Determination of elemental tissue composition following proton treatment using positron emission tomography

Positron emission tomography (PET) has been suggested as an imaging technique for in vivo proton dose and range verification after proton induced-tissue activation. During proton treatment, irradiated tissue is activated and decays while emitting positrons. In this paper, we assessed the feasibility of using PET imaging after proton treatment to determine tissue elemental composition by evaluating the resultant composite decay curve of activated tissue. A phantom consisting of sections composed of different combinations of ¹H, ¹²C, ¹⁴N, and ¹⁶O was irradiated using a pristine Bragg peak and a 6 cm spread-out Bragg-peak (SOBP) proton beam. The beam ranges defined at 90% distal dose were 10 cm; the delivered dose was 1.6 Gy for the near monoenergetic beam and 2 Gy for the SOBP beam. After irradiation, activated phantom decay was measured using an in-room PET scanner for 30 min in list mode. Decay curves from the activated ¹²C and ¹⁶O sections were first decomposed into multiple simple exponential decay curves, each curve corresponding to a constituent radioisotope, using a least-squares method. The relative radioisotope fractions from each section were determined. These fractions were used to guide the decay curve decomposition from the section consisting mainly of ¹²C + ¹⁶O and calculate the relative elemental composition of ¹²C and ¹⁶O. A Monte Carlo simulation was also used to determine the elemental composition of the ¹²C + ¹⁶O section. The calculated compositions of the ¹²C + ¹⁶O section using both approaches (PET and Monte Carlo) were compared with the true known phantom composition. Finally, two patients were imaged using an in-room PET scanner after proton therapy of the head. Their PET data and the technique described above were used to construct elemental composition (¹²C and ¹⁶O) maps that corresponded to the proton-activated regions. We compared the ¹²C and ¹⁶O compositions of seven ROIs that corresponded to the vitreous humor, adipose/face mask, adipose tissue, and brain tissue with ICRU 46 elemental composition data. The ¹²C and ¹⁶O compositions of the ¹²C + ¹⁶O phantom section were estimated to within a maximum difference of 3.6% for the near monoenergetic and SOBP beams over an 8 cm depth range. On the other hand, the Monte Carlo simulation estimated the corresponding ¹²C and ¹⁶O compositions in the ¹²C + ¹⁶O section to within a maximum difference of 3.4%. For the patients, the ¹²C and ¹⁶O compositions in the seven ROIs agreed with the ICRU elemental composition data, with a mean (maximum) difference of 9.4% (15.2%). The ¹²C and ¹⁶O compositions of the phantom and patients were estimated with relatively small differences. PET imaging may be useful for determining the tissue elemental composition and thereby improving proton treatment planning and verification.