J Darko

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).

Investigation of an efficient source design for Cobalt-60-based tomotherapy using EGSnrc Monte Carlo simulations

Recent investigations demonstrate a strong potential for Cobalt-60 (Co-60)-based tomotherapy. Reported work suggests that Co-60-based tomotherapy offers a clinically and commercially viable alternative to megavoltage x-ray-based tomotherapy. Tomotherapy applications use a combination of intensity-modulated fan beams to deliver highly conformal radiotherapy. However, conventional Co-60 units are designed to give large uniform rectangular fields using an isotropic radioactive source in a cylindrical geometry. Such cylindrical source geometry likely provides a sub-optimal use of the radioactivity within the source volume for tomotherapy applications due to a significant loss of radiated energy outside the fan beam collimation system. To investigate a more efficient source geometry suitable for Co-60 tomotherapy applications, a computer code was written to model an isotropic source in a 6-faced polyhedron geometry such as cube, parallelepiped, prism and truncated pyramid. This code was integrated with the existing EGSnrc/BEAMnrc Monte Carlo (MC) code. The integrated source code was thoroughly tested, validated and used to investigate the energy spectra, radiation output and self-shielding properties of various rectangular-shaped (RS) Co-60 sources. Fan beam dose profiles were calculated for various cylindrical and RS Co-60 sources for a range of source-to-axis distances (SAD), multi-leaf collimator-to-isocentre distances (CID) and modified collimator systems. Fringe and penumbra distances were analysed for the simulated dose profiles. Our results demonstrate that clinically acceptable fringe and penumbra distances can be achieved by a careful selection of SAD, CID, source shape and dimensions and modified collimator system. Significant overall gain in radiation output of the 20 x 1 cm(2) fan beams can be achieved by an optimal selection of the source geometry for a given active volume of Co-60. The overall gain includes the effects of change in packing density (accounting for self-absorption) and change in source shape.

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

PURPOSE Investigations 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). METHODS For 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. RESULTS The 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. CONCLUSIONS These 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.

Dosimetry of interface region near closed air cavities for Co-60, 6 MV and 15 MV photon beams using Monte Carlo simulations.

Underdosing of treatment targets can occur in radiation therapy due to electronic disequilibrium around air-tissue interfaces when tumors are situated near natural air cavities. These effects have been shown to increase with the beam energy and decrease with the field size. Intensity modulated radiation therapy (IMRT) and tomotherapy techniques employ combinations of multiple small radiation beamlets of varying intensities to deliver highly conformal radiation therapy. The use of small beamlets in these techniques may therefore result in underdosing of treatment target in the air-tissue interfaces region surrounding an air cavity. This work was undertaken to investigate dose reductions near the air-water interfaces of 1×1×1 and 3×3×3 cm3 air cavities, typically encountered in the treatment of head and neck cancer utilizing radiation therapy techniques such as IMRT and tomotherapy using small fields of Co-60, 6 MV and 15 MV photons. Additional investigations were performed for larger photon field sizes encompassing the entire air-cavity, such as encountered in conventional three dimensional conformal radiation therapy (3DCRT) techniques. The EGSnrc/DOSXYZnrc Monte Carlo code was used to calculate the dose reductions (in water) in air-water interface region for single, parallel opposed and four field irradiations with 2×2 cm2 (beamlet), 10×2 cm2 (fan beam), 5×5 and 7×7 cm2 field sizes. The magnitude of dose reduction in water near air-water interface increases with photon energy; decreases with distance from the interface as well as decreases as the number of beams are increased. No dose reductions were observed for large field sizes encompassing the air cavities. The results demonstrate that Co-60 beams may provide significantly smaller interface dose reductions than 6 MV and 15 MV irradiations for small field irradiations such as used in IMRT and tomotherapy.

Aperture superposition dose model versus pencil beam superposition dose model for a finite size Cobalt-60 source for tomotherapy deliveries

PURPOSE The finite size pencil beam (FSPB) superposition method is a commonly used dose calculation method in intensity modulated radiation therapy (IMRT). The FSPB model assumes that dose for a broad intensity modulated beam can be calculated by superposition of dose from small, pencil-like beams. However, this model is limited to point-like radiation sources and is not valid for finite size sources, such as a Cobalt-60 (Co-60) source of 2 cm diameter. In this paper, the authors present results that show the limitation of this model and propose an alternative model, namely the aperture superposition (AS) model, to calculate photon dose for intensity modulated beams arising from finite size radiation sources. METHODS The AS model is based on adding beam apertures rather than pencil beams. Each aperture is defined as a series of adjacently opened leaves of a multileaf collimator with no closed leaves in between them. The apertures are calculated using the EGSnrc Monte Carlo program. The accuracy of the AS model was tested for dose calculations of fan beams, as encountered in tomotherapy treatment plans. The results were compared with the FSPB model and GafChromic film measurements. The measurements and simulations were performed for a clinical Theratronics T780C Co-60 unit with MIMiC binary multileaf collimator mounted on it. RESULTS The comparisons between the AS model and film measurements show agreement better than 1.5% in the high dose regions and 3.7% in the low dose regions. On the contrary, film measurement comparisons to the FSPB model show that the FSPB model underestimates the dose by up to 7% for small field sizes such as 2 × 2 cm(2) and 20% for larger field sizes such as 20 × 2 cm(2). CONCLUSIONS The results presented in this paper indicate that the AS model provides better accuracy than the FSPB model when calculating dose for fan beams from large radiation sources. The implementation of this model to the current treatment planning systems has the scope of advancing Co-60 based IMRT and tomotherapy.

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.

Sci—Fri PM: Delivery — 10: Imaging Developments for Broad Beam Co-60 Radiation Therapy

Accessibility is essential to providing quality health care. Our research work has shown that Cobalt-60 (Co-60) treatment can be modernized to provide intensity modulated radiation therapy (IMRT). The use of Co-60 as a radiation source could provide a solution in parts of the world with limited infrastructure to support linac therapy. Previous work has been focused on tomotherapy; however, we have recently expanded to broad-beam IMRT. This could potentially allow the existing fleet of over one thousand Co-60 units to be upgraded instead of replaced. On-board image guidance would be necessary to ensure tumour localization for precise IMRT treatment. Recent acquisition of an amorphous silicon (a-Si) PortalVision aS500 (Varian Medical Systems, Palo Alto, CA) imaging panel has allowed testing of broad beam imaging modalities using a Co-60 therapy unit (Best Theratronics T780C, Kanata, ON). Imaging with the therapy source could avoid the requirement of an additional lower activity source or kV imaging system. Portal imaging and cone-beam computed tomography (CBCT) are widely used for image guidance with broad beam IMRT. Tomosynthesis imaging can provide depth discrimination information using a limited number of projections. Preliminary results clearly demonstrate that all three modalities are feasible with a therapy Co-60 source and a-Si imaging panel. Co-60 CBCT resolution and image quality are comparable to previous fan-beam scans. Co-60 digital tomosynthesis (DT) is shown to enhance anatomical features at arbitrary depths. Thus DT has the potential to generate more useful information for patient setup verification than portal images while delivering less dose than CBCT.

Poster — Wed Eve—56: Megavoltage Digital Tomosynthesis Using a Radioactive Cobalt‐60 Gamma Ray Source for Radiation Therapy Treatment Verification

The ability for megavoltage computed tomography patient setup verification using a cobalt‐60 (Co‐60) gamma ray source has already been established in the context of cobalt tomotherapy by our group. However, it would be beneficial to establish improved cobaltimaging that could be used on more conventional units. Digital tomosynthesis (DT) is an imaging modality that may provide depth localization and improved contrast and visibility for treatment verification in modern Co‐60 radiation therapy compared with conventional portal imaging. DT is a practical and affordable method of achieving volumetric data from a limited number of projections. Preliminary investigations of MVDT using a Co‐60 source in linear‐scan parallel‐path and isocentric acquisition geometries demonstrate the ability to achieve volumetric data from a limited number of projection over 20cm translation and 40° rotation, respectively. The potential for MVDT using a Co‐60 source in radiation therapytreatment verification has been demonstrated by the simplicity and clarity with which the volumetric data is attained. Further work is currently in progress to improve upon the Co‐60 image quality by the addition of an amorphous silicon detector upgrade and the removal of out‐of‐plane blur using Fourier analysis.

TH‐C‐AUD‐08: Comparison of Tomotherapy Dose Distributions for 6MV X‐Rays and Different Cobalt‐60 Source Designs Using Monte Carlo Methods

Purpose: To investigate intensity modulated dose distributions for Co‐60 based tomotherapy with cylindrical and rectangular shaped source geometries for a typical head and neck case using Monte Carlo(MC) methods. Method and Materials: EGSnrc/BEAMnrc MC code has been used to model three Co‐60 tomotherapy units and a 6 MV (Varian 2100EX) unit. Two of the Co‐60 units, consisting of modified collimator systems with customized binary multi‐leaf collimator (BMLC) were modeled for 3 and 5 cm long rectangular sources and SADs of 70 and 80 cm, respectively. The other units were a conventional Co‐60 unit (T780c) with 2 cm diameter cylindrical source (MDS Nordion, Canada) and a 6 MV Linac with an addon MIMiC BMLC (NOMOS, USA). All the Co‐60 sources had identical active volumes. EGSnrc/DOSXYZnrc MC code was used to calculate the intensity modulated beamlets and fan beam dose profiles in a water phantom. The intensity modulated energy fluence profiles from a 6 MV Linac and two Co‐60 units using the same beam segments will be compared. Tomotherapy treatment plans for a typical H&N case were calculated. All plans were optimized for PTV, two neck nodes and the spinal cord using the same optimization parameters. The treatment planning was provided by an in‐house inverse planning system based on a multi‐objective gradient‐search approach using MC calculated dose data. Results: Comparisons of optimized dose distributions, dose difference maps and dose area histogram will be presented. Despite significant differences in fluence profiles for Co‐60 and 6 MV beams, optimized dose distribution for all plans met the dose‐volume and tolerance criteria. However, the integral doses are potentially higher for Co‐60 plans particularly with longer source size and shorter SAD.Conclusions: Co‐60 based tomotherapy with appropriate source and BMLC design is dosimetrically viable. Research supported (in‐kind) by MDS Nordion, Canada.