Considerations on the miniaturization of detectors for in vivo dosimetry in radiotherapy: A Monte Carlo study

The evolution of technology in radiotherapy nowadays allows us to deliver much higher doses to the target volumes, thanks to better absorbed dose distribution accuracy and conformation, while better sparing healthy tissues. In photon radiotherapy, higher precision usually entails employing small and moving beams. This emphasizes the role of in vivo dosimetry, which assesses internal absorbed dose rather than entrance dose. This, along with advances in materials science, results in a tendency towards the miniaturization of dosimeters. However, the stochastic nature of the radiation-matter interaction takes on greater importance at smaller scales, resulting in fluctuations in energy deposition whose effect may not be negligible. Miniaturization needs to take this into account. We estimated such fluctuations by Monte Carlo simulations considering energy deposition in cylindrical volumes of different sizes and for several absorbed dose values. We not only present and discuss the probability distributions of absorbed doses for a large range of target sizes (0.1-10 μm) and clinically relevant doses (0.1-10 Gy), but also derive an estimation of the risk of measuring an absorbed dose with a value outside a given interval of tolerance (3, 5 and 10%) around the expected dose. The distributions presented features consistent with the theory of microdosimetry. Those for dosimeter sizes smaller than 0.3 μm showed a very high dispersion in specific energy, while those for 10 μm dosimeters tended to become Gaussian and narrower with increasing absorbed dose. The probability of measuring an absorbed dose outside the defined interval of tolerance is close to 100% for the smallest size, regardless of the dose and the interval width considered. It decreases with increasing dose, dosimeter size and width of the interval of tolerance. The best results were obtained with 10 μm dosimeters, for which the probability of doses outside the tolerance range is always zero for absorbed doses equal or greater than 1 Gy and for all the absorbed doses in the case of the largest interval of tolerance. The ability of a small dosimeter to give measurements within a suitable range of values depends on its size and on the absorbed dose to be measured. An excessively small dosimeter size yields a distribution of specific energies spread over a very large range of values, which makes it harder to obtain accurate measurements of absorbed dose. On the other hand, a dosimeter of about 10 μm in size may allow for good accuracy of the measurements in a wider range of doses and with different intervals of tolerance.