differences in the scattering distribution should show significant changes in the backscatter images.

The role of Cobalt-60 in modern radiation therapy: Dose delivery and image guidance.

The advances in modern radiation therapy with techniques such as intensity-modulated radiation therapy and image-guided radiation therapy (IMRT and IGRT) have been limited almost exclusively to linear accelerators. Investigations of modern Cobalt-60 (Co-60) radiation delivery in the context of IMRT and IGRT have been very sparse, and have been limited mainly to computer-modeling and treatment-planning exercises. In this paper, we report on the results of experiments using a tomotherapy benchtop apparatus attached to a conventional Co-60 unit. We show that conformal dose delivery is possible and also that Co-60 can be used as the radiation source in megavoltage computed tomography imaging. These results complement our modeling studies of Co-60 tomotherapy and provide a strong motivation for continuing development of modern Cobalt-60 treatment devices.

Practical and clinical considerations in Cobalt-60 tomotherapy

Cobalt-60 (Co-60) based radiation therapy continues to play a significant role in not only developing countries, where access to radiation therapy is extremely limited, but also in industrialized countries. Howver, technology has to be developed to accommodate modern techniques, including image guided and adaptive radiation therapy (IGART). In this paper we describe some of the practical and clinical considerations for Co-60 based tomotherapy by comparing Co-60 and 6 MV linac-based tomotherapy plans for a head and neck (HandN) cancer and a prostate cancer case. The tomotherapy IMRT plans were obtained by modeling a MIMiC binary multi-leaf collimator attached to a Theratron-780c Co-60 unit and a 6 MV linear accelerator (CL2100EX). The EGSnrc/BEAMnrc Monte Carlo (MC) code was used for the modeling of the treatment units with the MIMiC collimator and EGSnrc/DOSXYZnrc code was used for beamlet dose data. An in-house inverse treatment planning program was then used to generate optimized tomotherapy dose distributions for the H and N and prostate cases. The dose distributions, cumulative dose area histograms (DAHs) and dose difference maps were used to evaluate and compare Co-60 and 6 MV based tomotherapy plans. A quantitative analysis of the dose distributions and dose-volume histograms shows that both Co-60 and 6 MV plans achieve the plan objectives for the targets (CTV and nodes) and OARs (spinal cord in HandN case, and rectum in prostate case).

The Potential for Image Guided Radiation Therapy with Cobalt-60 Tomotherapy

Helical tomotherapy, a new approach for Intensity Modulated Radiation Therapy, employs a fan-beam of radiation from a source mounted in a CT-like ring gantry. Complex conformal dose delivery is achieved by modulating the intensity of the radiation beam as the source revolves about the patient. A particular benefit of helical tomotherapy is the ability to perform in-situ CT imaging to confirm patient set-up, and to reconstruct the dynamically delivered dose distributions. In this paper we present the results of ongoing work to establish the potential for tomotherapy using a Cobalt-60 radioactive source. Both dose delivery and megavoltage CT imaging data confirm the feasibility of image guided radiation therapy using Cobalt-60 tomotherapy.

Cobalt-60 tomotherapy: Clinical treatment planning and phantom dose delivery studies

PURPOSEnInvestigations have shown that a Cobalt-60 (Co-60) radioactive source has the potential to play a role in intensity modulated radiation therapy (IMRT). In this paper, Co-60 tomotherapys conformal dose delivery potential is evaluated by delivering conformal dose plans on a cylindrical homogeneous phantom containing clinical structures similar to those found in a typical head and neck (H&N) cancer. Also, the clinical potential of Co-60 tomotherapy is investigated by generating 2D clinical treatment plans for H&N and prostate anatomical regions. These plans are compared with the 6 MV based treatment plans for modalities such as linear accelerator-based tomotherapy and broad beam IMRT, and 15 MV based 3D conformal radiation therapy (3DCRT).nnnMETHODSnFor experimental validation studies, clinical and nonclinical conformal dose patterns were delivered on circular, homogeneous phantoms containing GafChromic film. For clinical planning study, dose calculations were performed with the EGSnrc Monte Carlo program, where a Theratronics 780C Co-60 unit and a 6 MV linear accelerator were modeled with a MIMiC binary multileaf collimator. An inhouse inverse treatment planning system was used to optimize tomotherapy plans using the same optimization parameters for both Co-60 and 6 MV beams. The IMRT and 3DCRT plans for the clinical cases were generated entirely in the Eclipse treatment planning system based on inhouse IMRT and 3DCRT site specific protocols.nnnRESULTSnThe doses delivered to the homogeneous phantoms agreed with the calculations, indicating that it is possible to deliver highly conformal doses with the Co-60 unit. The dose distributions for Co-60 tomotherapy clinical plans for both clinical cases were similar to those obtained with 6 MV based tomotherapy and IMRT, and much more conformal compared to 3DCRT plans. The dose area histograms showed that the Co-60 plans achieve the dose objectives for the targets and organs at risk.nnnCONCLUSIONSnThese results confirm that Co-60 tomotherapy is capable of providing state-of-the-art conformal dose delivery and could be used for the treatment of targets in both small and larger separation anatomical regions.

Metal artifact suppression in megavoltage computed tomography

There has been considerable interest in megavoltage CT (MVCT) imaging associated with the development of image guided radiation therapy. It is clear that MVCT can provide good image quality for patient setup verification with soft tissue contrast much better than noted in conventional megavoltage portal imaging. In addition, it has been observed that MVCT images exhibit considerably reduced artifacts surrounding metal implants (e.g., surgical clips, hip implants, dental fillings) compared to conventional diagnostic CT images (kVCT). When encountered, these artifacts greatly limit the usefulness of kVCT images, and a variety of solutions have been proposed to remove the artifacts, but these have met with only partial success. In this paper, we investigate the potential for CT imaging in regions surrounding metal implants using high-energy photons from a Cobalt-60 source and from a 4 MV linear accelerator. MVCT and kVCT images of contrast phantoms and a phantom containing a hip prosthesis are compared and analysed. We show that MVCT scans provide good fidelity for CT number quantification in the high-density regions of the images, and in the regions immediately adjacent to the metal implants. They also provide structural details within the high-density inserts and implants. Calculations will show that practical clinical MVCT imaging, able to detect 3% contrast objects, should be achievable with doses of about 2.5cGy. This suggests that MVCT not only has a role in radiotherapy treatment planning and guidance, but may also be indicated for surgical guidance and follow-up in regions where metal implants cannot be avoided.

Examination of Jeltrate®Plus as a tissue equivalent bolus material

A product available commercially for making dental impressions, Jeltrate®Plus, was evaluated as a tissue equivalent bolus material. Jeltrate®Plus was found to be tissue equivalent in 6 and 15 MV photon energy beams and 6, 12, and 20 MeV electron energy beams. As a first step, different preparations for making the bolus material were investigated and an optimal mixture was determined to be two parts Jeltrate®Plus powder to three parts water by weight. A suitable method for storing the material was found to be in a water filled plastic container. Since the product is fairly inexpensive and is easily and quickly made and moulded into different shapes, it is an excellent bolus material to use when treating irregular patient contours. PACS number(s): 87.53.–j, 87.66.–a

Poster — Thurs Eve‐09: Evaluation of a commercial 2D ion‐chamber array for intensity modulated radiation therapy dose measurements

Experimental verification of calculated dose from a treatment planning system is often essential for quality assurance (QA) of intensity modulated radiation therapy (IMRT). Film dosimetry and single ion chamber measurements are commonly used for IMRT QA. Film dosimetry has very good spatial resolution, but is labor intensive and absolute dose is not reliable. Ion chamber measurements are still required for absolute dose after measurements using films. Dosimeters based on 2D detector arrays that can measure 2D dose in real-time are gaining wider use. These devices provide a much easier and reliable tool for IMRT QA. We report the evaluation of a commercial 2D ion chamber array, including its basic performance characteristics, such as linearity, reproducibility and uniformity of relative ion chamber sensitivities, and comparisons between measured 2D dose and calculated dose with a commercial treatment planning system. Our analysis shows this matrix has excellent linearity and reproducibility, but relative sensitivities are tilted such that the +Y region is over sensitive, while the -Y region is under sensitive. Despite this behavior, our results show good agreement between measured 2D dose profiles and Eclipse planned data for IMRT test plans and a few verification plans for clinical breast field-in-field plans. The gamma values (3% or 3 mm distance-to-agreement) are all less than 1 except for one or two pixels at the field edge This device provides a fast and reliable stand-alone dosimeter for IMRT QA.

Quantitative investigations of megavoltage computed tomography

Megavoltage computed tomography (MVCT) has been an active area of research and development in image guided radiation therapy. We have been investigating a particular implementation of MVCT in conjunction with studies of the potential for tomotherapy with a Cobalt-60 radiation source. In this paper, we present results comparing MVCT using a Co-60 source and a 4 MV linear accelerator to conventional kVCT imaging. The Co-60 and linac MVCT measurements were obtained with a first generation benchtop CT imager; the KVCT measurements were obtained using a Philips AcQSim CT Simulator). Phantoms containing various inserts ranging in density from air, through lung, soft tissue and bone equivalent materials and extending to high atomic number metals were imaged with the three modalities. The results enable characterization of image artifacts, CT number linearity and beam hardening. The MVCT images have sufficient contrast that soft tissue regions with 2.8% difference in electron density can be visualized. In MVCT, a linear relationship between CT numbers and electron densities extends to materials with Z ≈ 60. In the 4MV CT imaging there is a position dependence of the CT numbers within a uniform water phantom, which is absent in Co-60 CT images, indicating the presence of beam hardening artifacts in the linac MVCT images. The differences between kVCT and MVCT will be discussed considering the variation of the photon interactions dominating the images. Our investigations indicate that MVCT has properties that may potentially extend its utility beyond radiation therapy.

SU‐EE‐A1‐06: A Comparative Study of Cobalt‐60 Based Tomotherapy versus 6 MV Linac‐Based Tomotherapy, IMRT, and 3DCRT for the Treatment Planning of Prostate and Head and Neck Cases

Purpose: Cobalt‐60 (Co‐60) based radiation therapy continues to play a significant role in a large number of countries due its simplicity and robustness. However, it has not been developed to accommodate modern techniques that provide intensity modulated radiation therapy(IMRT). In this paper we present the results of investigations of Co‐60 based tomotherapy. Particularly, we generate clinical plans for prostate and head and neck (H&N) anatomical regions and compare them with the plans obtained with 6MV based linac tomotherapy, standard 6MV IMRT, and 3D conformal radiation therapy (3DCRT) techniques. Method and Materials: The tomotherapy plans were obtained by modeling a MIMiC binary multileaf collimator attached to a Theratron‐780C Co‐60 unit and a 6MV linear accelerator. The EGSnrc/BEAMnrc Monte Carlo code was used to model the treatment units with the MIMiC collimator while EGSnrc/DOSXYZnrc code was used for calculating dose on prostate and H&N CT datasets. All heterogeneities and patient contours were considered. An in‐house inverse treatment planning program was then used to optimize all 2D tomotherapy plans. The IMRT and 3DCRT plans were generated in Eclipse treatment planning system based on our in‐house IMRT and 3DCRT clinical protocols for prostate and H&N treatment.Results: A quantitative analysis of the dose distributions and dose area histograms (DAHs) showed that the Co‐60 plans achieve the dose objectives for the targets and OARs. The dose distributions and DAHs for Co‐60 tomotherapy plans for both cases are very similar to those obtained with 6MV based tomotherapy and IMRT, and are much more conformal compared to 3DCRT plans. Conclusion: Our investigations confirm that Co‐60 tomotherapy is indeed capable of providing state‐of‐the‐art conformal dose delivery and could be used for the treatment of targets in both small and larger separation anatomical regions.