source head 3ch1.pdf

Cobalt-60 teletherapy machine ❑ The source head consists of: ❑ Steel shell with lead for shielding purposes ❑ Mechanism for bringing the source in front of the collimator opening to produce the clinical gamma ray beam. ❑ Currently, two methods are used for moving the tele-therapy...

Cobalt-60 teletherapy machine ❑ The source head consists of: ❑ Steel shell with lead for shielding purposes ❑ Mechanism for bringing the source in front of the collimator opening to produce the clinical gamma ray beam. ❑ Currently, two methods are used for moving the tele-therapy source from the BEAM-OFF into the BEAM-ON position and back: ❑ Source on a sliding drawer ❑ Source on a rotating cylinder M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Methods for moving the teletherapy source from the BEAM-OFF into the BEAM-ON position and back: Source on a sliding drawer Source on a rotating cylinder M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine Teletherapy source housing Both methods (source-on-drawer and source-on-cylinder) incorporate a safety feature in which the beam is terminated automatically in the event of power failure or emergency. When the source is in the BEAM-OFF position, a light source appears in the BEAM-ON position above the collimator opening, allowing an optical visualization of the radiation field, as defined by the machine collimator. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Teletherapy source housing ❑ Some radiation (leakage radiation) will escape from the teletherapy machine even when the source is in the BEAM-OFF position. ❑ Head leakage typically amounts to less than 1 mR/h (0.01 mSv/h) at 1 m from the source. ❑ International regulations require that the average leakage of a teletherapy machine head be less than 2 mR/h (0.02 mSv/h). M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Cobalt-60 teletherapy machine Dose delivery with teletherapy machines ❑ The prescribed dose is delivered to the patient with the help of two treatment timers: primary and secondary. ❑The primary timer actually controls the treatment time and turns the beam off upon reaching the prescribed beam-on time. ❑The secondary timer serves as a backup timer in case of the primary timer’s failure to turn the beam off. ❑ The set treatment time should incorporate the shutter correction time to account for the travel time of the source from the BEAM-OFF to the BEAM-ON position at the start of the irradiation and for the reverse travel at the end of irradiation. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN