Project entitled: “Development of a Dosimeter for Radiotherapy Using Integrated Optics Technology (Opracowanie dawkomierza dla radioterapii w technologii optyki zintegrowanej)” implemented under the Polska Metrologia II programme and funded by the Ministry of Science and Higher Education. The project is carried out in cooperation with the Świętokrzyskie Oncology Centre – Independent Public Healthcare Institution and the Henryk Niewodniczański Institute of Nuclear Physics of the Polish Academy of Sciences.
Project reference: PM-II/SP/0042/2024/02
Total funding: PLN 1,000,000
Radiotherapy is one of the principal methods of oncological treatment employing ionizing radiation. It is used not only for the eradication of malignant tumours but also for pain palliation in cases of metastatic disease. Annually, approximately 100,000 patients in Poland undergo radiotherapy. The success of radiotherapeutic treatment depends on the precise and homogeneous delivery of the dose to the treated volume. An optimally planned dose distribution should ensure damage to cancer cells while sparing the healthy tissue surrounding the target region. An insufficient dose leads to tumour cell survival and disease recurrence, whereas an excessive dose results in damage to healthy tissues. The range of dose values that enable local tumour control without inducing severe reactions in healthy tissues is narrow. It is estimated that a 5% variation in dose may affect the probability of cure by up to 25%. Maintaining dose accuracy at the level of 3.5% is difficult to achieve. One of the key conditions for treatment success is the accurate realization of the absorbed dose to water and its transfer, through calibration procedures, to therapeutic dosimeters. This process is performed in primary and secondary standards laboratories, which may operate within National Metrology Institutes. Dose realization is performed using one of three primary methods: chemical (Fricke dosimeter), ionometric, or calorimetric (graphite or water calorimeter). Each of these methods has advantages and limitations, which become particularly evident in the case of novel radiotherapy techniques, such as FLASH or VHEE. Chemical methods are characterized by low sensitivity and high measurement uncertainty. Ionometric methods, when applied to FLASH, VHEE, or small radiation fields, suffer from dimensional limitations that prevent the construction of a primary standard due to the large uncertainty in determining the effective volume of the gas-filled detector. Calorimetric methods are widely used as primary standards; however, their limitations arise from the use of thermistors, their response time, and unwanted thermal effects. In addition, the aforementioned methods are dedicated to the measurement of relatively small doses (up to 100 Gy). Emerging treatment techniques are based on significantly higher doses delivered within a very short time interval (seconds or even milliseconds). These challenges necessitate the search for new dose measurement methods or the modification of existing ones, particularly calorimetric techniques.
The use of sensors based on planar photonics and fibre-optic technologies will enable modifications of calorimetric methods for dose realization, real-time dose monitoring during irradiation, and the development of reference dosimeters for radiotherapy that are simple to operate. The objective of the project is to design, fabricate, and test a calorimetric dosimeter intended for radiotherapy applications. To this end, the development of an optical transducer capable of measuring extremely small temperature changes is planned. In the first configuration, the transducer will be based on a single planar ring waveguide coupled to a strip waveguide. As a result of temperature variations, the resonant wavelength at which light is coupled into the ring—forming a minimum in the spectral transmission characteristic—will shift. The second configuration will employ two coupled ring resonators. The geometric parameters of the first resonator will be selected such that the sensitivity of its resonant wavelength to temperature changes is lower than that of the second resonator, while its free spectral range is larger. In the system of two coupled interferometers, an increase in the sensitivity of the resonant wavelength to temperature variations is expected due to the Vernier effect. It is assumed that the structures will be passivated with a SiO₂ layer in order to prevent changes in sensor properties resulting from silicon oxidation under exposure to penetrating radiation.
