The present disclosure generally relates to systems and methods for locating wave sources, and in particular, to systems and methods for thermoacoustic range verification in ion therapy.
Ion therapy takes advantage of the energy deposition profile of high energy ions to provide highly conformal irradiation of tumors while sparing critical structures. Specifically, an ion beam has a finite penetration depth inside a patient, and depends on the ion energies. In contrast to conventional photon radiotherapy, most of the radiation dose of a mono-energetic ion beam is delivered within a short distance of its end range in tissue, also known as the Bragg peak. Superficial tissues receive less radiation dose compared to those near the Bragg peak. In addition, the dose profile drops dramatically beyond the Bragg peak, with those tissues receiving practically no radiation dose thereafter. Such dose deposition characteristics allow high doses to be delivered to deep-seated tumors with reduced normal tissue doses and toxicities. To provide volume coverage, often a combination of multiple proton beams with different energies are utilized, resulting in a spread-out Bragg peak with increased dose received by superficial tissues.
Regardless of whether single or multi-energy beams are used, ion range inaccuracies are often of concern due to the steep dose drop-off near the Bragg peak, which can reduce target coverage and increase toxicities to critical structures. End range inaccuracies can occur from positioning errors, changes or movement in a patient's anatomy and patient motion. In some cases, specific anatomical features, such as large boney structures or gas pockets in the abdomen, can move in or out of the beam path and introduce large uncertainties due to significant differences in beam stopping power.
Although positioning is verified prior to treatment and interlocks can halt treatment immediately upon detecting equipment failure, no safety mechanism exists to defend against anatomical changes and patient motion during treatment. As a result, many proton treatment plans are developed to favor certain beam arrangements that are robust to errors induced by range inaccuracies. For example, prostate treatments typically use parallel-opposed lateral beams. However, many such plans can often be suboptimal with respect to the overall target coverage and exposure of healthy tissue, thereby decreasing treatment effectiveness and increasing toxicity risk.
Therefore, several techniques have been developed with the goal of estimating or inferring ion range prior to, during and after treatment field delivery. For instance, one approach utilizes short-lived isotopes that are produced by nuclear interactions of an ion beam with tissue. The isotopes decay to positrons that annihilate and generate coincident gamma rays that are measurable with a PET scanner. By overlaying PET data on CT images, a good indicator of the delivered radiation can be obtained. However, such passive scattering methods are only valid for pretreatment verification with selected beam angles. Also, although prompt gamma emissions can provide fast and real-time feedback, they cannot yet achieve millimeter accuracy due to the decaying nature of the isotopes and the resolution limits on PET imaging. In addition, an automated method for correlating PET data to underlying anatomy in CT images acquired days prior to treatment is slow and precludes online range verification. Furthermore, ion range is determined based on room coordinates, which are subject to setup uncertainty and intra-fractional motion when registered to underlying anatomy.
Another approach for range verification relies on the thermoacoustic effect. For instance, when an ion beam is incident on a tissue, thermoacoustic pressure waves are generated due to the localized energy deposition near the Bragg peak. Although in principle such pressure waves may be measurable using ultrasound detection techniques, many challenges remain for translating the approach to a clinical setting. For example, in soft tissue, an instantaneous delivery of 2 Gy generates a pressure of 200 Pa at the Bragg peak, whereas a reduced dose of 1 cGy generates only 1 Pa. Pressure amplitudes, however, decay with distance from the Bragg peak, and detectors must therefore be positioned relatively close. Also, since tissues are subject to radiation dose constraints, there are limits on measurable pressure amplitudes, as well as the ability to perform significant signal averaging. Furthermore, range verification also requires dose depositions to be fast enough to ensure that pressure is generated and detected before it can dissipate.
In addition, ion scattering blurs the location of a thermoacoustic source and bandlimits signals detected by transducers located laterally or distally. Specifically, although ions scatter less than photons in soft tissue, a thermoacoustic source is determined longitudinally by the linear energy transfer (LET) or Bragg curve, and laterally by beam diameter, both of which often have full width at half maximum (FWHM) greater than about 7.5 mm. Acoustic travel time across a 7.5 mm beam is approximately 5 μs, which bandlimits thermoacoustic emissions below 200 kHz. Additionally, spill times of clinical ion therapy systems currently exceed 6 μs, further bandlimiting spectra and suppressing pressure amplitudes. Simulations of 230 MeV proton beams in water targets indicate thermoacoustic emissions are bandlimited below 150 kHz, assuming instantaneous deposition. Finally, resolving the range or Bragg peak location with better than 3 mm accuracy requires methods beyond the scope of typical inverse source techniques, which are typically accurate to within one-half wavelength. The acoustic wavelength corresponding to 150 kHz is 10 mm, so 5 mm accuracy is expected from traditional one-way beamforming of thermoacoustic emissions.
Although ion therapy has long been recognized as advantageous for treating various adult and pediatric tumors, including those associated with the base of the neck, spine, eye, prostate and others, inadequate range verification has limited its clinical utility. Therefore, there is an urgent need to develop accurate and fast range verification for the treatment of many cancers.