Sunday, February 7, 2010

Overview on Cobalt-60 - From the reactor to the teletherapy machine

Cobalt is a heavy metal well known for more than 300 years. Since all the forms of cobalt is highly unstable, no free form of it persists in nature. Cobalt is available in the form of ore.

40% of the world's cobalt comes from Zambia and Congo. There are many forms of isotope available for cobalt of which only Co60 has the longest half life of 5.27years. Artificial Co60 is created by bombarding a Co59 target with a slow neutron source, usually 252Cf moderated through water to slow the neutrons down, or in a nuclear reactor such as CANDU, where adjuster rods usually made of steel are instead made of 59Co.

Usually the Co60 used in medicine is produced in the size of 2 x 1.5cm cylinder. In India Co60 is produced by Board of Radio Isotope Technology(BRIT), Mumbai which supplies the cobalt source all over India and sometimes exports to foreign countries.

The supply and use of all the isotopes supplied by BRIT is supervised by Atomic Energy Regulatory Board(AERB). When a registered medical institute orders for the isotope with BRIT, it receives the total or part of the payment and place the orders with the Lab.

When the isotope is ready for delivery, the customer is informed to receive the source by paying the full amount(At present it costs Rs.40laks including transportation)The transport and loading process will be undertaken by the Teletherapy machine's manufacturer. The package will be packed in a lead shield and transported to the medical institute.

Under the supervision of the physicist authorized by AERB, the source loading will be done.

Since the half life of Co60 is 5.27 yrs it will emit radiation that can be used for treatment atleast for 10yrs. After that a new source should be purchased. While buying the teletherapy machine itself they will load the Co60 source,so 10 yrs after buying a new machine only, there will be need for source replacement.

The old decayed source will be taken back by AERB for safe disposal as per the guidelines of International Atomic Energy Agency(IAEA).Cobalt from radiotherapy machines has been a serious hazard when not disposed of properly, and one of the worst radiation contamination accidents in North America occurred in 1984, after a discarded cobalt-60 containing radiotherapy unit was mistakenly disassembled in a junkyard in Juarez,Mexico.

In Madras Medical College, the present machine is purchased in 1994 which is preloaded with the Co60 source which is now replaced after completing its third half life. Before that, a old form of teletherapy machine called Janus was used in which the source is loaded in the early 80s. SO its been more than 25 yrs for MMC to witness the source loading.

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