Suppression of bony growth of the orbit after radiotherapy for retinoblastoma.

Suppression of the growth of orbit and skull in 13 retinoblastoma patients treated by radiotherapy was investigated. The age of patients at the time of treatment ranged from 1 month to 6 years and 11 months. They were examined after three to 18 years. Suppression of the growth in involved areas and other parts of the skull within the beam of radiation were estimated by inspection, X-ray findings, and X-CT examinations. The involved orbit received more than 32 Gy, and the suppression of bone growth was prominent. At the opposite temporal bone, suppression of growth was prominent in eight cases out of 11, in these cases, the radiation dose was at least 14 Gy. Two cases, in whom the radiation dose was less than 12.6 Gy, have not shown any suppression of bony growth. Results were also obtained for a nominal single dose (ret). In all cases of more than 1,000 ret, suppression was prominent, but at less than 1,000 ret, four out of nine cases, and at less than 600 ret, two out of four cases showed suppression of bony growth. If the safety dose to avoid the suppression of bony growth is 400 ret, this is equivalent to three times the radiation dose of 200 cGy. In conclusion, the suppression of bony growth of the orbit and temporal area of the skull of young patients is caused by very small doses of radiation.