Thermoacoustic Range Verification During Delivery of a Clinical Plan by a Synchrocyclotron: transition from research prototype to turnkey clinical device

Particle therapy is a technique for treating solid tumors that is potentially more precise than x-ray radiation therapy. Charged particles enter the patient at relativistic speeds, depositing increasingly more dose as they come to rest inside the patient. X-ray photon beams deliver an exponentially decaying dose along the beam path, dosing healthy tissue before and after the tumor. Proton therapy causes less collateral damage, and when patient alignment is accurate, delivers less dose to healthy proximal tissue and spares distal tissue altogether. Today, patients are positioned by matching bony structures in daily x- ray images to planning CT volumes acquired days (or weeks) prior. Inter-fraction changes to soft tissue (weight loss, edema) are common and intra-fraction changes (digestion, respiration, etc.) are unavoidable. Range errors incorrectly dose healthy tissue and undertreat the tumor, limiting benefits of proton therapy. Therefore, the American Society for Therapeutic Radiation Oncology currently supports proton therapy for tumors near the base of the neck, spine, eye and in pediatric patients, and only in the context of research studies for most other tumor sites. Thermoacoustic range verification relies upon stress confinement and is suitable for compact synchrocyclotrons recently introduced by two manufacturers, IBA and Mevion. A prototype system has been validated for use with IBA systems and the same receive chain has been validated for Mevion. System design for a thermoacoustic range verification (tARV) device will be finalized and validated during Phase II. The tARV consists of a wireless ultrasound array around which six thermoacoustic receivers are packed. A compact (65mm x 55mm) data acquisition (DAQ) board provides 56-59 dB gain over the thermoacoustic frequency range of 10-100 kHz, samples at 500 ksps to minimize aliasing of high frequency noise, and stores data to a secure data card onboard. A compact gamma detector monitors beam intensity and provides a trigger to the DAQ. End-to-end validation requires software integration with the treatment planning system (TPS) software used in radiotherapy clinics. Furthermore, QA and clinically realistic test objects, called phantoms, are required. Thermoacoustics is a hybrid technique that combines aspects of ultrasound imaging and radiation therapy. Commercial phantoms designed for ultrasound imaging have not yet been vetted for radiation therapy. We will integrate software, quantify materials in ultrasound phantoms already produced by CIRS, Inc and add dosimetric capability to enable end-to-end validation in a clinical environment.