Contribution to dose in healthy tissue from secondary target fragments in therapeutic proton, He and C beams measured with CR-39 plastic nuclear track detectors

Abstract

The linear energy transfer (LET) spectrum, absorbed dose and dose equivalent from secondary particles of LET_∞H₂O ≥15 keV/μm deposited within the plateau of the Bragg curve in primary particle-induced nuclear target fragmentation reactions in tissue during proton and heavy ion radiotherapy were measured using CR-39 plastic nuclear track detectors and analyzed by means of atomic force microscopy. It was found that secondary target fragments contributed 20% to dose equivalent for primary protons (157 MeV), 13% for primary helium ions (145 MeV/n) and 4% for primary carbon ions (383 MeV/n), respectively. Little research has been done on the contribution from these particles to primary given dose. The smaller contribution measured for energetic carbon ion beams compared to proton beams can be considered an advantage of carbon ion radiotherapy over proton radiotherapy.

Figure 1

Typical AFM images (25 μm × 25 μm) of etched CR-39 PNTD exposed at three incident angles (δ = 90°, 45° and 15°) showing secondary target fragment nuclear tracks from primary proton, helium and carbon ion beams.

Figure 2

(A) LET spectra of secondary target fragments for each primary ion beam and (B) LET spectra in which the fluence is normalized to primary beam dose.

Figure 3

Variations of dose contribution of secondary target fragments to primary irradiation dose (D_s/D_p and H_s/H_p) and secondary production rate (F_s/F_p) as a function of primary Z/β (Z: nuclear charge and β: velocity).