Biological effects of ingested asbestos: report and commentary.

The concept of whole retina radiotherapy for infants and youngsters with retinoblastoma is not new. Such treatment was pioneered in the early 1960's shortly after the linear accelerator became available for medical use (Bagshaw & Kaplan, 1966). The linear accelerator provided the possibility of using focused small field megavoltage radiation beams to treat the entire retina while simultaneously sparing nearby critical normal structures. This external beam radiotherapy provided the delivery of a homogeneous dose distribution across the area of interest, the entire retina. Such treatment was immediately seen as an advantage over the use of even more focal radiation as delivered using a radioactive cobalt plaque (Stallard, 1968). While the brachytherapy plaque technique delivered extraordinarily high doses of radiation to the area under the plaque, with a rapid fall off in dose so that points within millimeters of the plaque were calculated to receive almost negligible doses, it is of limited application when the entire retina is at risk of developing disease, as in the case in heritable retinoblastoma. Most radiation oncologists have chosen to use a single lateral radiation field for unilateral retinoblastoma and bilateral opposed fields for bilateral retinoblastoma. However the investigators at Utrecht, the Netherlands and those at St Bartholomew's Hospital in London have employed a technique to spare the contralateral eye by using beams angled obliquely in a cranio-caudal or caudo-cranial direction (Harnett et al., 1987a,b; Schipper, 1983). These obliqued beams have been criticised because the radiation beam exits through the opposite frontal lobe of the brain or the opposite mandible and developing teeth, thus exposing these normal developing structures to unnecessary radiation. By contrast a single direct lateral megavoltage radiation beam of 4-6 MV, calculated to 3 cm depth ensures that the ipsilateral eye lies within the 90% isodose curve, whereas, the retina of the opposite eye receives approximately 70% of the delivered dose (Donaldson et al., 1993). What is new is that Plowman et al. have utilised this direct lateral technique in the treatment of an infant at high risk to develop contralateral disease, and have demonstrated the ability to prevent disease by this 'prophylactic' radiation field. Using a direct lateral beam in a prophylactic fashion, in a patient with heritable disease, assumes that subclinical disease can be controlled with lower radiation doses than that used for patients with gross disease. Dose response data have been hard to obtain in retinoblastoma because only narrow …