Short-lived positron emitters in beam-on PET imaging during proton therapy

Area of Science:

• Nuclear Physics and Medical Imaging
• Radiotherapy and Radiation Oncology

Background:

• Current in vivo proton beam radiotherapy dose verification relies on positron emission tomography (PET) detecting positron emitters produced during irradiation.
• Beam-on PET, acquiring data during irradiation, offers advantages like maximal counts, minimal biological washout, and rapid feedback for all nuclides.

Purpose of the Study:

• To assess the relevance of short-lived nuclides (half-life < 19s) for in vivo dose verification using beam-on PET.
• To measure the production rates of these short-lived nuclides when 55 MeV protons stop in water, carbon, phosphorus, and calcium.

Main Methods:

• Measured production rates of short-lived nuclides (e.g., Nitrogen-12, Phosphorus-29, Potassium-38m) from proton interactions with water, carbon, phosphorus, and calcium.
• Determined the integrated number of decays over time during irradiation of PMMA and tissue-equivalent materials.
• Compared the contribution of short-lived nuclides to total PET counts and proton range information against long-lived nuclides and prompt gamma imaging.

Main Results:

• Identified key short-lived nuclides: Nitrogen-12 (11 ms) on carbon (9% of Carbon-11), Phosphorus-29 (4.1 s) on phosphorus (20% of Phosphorus-30), and Potassium-38m (0.92 s) on calcium (113% of Potassium-38g).
• Nitrogen-12 dominated dose verification signals in carbon-rich adipose tissue up to 70s and in bone tissue for the initial 8-15s.
• Short-lived nuclides from phosphorus and calcium increased beam-on PET counts by 2.5 times compared to long-lived nuclides over a 70s irradiation.

Conclusions:

• Short-lived nuclides, particularly Nitrogen-12, are highly relevant for in vivo dose verification in proton therapy using beam-on PET.
• Nitrogen-12 PET imaging offers potentially superior or equal proton range information compared to prompt gamma imaging.
• The study supports the practical implementation of Nitrogen-12 PET imaging for enhanced proton therapy precision.