Dosimetry

Radioactive material is implanted into tissues according to distribution rules that vary according to the system used. In Europe the classical Parker-Paterson and Quimby systems have largely been superseded by the Paris system which is particularly suitable for iridium wire implants. Wire of the same linear intensity is used and sources are arranged in parallel, straight, equidistant lines, 8-20 mm apart. To compensate for 'uncrossed' ends, the wires are 20-30% longer than the length required to treat the tumour. In a volume implant, sources in cross-section should be arranged in either equilateral triangles or squares.

The dose to the tumour can be calculated manually, using graphs such as Oxford cross line curves, or by computer. The basal dose rate (the mean of minimum values between sources) is first calculated. The treatment dose (e.g. 65 Gy in 7 days) is prescribed to the reference dose line (85% of the basal dose).

The prescription point for surface applicators such as moulds and some intracavity treatment is usually 0.5-1 cm from the applicator. A special case is intracavitary gynaecological treatment. The most frequently used prescribing point is the Manchester A point, defined as a point 2 cm lateral to the uterine canal and 2 cm above the cervical os. The dose calculated at this point is a good predictor of late radiation damage to the ureter, bladder, rectum, and other pelvic organs. The International Commission of Radiation Units (ICRU) Report 381 has proposed that the volume (defined in height, thickness, and width enclosed by a 60 Gy isodose line) should be used for reporting absorbed dose following gynaecological treatments.

0 0

Post a comment