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Nuklearmedizin 2008; 47(03): 120-126
DOI: 10.3413/nukmed-0129
Originalarbeiten - Original Articles
Schattauer GmbH

Geometric models in dosimetry of thyroid remnant mass

Geometrische Modelle zur Dosimetrie der Schilddrüsenrestmasse
D. Grošev
1   Clinical Department of Nuclear Medicine and Radiation Protection, Clinial Hospital Center Zagreb, Zagreb, Croatia
,
S. Lončarić
1   Clinical Department of Nuclear Medicine and Radiation Protection, Clinial Hospital Center Zagreb, Zagreb, Croatia
,
D. Huić
1   Clinical Department of Nuclear Medicine and Radiation Protection, Clinial Hospital Center Zagreb, Zagreb, Croatia
,
D. Dodig
1   Clinical Department of Nuclear Medicine and Radiation Protection, Clinial Hospital Center Zagreb, Zagreb, Croatia
› Author Affiliations
Further Information

Publication History

Received: 01 June 2007

accepted in revised form: 23 October 2007

Publication Date:
04 January 2018 (online)

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Summary

Aim: Absorbed dose to thyroid remnant tissue after 131I ablation becomes mass/size-dependent. This is a direct consequence of the small remnant size and radiation escape starts to be relevant. The self-absorbed fraction becomes mass/size-dependent. We have used Monte Carlo simulations to investigate the influence of the thyroid remnant shape upon the absorbed fraction calculation. Methods: Thyroid residue was modeled using spherical, cylindrical and elliptical shapes. Uniform beta activity distribution and unit density medium (water) within a remnant was assumed. For each of the geometrical models beta self-absorbed fraction (ϕγ) was calculated using Monte Carlo codes, while the mean absorbed dose per unit cumulated activity (Sγ) was calculated using MIRD formalism. Results: For spherical objects Ömono for mean beta energy (E = 0.182 MeV) of 131I is always greater than ϕγ calculated for the complete beta spectrum. For spheres having diameters 2–6 mm and assumption ϕγ=1, Sγ is overestimated by 11–37%. For cylinder and prolate spheroid of the same length and thickness, Sγ for cylinder is 30% smaller because of the greater mass. Similarly, elliptical cylinder and general ellipsoid of the same length and the same perpendicular dimensions (width and breadth), have similar ϕγ, while Sγ for elliptical cylinder is correspondingly smaller. Conclusion: For accurate dosimetry of thyroid remnants having masses <1 g and chordal lengths <1 cm it is necessary to calculate ϕγ for the full beta spectrum, or Sγ will be overestimated. The shape of the remnant may also be important since elongated non-spherical objects may also have ϕγ < 1.

Zusammenfassung

Ziel: Die absorbierte Dosis fur das Restgewebe der Schilddruse nach 131I-Ablation hangt von Masse und Grose ab. Dies ist die direkte Konsequenz der kleinen Restgrose. Der Strahlungsverlust wird dadurch relevant. Wir haben Monte- Carlo-Simulationen verwendet, um den Einfluss der Form des Schilddrusenrestgewebes auf die absorbierte Dosis zu untersuchen. Methoden: Der Schilddrusenrest wurde durch spharische, zylindrische und elliptische Formen modelliert. Fur den Schilddrusenrest wurde eine gleichformige Verteilung der β-Aktivitat und ein Medium mit Einheitsdichte (Wasser) angenommen. Fur jedes der geometrischen Modelle wurde eine β-Selbstabsorptionsfraktion (ϕβ) mithilfe von Monte-Carlo-Codes berechnet. Die mittlere absorbierte Dosis pro Zerfall (Sβ) wurde mit dem MIRD-Formalismus berechnet. Ergebnisse: Fur spharische Objekte ist ϕmono berechnet fur die mittlere Betaenergie (E = 0.182 MeV) des 131I immer groser als ϕβ berechnet fur das gesamte Betastrahlenspektrum. Fur Kugeln mit Durchmessern von 2.6 mm und die Annahme ϕβ=1 wird Sβ um 11.37% uberschatzt. Fur Zylinder und prolate Spharoide der gleichen Lange und Dicke ist Sβ fur den Zylinder auf Grund der groseren Masse 30% kleiner. Gleichsam haben elliptische Zylinder und allgemeine Ellipsoide gleicher Grose und gleiche ausere Dimensionen (Breite und Hohe) ahnliche ϕβ, wahrend Sβ fur elliptische Zylinder entsprechend kleiner ist. Schlussfolgerung: Fur eine akkurate Dosimetrie des Schilddrusenrests mit Massen <1 g und Sehnenlangen <1 cm ist es notwendig, ϕβ fur das gesamte β-Spektrum zu berechnen, da sonst Sβ uberschatzt wird. Die Form des Restgewebes kann wichtig sein, weil gestreckte nicht spharische Objekte auch ein ϕβ < 1 aufweisen konnen.

Keywords

Internal dosimetry - radioiodine ablation - absorbed fraction - Monte Carlo calculation

Schlüsselwörter

Interne Dosimetrie - Radioiod-Ablation - absorbierten Fraktion - Monte-Carlo-Rechung
 

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