11.10.9 - In Vivo Dosimetry

In Vivo dosimetry refers to measuring the dose received by the patient during treatment. This is different to in vitro dosimetry, which refers to most other physics measurements in phantoms. Doses at depth are difficult, if not impossible, to obtain without invasive procedures.

Useful In Vivo Dosimeters

Thermoluminescent dosimeters

TLDs can be made into different sizes, are small and do not require electrical wiring to operate. This makes them ideal for in vivo dosimetry, where they can placed on the patient surface to record entry doses. If bolus is used, TLDs may be placed between the bolus and the skin surface with insignificant effect on the dose distribution.

Silicon Diodes

Diodes are also relatively small and have the benefit of an instantaneous readout. Their disadvantage is the need to connect them to an electrometer during dose delivery.


MOSFETs are small and can be read more quickly than a TLD. They do not require a power source during irradiation. They are, however, relatively expensive and can only be used once per dose.

Purpose of In Vivo Dosimetry

Computer simulation of radiation dose is not absolutely accurate, and particularly for correction based algorithms. Even modern treatment planning software with superposition/convolution techniques may have errors, particularly at the interfaces of tissue inhomogeneity. The development of IMRT has given computers more responsibility for the delivery of radiation, including field placement and weighting.
In vivo dosimetry provides an extra 'check' to ensure that systematic errors have not been acquired during the treatment planning process. If an inconsistency is found, the plan can be corrected before the patient begins to suffer irreversible problems from the error.