Auditore Lucrezia, Amato Ernesto, Pistone Daniele, Italiano Antonio
Department of Biomedical and Dental Sciences and of Morphologic and Functional Imaging (BIOMORF), University of Messina, Italy.
INFN, National Institute for Nuclear Physics, Section of Catania, Italy.
Med Phys. 2023 Mar;50(3):1865-1870. doi: 10.1002/mp.16171. Epub 2023 Jan 7.
Internal dosimetry has an increasing role in the planning and verification of nuclear medicine therapies with radiopharmaceuticals. Dose Point Kernels (DPKs), quantifying the energy deposition all around a point source, in a homogenous medium, are extensively used for 3D dosimetry and nowadays are mostly evaluated by Monte Carlo (MC) simulation. To our knowledge, DPK for beta emitters is estimated neglecting the continuous photon emission due to the Internal Bremsstrahlung (IB), whose contribution to the absorbed dose can be relevant beyond the maximum range of betas, as evidenced in recent works.
Aim of this study was to investigate and quantify, by means of MC simulations, the contribution of IB photons to DPK calculated for Y and provide the updated Y DPK.
The overall radiation due to the decay of a Y point source, placed at the centre of concentric water shells of increasing radii from 0.02 cm to 20 cm, was simulated with GAMOS, including the IB source term whose spectral distribution was described by an analytical model. Energy deposition was scored in the shells as a function of the distance from the source, R, and DPK was estimated in terms of the scaled absorbed dose fraction, F(R/X ), where X is the range within which the beta particles deposit 90% of their energy.
A comparison between the two simulated absorbed dose distributions, calculated with or without IB, clearly shows that the latter (incomplete) choice is consistent with the findings of other Authors and systematically underestimates the absorbed dose imparted to the tissue. Y DPK values currently used are underestimated by 20%-34% for R>2X .
The revised values provided in this work suggest that the inclusion of IB emission in DPK evaluations is advisable for pure beta emitters.
内照射剂量学在放射性药物核医学治疗的规划和验证中发挥着越来越重要的作用。剂量点核（DPK）用于量化均匀介质中围绕点源的能量沉积，广泛应用于三维剂量学，目前大多通过蒙特卡罗（MC）模拟进行评估。据我们所知，β发射体的DPK估计忽略了内韧致辐射（IB）产生的连续光子发射，近期研究表明，其对吸收剂量的贡献在β粒子的最大射程之外可能相当可观。
本研究旨在通过MC模拟研究并量化IB光子对为钇计算的DPK的贡献，并提供更新后的钇DPK。
使用GAMOS模拟了位于半径从0.02 cm到20 cm逐渐增大的同心水壳中心的钇点源衰变产生的总辐射，包括用解析模型描述其光谱分布的IB源项。在壳层中根据距源的距离R对能量沉积进行评分，并根据缩放吸收剂量分数F(R/X)估计DPK，其中X是β粒子沉积其90%能量的范围。
对有或无IB情况下计算的两种模拟吸收剂量分布的比较清楚地表明，后一种（不完整）选择与其他作者的研究结果一致，并且系统性地低估了赋予组织的吸收剂量。对于R>2X，目前使用的钇DPK值被低估了20%-34%。
本研究提供的修订值表明，对于纯β发射体，在DPK评估中纳入IB发射是可取的。
