INSTALLATION OF CO-60 100-CM SOURCE-TO-AXIS DISTANCE TELETHERAPY UNITS IN VAULTS DESIGNED FOR 80-CM UNITS

Co-60 teletherapy units featuring 100 cm source-to-axis distances and with nominal source activities of 481 TBq (13,000 Ci) are available as replacements for older model 80 cm source-to-axis teletherapy units t hat generally have source activities of less than 370 TBq (10,000 Ci), We have redesigned and renovated two vaults - one of which required s upplemental shielding-designed for older 80 cm source-to-axis units wi th 333 TBq (9,000 Ci) sources and installed modern 100 cm source-to-ax is units with 481 TBq (13,000 Ci) sources. Renovation costs (in 1991 U .S. dollars) were about $40,000 and $160,000; the latter, including sh ielding tests, was about two-thirds of projected costs for installatio n of a 6 MV electron beam linear accelerator (linac), We have designed the treatment heads of these 100 cm Co-60 Units to accept the seconda ry field shaping blocks used on 100 cm source-to-axis linacs, allowing the (CO)-C-60 units to be used as emergency back-up units for linacs when they are inoperable.