Abstract
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Objectives Proton range uncertainty places limits on the accuracy of proton therapy. PET imaging of activated endogenous tissue following proton treatment has been suggested as a tool for proton range verification. However, this approach is also limited due to the weak activation of tissue near the proton distal fall-off. To overcome this limitation, we investigated the use of patient-implantable proton activated elements such as 63Cu which is strongly activated by low energy protons and decays by positron emission with a relatively long half-live (T½ = 38 min). This study compares PET signals from irradiated copper foil (63Cu = 69.15%) and a tissue substitute - polycarbonate, (C16O3H14).
Methods Two sets of 0.1 mm thick copper foils simulating pellets of 40, 10, and 2.5 mm3 (2x2, 1x1, 0.5x0.5 cm) and two sets of 0.76 mm thick polycarbonate sheets simulating volumes of 305 and 76 mm3 (2x2, 1x1 cm) were irradiated by 160 MeV, 6-cm SOBP proton beam. Each of the pieces was placed at four different depths over the distal fall-off (100% ~ 29% of SOBP dose). 15 min following activation using a 10 Gy proton beam, a 3 hour PET scan was acquired and the corresponding images were reconstructed. The PET signal intensity of each pellet/volume was then measured. Monte Carlo simulation was also performed for comparison.
Results Relative mean activities per unit volume at the distal fall off region at 100~99%, 99~95%, 87~71%, 49~29% dose were as follows: For 2x2 cm copper (polycarbonate) - 100% (23%), 80% (7%), 28% (0.5%), 6% (0.1%). For 1x1 cm: copper (polycarbonate) - 100% (35%), 79% (10%), 14% (0.4%), 11% (0.5%). The 0.5x0.5 cm copper foils (2.5 mm3 volume) also had a detectable PET signal. Monte Carlo simulation agreed with these measurements.
Conclusions The higher activation of copper over the distal fall-off suggests the possibility of using implanted copper for proton range verification. Other elements such as 68Zn with higher activation cross section and longer half-life are expected to give greater PET signal under similar experimental conditions.