Diodes and In Vivo Dosimetry

Diodes

Diodes are used during the first few occasions you attend for radiotherapy. They are basically small detectors attached to a long wire, and are used to measure the dose, you are receiving in 'real time', whilst having your radiotherapy treatment. They are normally attached to your body with tape at specific points, where the treatment beam enters your body. Many professionals feel they are important because they have the potential to detect any errors that may have slipped through the quality safety net. Every part of your radiotherapy treatment is checked and double checked and then checked again and even the treatment machine itself has a range of interlocks that activate should any parameter appear out of tolerance for even a moment. Therefore the chance of any error getting through this vigorous checking process, is incredibly unlikely. However, if the 'incredibly unlikely' did happen, then the diodes would detect the error so it could be rectified.

The argument against using diodes is that the quality process should pick up any error that may occur, which means the money needed to buy, calibrate and investigate the diode readings could be better used elsewhere.

Block diagram of in vivo dosimetry
 application

The dosimeter can be used as a tool for quantifying dose at depth, as well as to evaluate adherence between doses from the treatment planning and the delivered dose. Dependence of small filed sizes (less than 4 × 4 cm) would be of future interest, especially when small radiation segments are used during IMRT. The DVS dosimeter does not identify the specific cause of the difference in dose, but it can reliably alert the physician that a deviation between the planned and delivered dose has occurred. As such, the dosimeter can act as a fail-safe device with the potential to catch an over- or underdose situation before the mistake could be repeated.

There are important advantages to DVS dose verification. It can be reliably used for the verification of any possible change of the dose to the target or nearby organs. The system is capable of keeping a permanent patient dose record by monitoring the daily dose delivered. Based on the dosimeter readings and trends, the dose changes might be predicted. If the weekly average readings were outside the clinically implemented tolerance, the physician can investigate the patient position, clinical protocols, internal anatomy, and treatment plan, then apply corrections, if necessary. Furthermore, the DVS system provides medical physicists with an independent QA verification of machine performances. The extensive commissioning and implementation strategy detailed above can improve the usage of implantable dosimeters and may lead to improvements in patient treatment outcomes.