Dosimetry of small stereotactic fields

Dosimetry of small stereotactic fields involves:

The problem of lack of charged particle equilibrium be given special consideration and also that TMR and off axis ratios be measured for small fields.

With Co-60 beam treatment, one requires about 5 mm of tissue material for charged particle equilibrium. So a field size of less than about 1 cm will give reduced output due to lack of CPE. Similarly for a 6MV beam, the minimum field size for CPE is about 3 cm; though in practice, the condition may hold slightly smaller field sizes as well since the buildup curve is very steep.

So dosimetric functions must be measured for very small field sizes, for treatment purposes. Also such small fields require microchambers for measurement.

A Farmer type chamber has typical dimensions of about 16 to 18 mm diameter by about 22 mm length, which is not suitable for the dosimetry of small fields.

The solution is a PTW pinpoint chamber (SEE OR INSERT DIAGRAM)