Installation of 60Co 100 cm source-to-axis distance teletherapy units in vaults designed for 80-cm units

60Co 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 that generally have source activities of less than 370 TBq (10,000 Ci). We have redesigned and renovated two vaults--one of which required supplemental shielding--designed for older 80 cm source-to-axis units with 333 TBq (9,000 Ci) sources and installed modern 100 cm source-to-axis 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 shielding costs, was about two-thirds of projected costs for installation of a 6 MV electron beam linear accelerator (linac). We have designed the treatment heads of these 100 cm 60Co units to accept the secondary field shaping blocks used on 100 cm source-to-axis linacs, allowing the 60Co units to be used as emergency back-up units for linacs when they are inoperable.