An applicator attached to a 6 MV linear accelerator has been adapted to produce a well defined beam for the treatment of retinoblastoma. By using a contact lens as the reference point on the patient's cornea, the beam edge is aligned to just behind the posterior margin of the crystalline lens, thus sparing the anterior chamber and lens from the radiation field. In addition to basic dosimetric measurements made using an ionisation chamber and film densitometry to quantify the output and dose distribution from the applicator, extensive use was made of thermoluminescent dosemeters and a tissue-equivalent anthropomorphic head phantom. This was designed for detailed dosimetry on the eye and was used to confirm the correct positioning and reproducibility of the clinical set-up. We present data that corroborate and extend the dosimetric analyses documented in an earlier Dutch publication. In addition to the improved dosimetric analysis, other advantages include patient immobilisation, anaesthetic procedure and finer couch control as well as a recommendation on the dose prescription.

A coin-shaped ionisation chamber, orientated with its thin window facing away from the radiation source, was used to investigate the dose perturbations caused by the absence of back-scattering material near the exit surface of solid phantoms. Cobalt 60 and 4, 8 and 16 MV X-ray beams were used in the study. With no scattering material beyond the chamber window the ionisation was found to be as much as 17% less than the full scatter value. This was attributed to the absence of both back-scattered electrons and back-scattered photons. Full electron back-scattering could be restored by placing between 1.0 and 2.7 mm of unit density material beyond the chamber, depending on the primary beam energy. Under these circumstances the reduction in dose, now due to the absence of back-scattered photons only, was found to be small.

The total-body irradiation (TBI) technique at St Bartholomew's Hospital has been developed to improve dose homogeneity within the patient. Using a standard 6 MV linear accelerator in an orthodox-sized treatment room, the midpoint doses in head, neck, shoulders, mid-mediastinum, pelvis, knees and ankles are +/- 5% of that of the umbilicus in our current technique. This homogeneity has been achieved by a four-field technique, a reproducible patient set-up, careful use of a new bolus material and an additional beam-flattening filter mounted near the machine head. In addition, thermoluminescent dosimetric data collected at a test irradiation before TBI are used to influence field weightings and further improve dosimetry. This individualised and empirical TBI technique has dosimetric advantages over theoretical TBI dosimetric considerations in reducing dose gradients within the patient. These advantages are discussed.

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A standard 6 MV linear accelerator X-ray beam has been adapted to produce a non-divergent and almost penumbra-free beam edge by a beam splitting and extended collimation system. Using a contact lens to provide the reference point on the front surface of the eye and an attached rod-and-scale measuring system that is linked to the sharp beam edge system, it has proved possible to place this field border with an exactitude of within 0.5 mm at a required distance behind the front surface of the eye. This system has been developed for the treatment of small retinoblastomas not amenable to focal treatment methods; the technique may have other applications. Data in this manuscript corroborate an earlier Dutch publication and extend the observations on the physics beam profile obtainable, the immobilization of the patient, the anaesthetic procedure, the contact lens system and the dose prescription. Both scanning densitometry and TLC measurements in an anthropomorphic head shell (with extractable eye) confirm the extreme precision and sharp beam profile obtainable by this technique.