Proton beam therapy (PBT) is an advanced radiotherapy technique that delivers highly targeted radiation doses to tumours while minimising exposure to surrounding healthy tissue. This is achieved due to the Bragg peak phenomenon, where protons deposit most of their energy at a specific depth in tissue, followed by a rapid dose fall-off. However, precise dose delivery requires rigorous verification to ensure that the prescribed radiation reaches the intended location. Proton beam dose verification imaging plays a crucial role in this process, helping to confirm treatment accuracy and improve patient safety.
Importance of Dose Verification
Proton therapy relies on treatment planning systems (TPS) that use imaging data from computed tomography (CT) scans to calculate the proton range and dose distribution. However, uncertainties such as anatomical changes, patient positioning errors, and variations in tissue density can affect dose deposition. Verification imaging allows clinicians to detect and correct such deviations, ensuring that the delivered dose matches the planned dose. Without accurate verification, there is a risk of underdosing the tumour or overdosing surrounding healthy tissues, leading to compromised treatment effectiveness or unwanted side effects.
Methods of Proton Beam Dose Verification Imaging
Several imaging techniques are used to verify proton dose delivery. These include:
Prompt Gamma Imaging (PGI)
When protons interact with atomic nuclei in tissue, they produce prompt gamma rays. PGI captures these emissions to estimate the proton range in real time. Since the energy and emission profile of prompt gamma rays are linked to proton interactions, this technique helps identify range shifts that could lead to incorrect dose placement.
Positron Emission Tomography (PET) Imaging
Proton interactions generate positron-emitting isotopes such as oxygen-15 and carbon-11. PET imaging detects these isotopes after treatment, creating a distribution map of the proton-induced activity. While PET provides post-treatment verification rather than real-time monitoring, it is useful for confirming whether the proton dose was delivered to the correct anatomical location.
Radiographic and Fluoroscopic Imaging
X-ray radiography and fluoroscopy can be used to monitor patient positioning before and during treatment. Although they do not provide direct information about proton dose deposition, they help minimise positioning errors that could affect the accuracy of treatment delivery.
Challenges and Future Developments
Despite advances in proton beam dose verification imaging, challenges remain. PGI and PET techniques require sophisticated equipment and calibration to account for biological and physical uncertainties. Additionally, real-time imaging methods need to balance accuracy with clinical feasibility. Emerging technologies, such as artificial intelligence-based image analysis and novel detector systems, may improve dose verification in the future, leading to even greater precision in proton therapy.
Proton beam dose verification imaging is essential for ensuring the safety and effectiveness of PBT. By continuously refining verification methods, clinicians can enhance treatment accuracy, reduce uncertainties, and improve patient outcomes.
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