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).

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.

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.

The ontogeny of photosensitivity has been studied in a holometabolous insect, the midge Chironomus ramosus. The life cycle of midges shifts from an aquatic environment to a non-aquatic environment. Extracellular electrical activity of photoreceptor organs was recorded at larval and adult stages. We found an increase in photosensitivity as the larva metamorphosed to the adult stage. This is the first report of changes in photosensitivity during the development of any insect described in an ecological context.

The FBX aqueous chemical dosimeter contains 0.2 mol m(-3) ferrous ammonium sulphate, 5.0 mol m(-3) benzoic acid and 0.20 mol m(-3) xylenol orange in 40.0 mol m(-3) sulphuric acid. The dosimeter can measure photon and electron doses in the range 0.1 to 3000 cGy in radiotherapy. The response of this dosimeter was measured for a 53.2 MeV carbon beam in the present work. Our initial result indicates that the sensitivity of the FBX system to the carbon beam as compared to cobalt-60 gamma rays is 25.5%, and thus we believe that the FBX system could be a useful dosimeter for carbon beams and similar heavy ions considered useful in radiotherapy.