Simulation of 90Y microspheres in selective internal radiation therapy (SIRT) reveals different heterogeneity profiles for glass and resin microspheres.

• Aim: Selective internal radiation therapy (SIRT) can supply normal tissue in the liver with radiation doses exceeding tolerance without causing toxicity, an effect possibly explained by microsphere clustering causing heterogeneity in the distribution. Using a simulation of microspheres transported through a simplified arterial structure, the aim of this study has been to evaluate how the number of injected microspheres affect the dose distribution. Materials and methods: The simulation was based on observations in biopsies and autoradiographies from resected liver tissue receiving 90YSIRT. Individual microspheres, with diameter and density from literature, were followed through a simplistic bifurcations model in which three parameters were introduced and optimised to obtain results consistent with observations: an artery coefficient of variation, a reduction parameter for the arterial diameter and a distribution volume parameter for the arterial tree. In the model the arterial diameter decreases for higher artery generations and the probability of microsphere clustering increases successively in the artery tree. After simulation a 90Y dose kernel was applied to the 940 cm3 sized 3D-matrix of microspheres and the dose distribution throughout the matrix was evaluated using varying resolutions and computing the coefficient of variation (CV). A smaller number of simulated microspheres (10^5) corresponds to the higher activity/sphere found in glass spheres while a larger number (10^7) resembles resin spheres. Results: As the number of microspheres increase from thousands to 106 the CV of the absorbed dose decrease from over 80 % to 32 % in a volume corresponding to a lobuli (2 mm^3) and at 106 microspheres a plateau was reached. The simulation was consistent with biopsy and autoradiography observations regarding the formation of clusters. A large majority of the clusters contain few microspheres and a minority of the clusters are significantly more numerous in microspheres. Conclusion: For the high microsphere concentrations used with resin microspheres the CVof the absorbed doses was constant (32 %) while for the lower microsphere concentrations used for glass spheres the CV varied between 90 % and 40 %. These results implies that small scale dosimetry for 90Y-SIRT differs between resin and glass spheres.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Cancer och onkologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Cancer and Oncology (hsv//eng)

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Radiologi och bildbehandling (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Radiology, Nuclear Medicine and Medical Imaging (hsv//eng)

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