penumbra 4ch1.pdf

Cobalt-60 teletherapy machine Collimator and penumbra ❑ Collimators of teletherapy machines provide square and rectangular radiation fields typically ranging from 5×5 to 35×35 cm2 at 80 cm from the source. ❑ Penumbra is the region, at the edge of radiation beam, over which the dose rate changes...

Cobalt-60 teletherapy machine Collimator and penumbra ❑ Collimators of teletherapy machines provide square and rectangular radiation fields typically ranging from 5×5 to 35×35 cm2 at 80 cm from the source. ❑ Penumbra is the region, at the edge of radiation beam, over which the dose rate changes rapidly as function of distance from the beam axis. ❑ TYPES OF penumbra: ❑ Transmission Penumbra ❑ Geometric Penumbra ❑ Physical Penumbra M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine The Transmission Penumbra resulting from the inner surface of the blocks, if it is parallel to the central axis of the beam. It is due the transmission through the edge of the collimator block. The transmission Penumbra increase with increase of SSD and the collimator opening. It can be reduce by shaping the collimator M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN Cobalt-60 teletherapy machine The geometric penumbra resulting from the finite source diameter, may be minimized by using: ◦ Small source diameter ◦ Penumbra trimmers as close as possible to the patient’s skin (z = 0) P(z) (SSD + z − SDD) = s SDD M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN  Physical Penumbra ❑ Physical penumbra is the later distance between specified isodose curves at a specific depth (90% and 20% at Dmax). Takes the scattered radiation into account in addition to the transmission and geometrical penumbra. Electron Contaminations ❑ The blocks should not be placed closer than 10 to 20cm form the patient because of the excessive electron contamination produced by the block. M.SC. MEDICAL PHYSICS DR. KHALID I HUSSEIN