Purpose: To perform detailed characterizations of the secondary radiation field produced in carbon ion beam interactions with tissue and investigate the feasibility of treatment verification or adaptive radiotherapy using this field.
Methods: Monte Carlo simulations were performed for an active spot-scanning (σx:1.5 x σY:1.5 mm, δE/E=1% at E=200 MeV/u, WET=8.3 cm) carbon ion beam (10⁶ histories/spot), delivered by a clinical gantry system, impinging on a 15x15x15 cm³ water phantom. The induced radiation field was characterized in type, yield, energy distribution, and emission direction using 40x40 cm² profilers surrounding the phantom (10-cm from each side). The production origins (POs) of the most abundant radiation species were studied at several therapeutic energies (100-300 MeV/u). A setup consisting of two coincident pixelated (50 μm pitch) detectors positioned downstream from the phantom was utilized to reconstruct the charged radiation POs. The POs axial distribution was investigated to verify incident carbon beam features.
Results: The simulations predicted a complex radiation field formed by neutrons, prompt gamma-rays, charged ions dominated by protons (20% all fragments) and ⁴He (12%), as well as trace activities of positron emitters (¹⁰⁻¹¹C). Only 61% of the energetic protons (84 MeV median) escaping the phantom forwardly originated from the primary carbon beam, compared to 91% of heavier ⁴He (77 MeV/u median) products. The reconstructed POs axial distribution exhibited a qualitative relationship to the carbon beam range, its entry into phantom, and the latter’s boundaries.
Conclusion: This study demonstrated that carbon radiotherapy produces a multi-type, cocktail-beam-like charged secondary radiation that escapes the irradiated volume, carrying with it crucial information about the incident carbon beam and the phantom. The proposed POs reconstruction method using a simple coincident detection setup provides the first step towards establishing potential quantitative treatment verification information (e.g., ion range, dose distribution) that could assist adaptive radiotherapy delivery.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by the Hitachi Professorship in Radiation Oncology.
Heavy Ions, Radiation Therapy, Treatment Verification
TH- External Beam- Particle/high LET therapy: Carbon ion therapy - Development (new technology and techniques)