PIN diodes for radiation therapy use: Their construction, characterization, and implementation.

PURPOSE: Development and implementation of a PIN diode measurement system that can measure dose on the surface of a patient, which can be compared to dose calculated by a treatment planning system. Measurements are to be possible for static or rotational photon beams and electrons. Simple calibration procedures are to be devised that require a minimum set of correction factors.

METHODS: Readily available PIN type photodiodes are fabricated into devices that can be used for detection of ionizing radiation. Single or dual PIN diodes are soldered onto flexible shielded cables for static and rotational irradiation use. Diode signals are measured with an electrometer with zero input bias voltage. Diode temperature is determined by operating it as a thermistor. Linac photon and electron dose is measured. A commercial treatment planning system is used for calculating dose expected in various test geometries.

RESULTS: The scanning and surface diodes have an intrinsic buildup of 0.3 mm water equivalence and the IMRT device a buildup of 1.5 mm. Correction factors are determined for changes in diode sensitivity with the following factors: dose-per-pulse, dose rate, angle of radiation incidence, temperature, and field size. The surface or IMRT diode detector measurements agree within 1.0% with Eclipse calculations.

CONCLUSIONS: Clinically useable dose detection devices can be fabricated from PIN type photodiodes. These diodes are used to measure dose at the surface of a patient or under bolus. Correction factors are unnecessary if a surface or IMRT type device is chosen for use with static orthogonal or rotational IMRT delivered beams, respectively.