Air pressure monitoring within the cancer treatment centre of hospitals

20-10-2023

In a cancer hospital where they treat patients using Linear Accelerators, the dose delivered must be calibrated, nominally to 2% of target, so the medical physics team use ionisation chambers to measure the dose.

In the photograph you can see a Farmer Ionisation Chamber with the sensitive volume (the radiation detector) being the black cylinder at the end of the aluminium stem. This is being placed inside a measurement phantom (equivalent to water in its radiation interaction properties). The radiation ionises the air in the sensitive volume, however how many air molecules there are in that volume depends upon the density of the air, higher pressure - more air molecules – more ionisation recorded, the air pressure is thus corrected for temperature and pressure hence the need for two T2514 barometers. The two T2514’s must read within 2mbar of each other and then an average reading is taken to make the correction and calibration.