The Radiotherapeutic ratio can be improved by rationalisation of fractionation regimes. Conventional fractionation in radiotherapy may not be the most optimal with respect to the cellular kinetics and radiosensitivity of proliferating tumour cells. We have tried hyperfractionated radiotherapy in cancer cervix and head and neck cases. In conventional fractionation (group 1) 200 cGy per fraction to a total dose of 6000-7000 cGy was delivered. In multifractionated group, 2 fractions per day of 120 cGy per fraction (group II), 140 cGy per fraction (group Ill) and 160 cGy per fraction (group IV) with 4-6 hours interfraction interval were delivered. For assessment of dose required to produce tolerable normal tissue reactions, semiempirical formulae such as NSD, PT, TDF and CRE are being used but they have some limitations and hence the linear quadratic (LQ) model is being tried to describe the biological effects. We have estimated the a /P values for tumour regression by Boer's method (D Vs. dD plot) and they are 12.0 Gy for squamous cell carcinoma of head & neck and 11.2 Gy for cancer cervix. The results thus obtained have been compared with Boer's computer programme.

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We present the results of attenuation measurements in water-equivalent material of high energy photon beams at off-axis points. The measurements were performed on four accelerators and for the energies of 4, 6, 12, 15 and 23 MV in narrow beam geometry with and without flattening filter. We found a linear increase of the attenuation coefficient as a function of the distance from the central axis for all the energies and types of linear accelerators. For the same nominal energy, this increase is different from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without flattening filter in the same experiment conditions. These results are tentatively interpreted considering the angular variation of bremsstrahlung energy spectra with and without flattening filter as calculated by a Monte-Carlo method.

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In 192Ir dosimetry, the decay of the source during treatment has to be taken into account due to its short half life. Otherwise, the error in dosimetry will result in underdosage. In this paper, two methods are suggested to incorporate the decay correction factor into dosimetry. The routine dosimetry and suggested methods are compared and analysed.

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Radiation dose was computed for five 131I and eleven 99mT radiopharmaceuticals utilizing the dose coefficients given by ICRP-53 and new tissue weighting factors for ICRP adult as well as Indian adult. The revised effective doses were found to be generally lower than the corresponding old values except for 131I labelled sodium iodide and Rose Bengal. The radiation doses to Indian adult were found to be mostly higher than from his ICRP counterpart.

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125I seeds employed for permanent and temporary interstitial implants exhibit significant radiation fluence anisotropy due to self absorption in the marker and oblique filtration through the encapsulation jacket. A silver wire 1251 seed (Model 6711), introduced in 1983 by 3M Company, failed to improve the photon distribution anisotropy. In addition to the known 1251 photon spectrum, the new seed emitted two silver characteristic x-rays, lowering the mean photon energy from 28.4 to 27.4 keV. A double wall uniform thickness encapsulated 125I source, laser welded at one end, has been developed for clinical use. The source uses a carbon coated thin tungsten filaments for enhanced radiographic visualization. Measurements made by Nal and intrinsic Ge detectors indicate that the 2300 series 125I source emits a pure 125I spectrum. The 6ngular dependence of individual photon peaks and total photon spectrum as well as the corresponding anisotropy factors were measured. The 4x averaged anisotropy factor for the total radiation fluence is 0.92 compared to 0.87 for model 6711 seed. The dose distribution around the new 125I source in water is very isotropic.

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