The present invention provides a method of determining the dose rate of a radiation source such as a seed/s used in bachytherapy, comprising three or more needles (12) in the vicinity of the seed/s piercing the skin of the patient in a pattern from an externally placed template around the area to be implanted with the seed/s, each needle having a series of detectors inside their length for providing an output indicative of the amount of radiation received from the seed/s and transmitting their signal optically (14) to a data collection unit (16). The determination of the location of the seed/s from the detector outputs is made by a dose planning and control computer (18), which requires as many of said detector outputs as possible to provide an acceptably accurate result are used in determining the spatial location of the seed. The dose of radiation received by the area requiring treatment from seed/s is determined by the computer (18) from determined location of the seed/s and either the known strength of the seed/s or by the signal strength detected by the detectors from the seed/s.
