Dosimetric aspects of a rotating beam splitter used in tangential field breast treatment

A rotating beam splitter was designed and fabricated for use in treating tangential breast fields on an AECL Theratron-80 cobalt teletherapy unit. Its dosimetric properties were studied using a 0.6-cm3 Baldwin-Farmer ionization chamber with Keithley electrometer and a Scanditronix RFA-3 three-dimensional water phantom scanner with semiconductor detector. An aluminum plate, which held the semicircular rotating 5-HVL (half-value layer) lead block, extended to the phantom surface (80-cm source-surface distance). The beam was blocked directly along the central axis and also at distances up to 7.5 mm off-axis, corresponding to the projected extent of the 1.5-cm-diam source. The penumbra at the central ray and at each off-axis point was measured at dmax and at 5-cm depth in water. A reduction in the penumbra from 8 to about 2 mm for 20 X 20 cm2 beam was observed regardless of the off-axis distance of the block. Isodose distributions obtained for various field sizes indicated that the percent depth doses of the split fields agree well with the equivalent squares of the irradiated field sizes. Output measurements in water and in air indicated that scatter from the aluminum plate more than compensates for the reduction in backscatter factor, due to the decrease in irradiated area when the beam splitter is used. Isodose curves in various planes were obtained at clinically useful rotational angles of the beam splitter. Computer generated isodose curves have been obtained that match the measured curves to be used in treatment planning.