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

Abstract

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.

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

“…[1] There have been a number of modeling studies that have suggested that Co-60 may be more effective in modern radiation therapy than perceived in the past. [2–4] However, aside from a limited number of studies, there has been little experimental validation of Co-60 conformal delivery. [5] Furthermore, while computed tomography imaging with Co-60 has been performed for basic medical physics studies,[67] there has been very little work done on the evaluation of the potential for image guidance with Co-60.…”

Practical and clinical considerations in Cobalt-60 tomotherapy

“…(2001, 2008) noted that the problem of low dose rate for fan beam applications, for a given source strength can be improved if the Co-60 unit is redesigned to include multiple sources, decreased source to axis distance (SAD) and/or using a different source shape and packing density. [910] Considering these solutions to be preceived problems, along with the robustness of the unit in terms of maintenance, safety and cost, put Co-60 in a better light with respect to linac based radiation therapy than held by conventional wisdom. [1–311] It has been suggested that Co-60 could have an increased role in radiation therapy if some new thought went into the design of improved clinical units.…”

Comment on Investigation of Photon Shielding Property Changes in Curing High Density Concrete

“…We have fully modeled this spectrum in previous Monte Carlo studies of source design considerations in potential modern 60 Co radiotherapy units (Fig. 4 in Joshi et al 2008).…”