Un nuovo sistema di rilevamento per il monitoraggio del fascio impulso per impulso per la dosimetria in radioterapia

Radiotherapy (RT) is a physical therapy that uses high-energy ionising radiation to damage the DNA of cancer cells, causing apoptosis and necrosis, thereby blocking their proliferation. The quality of the treatments is of exceptional importance and accurate dose administration is the key feature during a RT treatment. In particular, the accurate control of the delivered radiation-dose to the patient during cancer treatments is crucial, and the use of dosimeters capable of performing precise dose measurements is necessary for the calibration of the system, aimed at assessing its characteristics and validating the treatment plan to be used for therapy. Electrometers, typically used in clinical dosimetry for dose measurements, integrate the charge collected at the detector contacts in a continuous-time mode. On the other hand, LINAC sources commonly used for RT generate pulsed beam at a fixed repetition rate. With the spread of modern dynamic and conformal RT techniques, information on the released dose by each individual radiation pulse generated by the LINAC is becoming essential. More recently, dose-per-pulse monitoring is crucial for the emerging FLASH technique, where few pulses release high dose values, thus drastically reducing the treatment times and safeguarding surrounding healthy tissues. In this regard, single-pulse monitoring also becomes important to better understand the mechanism of tissue-radiation interaction, which is not yet completely known for the FLASH technique. A novel prototypal detection system for pulse-by-pulse measurements of radiation emitted by medical LINACs is described in this work, with the aim of providing an effective contribution to the demand for accurate dosimetry. The system is fundamentally based on the well-known gated-integrating measurement method. Proposed method is not currently employed in clinical dosimetry practice where standard instrumentation do not allow single pulse monitoring. The system is designed for synchronous integration of the charge collected at detector contacts in a time interval around the single pulse by exploiting the sync signal provided by the control unit of the LINAC. The proposed prototype represents a suitable solution for accurate dose measurements allowing significant attenuation of the noise contribution in the dead time between two consecutive pulses and inherently enabling pulse by-pulse dose monitoring. A synchronized dedicated front-end electronics was designed, assembled, and then validated both in-the-lab and on-the-field. The adopted embedded-circuit solution allowed ii for the acquisition of pulsed signals produced by an in-the-lab assembled diamond dosimeter (fast enough to be sensitive to microsecond pulses) irradiated by pulsed X-rays and electrons sourced by a Clinac iX LINAC system. The designed instrument performs single-pulse acquisition even at pulse-repetition-frequencies greater than 1 kHz, well above the conventional repetition rate of medical LINACs. Specifically, the front-end electronics is based on a mature and commercially available precision integrator. A very versatile microcontroller with a Cortex-M0+ core implements the firmware to generate the control signals for synchronous integration, to perform the analog-to-digital conversion, and to process and transfer the acquired data. The experimental results highlighted the excellent performance of the electronic prototype in terms of both resolution and input dynamics also demonstrating the advantages of synchronous measurement over continuous integration. The realized diamond detector showed an excellent stability despite the high 100 Gy absorbed dose during field experiments under 6 MV and 18 MV X-rays and electrons generated by the Clinac iX. In addition, a good sensitivity of the dosimeter was verified for both X-ray and electrons. Sensitivities of 302.2 nC·Gy-1 and 286.6 nC·Gy-1 at 6 MV and 18 MV, respectively, have been evaluated regardless of the type of impinging radiation (electrons or X-rays). The developed system is highly versatile and demonstrates its effectiveness as a tool for LINAC sources diagnostics in terms of both beam intensity and emission timing monitoring. The proposed method represents a good solution for dose measurements, meeting the quality assurance requirements of modern RT techniques.