Beddar Laboratory Research Projects

Proton radiation therapy is clinically used to deliver highly conformal dose distributions to treatment volumes. However, existing uncertainties in proton range calculations has prevented it from reaching the full potential of proton radiation therapy. These uncertainties mainly arise from target motion, patient setup, and dose calculations, which can cause the proton Bragg peak to miss the treatment volume. Such uncertainties are currently managed by applying treatment margins around the target volume, which negatively affects the conformity of the dose distribution and exposes additional healthy tissues/organs along the beam path.

Proton-based imaging systems can eliminate range uncertainties by directly generating a 3D distribution of proton stopping power ratios (SPRs). These SPR values can be incorporated into the treatment planning to accurately calculate proton ranges within tissues. In addition to treatment planning, a proton imager installed inside the proton therapy gantry room can provide 3D imaging at the treatment isocenter. Thus, proton systems can be useful for image-guided proton therapy as well as aid with quick and precise positioning of patients for proton therapy.

Our laboratory is currently developing a first-of-its-kind proton-integrating imaging system design based on a 3D scintillator detector, as seen in the figure. Unlike existing 2D scintillator detector systems, our approach can record a wide distribution of energy-attenuated proton beams exiting the patient from a single proton beam exposure. This design can potentially save time and reduce dose requirements for proton imaging. Our proton imaging system is capable of generating proton radiographs and tomograms.
 

While modern radiation therapy treatment fields have complex 3D features, the standard measurement tools are point detectors and 2D-detector arrays. As a result, verification procedures of treatment fields can be very time consuming, even when measuring the dose at only a few points. This is especially problematic in proton radiation therapy, because of the complex shapes of proton dose distributions. Ideally, 3D-dose distributions would be measured prior to treatment for all patients. Unfortunately, these measurements are impractical because of time constraints and limited availability of suitable detectors.

Large-volume scintillation detectors have the potential for high-resolution and 3D dose measurement of complex radiotherapy fields. Scintillators convert the energy from proton beams into visible light, which can be recorded with an optical camera and converted into radiation dose. By recording the scintillation light in a large detector from multiple angles, we can reconstruct the 3D shape of the dose distribution. Liquid scintillator-based detectors have features, such as quick response (nanosecond), dose rate independence, and linear dose response, that make them ideal for dosimetric measurements. Measurements with liquid scintillator-based detectors are also immediate and unlike other 3D dosimeters do not require readout after the dose delivery.

We have developed linear energy transfer (LET)-dependence and optical artifact correction techniques, 3D-reconstruction algorithms, and we are working on quality assurance procedures for scanned proton beams. Our long-term goal is to reduce radiation treatment errors and improve dose verification accuracy by developing a new, fast, reusable 3D detector for patient treatment verification.
 

The technology involved in delivering radiation therapy to patients is enormously complex and prone to human error. Therefore, safety checks have been introduced to minimize errors and have been generally very successful in catching errors. However, a small number of highly publicized misadministration occurrences resulting in severe patient injuries demonstrates that errors can still occur. In vivo dosimetry, or measuring the dose received by patients as they are receiving radiation treatment, is an effective method to catching any errors by verifying delivery at the point of treatment.

In vivo dosimetry is not practiced routinely because of the high cost of detectors suitable for in vivo dosimetry and the technical expertise required to use them. Furthermore, metal–oxide–semiconductor field-effect transistors (MOSFETS), used in in vivo dosimetry, experience radiation induced damage and must be replaced regularly. However, plastic scintillation detectors are relatively inexpensive and can be reused extensively. Plastic scintillation detectors are also free from response-altering effects due to factors other than the deposited dose (e.g., radiation energy, dose rate, and detector orientation), making them straightforward and accurate detectors for in vivo dosimetry.

We are currently developing in vivo dosimetry systems based on plastic scintillation detectors to establish the usefulness and ease of implementation of such systems. Our long-term goal is to facilitate increasingly common use of plastic scintillation detectors for routine clinical in vivo dosimetry, providing a higher standard of care to patients.
 

During brachytherapy treatments, one or multiple radioactive seeds are inserted within the tumor to eradicate it. Although this treatment is very conformal, it is still important to limit the dose of radiation delivered to healthy tissues. Plastic scintillation detectors show a great promise to measure in vivo dose delivered in real-time to the tumor and its surroundings in order to limit the dose of delivered radiation. Their sub-millimeter size, fast response (~nanoseconds), water-equivalency, and small dependence on temperature give plastic scintillation detectors an advantage over other detectors. Work was previously performed by our laboratory for iridium-192 high-dose rate brachytherapy, leading to multiple publications.

An ongoing project focuses on extending the application of plastic scintillation detectors to low-dose rate brachytherapy. For this, radioactive sources of low energy (e.g., Iodine-125 and Palladium-103), are used that possess polyenergetic emission spectra with average energies of 28 keV and 21 keV, respectively. The overall objective of this research project is to account for the energy dependence and scintillation quenching at these energies and develop a clinically reliable and accurate plastic scintillation detector system.

Accurate dose delivery of radiation therapy can improve treatment outcomes and reduce side effects. Unfortunately, respiratory motion can create inaccuracy and spatial uncertainty that can lead to insufficient dose to the tumor or/and overdose to normal tissue. 4D (time resolved volumetric) imaging techniques can account for respiratory motion and acquire images of a patient during a complete breathing cycle. As a result, one can account for the respiratory motion in the planning process of radiation treatment.

The major advantage of MR imaging over other medical imaging modalities is its superior soft tissue contrast, which makes visualization clearer and segmentation between tumor and normal tissue much easier. Therefore, development of 4D MR imaging techniques can be extremely beneficial to radiation therapy treatment planning.

We are currently interested in the development of 4D MR imaging protocols as well as new algorithms to sort the acquired MR images into a 4D volume.

Unlike x-rays beams, which pass through the body of a patient, proton beams have a finite range of penetration into tissues. Proton beams deliver a burst of radiation dose into tissues just before stopping in their trajectory. By appropriately selecting the initial energy, proton beams can be tuned to deliver the final burst, called the Bragg peak, primarily in the tumor tissue allowing for a highly conformal dose distribution. Unfortunately, there is a large uncertainty in the proton beam range in patients due to tissue heterogeneities. Therefore, proton treatment plans must include large margins around the tumor. The plans must also avoid placing the Bragg peak directly in front of critical organs. Therefore, in order to maximize the benefits provided by the Bragg peak of proton therapy, we need to reduce the range uncertainty.

We are investigating ways to verify proton beam range in vivo by producing images from secondary radiation emitted from tissues during treatment. In collaboration with Dr. Jerimy C. Polf, Associate Professor of Radiation Oncology at the University of Maryland School of Medicine, we are designing a multistage Compton scatter camera, gamma-ray detection system. This system works by reconstructing the likely origins of thousands of gamma rays that each scatter multiple times in the detector. The proton beam range can then be verified from images produced by the gamma emission origins.

Both the hardware and the imaging algorithms for the detection system are active areas of research. We are working to improve the imaging algorithms through the application of Monte Carlo methods, machine learning, and high-performance computing. Our long-term goal is to enable real-time monitoring of the beam range from the clinic’s treatment control room.