10.6 - Phantoms

Phantoms are tools used by physicists to measure radiation under different conditions. They are useful because they allow measurement of radiation in a controlled environment with minimal risk to staff and patients. They are limited because they can only replicate conditions.

Slab Phantoms

Slab phantoms are square blocks of varying thickness which may be build out of different materials. The most commonly used material is a water equivalent solid, but other phantoms representing lung, bone and metal may be used. Slab phantoms may be placed within a beam to simulate various conditions. Ionisation chambers may be placed within pre-hollowed holes to measure dose rates, or film may be placed between two slabs to measure beam profile and isodose distributions.
Slab phantoms are particularly useful as they are solid and easy to position, requiring minimal setup.

Water Phantoms

Water phantoms are the primary tool used for absolute dosimetry. The consist of a transparent plastic tub (about 60cm in all dimensions) filled with water. An waterproof ionisation chamber can be placed on a movable arm within the phantom. This can accurately maneuver the ionisation chamber to a number of positions to measure dose rate.
Water phantoms are useful for absolute dosimetry as they are homogenous and water equivalent, a close substitute for soft tissue and muscle. Water is almost freely available. Most absolute dosimetry protocols require tests to be carried out in a water phantom.
They are cumbersome to use due to their weight and the presence of water. Water also creates difficulty with electrical circuits which must be waterproofed.

Anthropomorphic Phantoms

Some phantoms may be shaped into a shape approximating a human torso, including internal inhomogeneities. These phantoms are typically formed by multiple slabs arranged in the axial plane. This allows film to be placed between the slabs. The slabs also contain holes for insertion of ionisation chambers.