Central Axis Percentage Depth Dose Research Articles

A kilo-curie caesium 137 beam unit at Westminister Hospital: physical aspects.

A new caesium 137 beam unit head designed by personnel of the United Kingdom Atomic Energy Authority is described together with modifications to the original control circuit. The activity of the source at installation was 1050 curies, giving a dose-rate in air with a 7 × 7 cm field of 21·8 r/minute at 45 cm from the source (March, 1958). Physical measurements reported include an assessment of the room and head protection, H.V.L.'s, ionization build-up, back-scatter, variation of dose-rate with field size and central axis percentage depth doses. Measured isodose curves are being made available through the Diagrams and Data scheme of the Hospital Physicists' Association.

Physical measurements on the London Hospital Picker C 3000 cobalt unit

Physical measurements on the Picker C 3000 cobalt unit installed at The London Hospital, England, are reported. The strength of the source at installation (May 1956) was approximately 1,500 curies. The measurements include accurate assessment of room protection, backscatter factors, build-up in the superficial layers, leading to the computation of over-all field factors relating the standard oulput in air to the dose rate in rads/min at the '100' level for any given field. Central axis percentage depth doses were in good general agreement with those of Johns. Extensive study of the effects of field elongation on depth doses showed excellent agreement with the data of Jones and of Day. (auth)

II. The Patient and the Röntgen. Part I

The problem involved in translating the rontgen from the standardising laboratory to the patient is reviewed. Some recent measurements of central axis percentage depth doses at 1·7 mm Cu and 1·06 mm Cu H.V.L. agree well with the H.P.A. Survey figures (1953) except at 20 cm2 for which the Survey values are significantly higher. Evidence is given for utilising the concept of surface dose-rate factor rather than back-scatter factor in the assessment of surface dose-rates. Measurements have been made of the effect on the tumour dose of a plaster shell and wax applicator seating. The optimum correction is considered to be zero rather than 5 per cent as hitherto widely assumed. Heterogeneous phantoms representing the skull and the thorax have been built and dosage has been investigated on and in these phantoms. The results are expressed in terms of both percentage depth doses and absolute doses, as compared with corresponding points in a water phantom.