Strahlenbiologie

Summary

Radiotherapy with unsealed radionuclides differs from external radiotherapy with regard to the radiation quality and energy range, the regional dose uniformity and the time course of irradiation regimen. External radiotherapy is planned precisely and can be applied to a target volume independently from blood flow during a course of irradiation fractions. In contrary, administered radiopharmaceuticals distribute according to their pharmacokinetic properties and generate a continuous irradiation corresponding to the effective halflife. The resulting dose rates are approximately 1 Gy/min and 1 Gy/h, respectively. The bio – kinetics of radiopharmaceuticals involves cellular accumulation and retention with highly variable affinity to specific organs that can be modulated as well. A remarkable dose gradient is found at the edge of volumes with enhanced uptake. The biological effect of an irradiation with decreasing intensity can be compared with the radiation effect caused by conventional fractionation with 2 Gy a day in external beam therapy by means of the linear-quadratic model. However, the experimental validation of this translation is still under investigation. Radionuclide therapy is usually performed in several cycles some month apart. This procedure fails to meet external radiotherapy. The vision of a combined external-internal radiotherapy requires efforts for a common dosimetry approach both in vitro and in vivo with a physical and biological verification of the results.

Zusammenfassung

Die Radiotherapie mit offenen Radionukliden unterscheidet sich neben Strahlungsqualität und -energie auch in der regionalen Dosis – homogenität und den zeitlichen Dimensionen der Bestrahlung von der externen Radiotherapie. Externe Radiotherapie kann exakt geplant, gezielt und lokal umschrieben unabhängig von regionaler Durchblutung, wiederholt in kurzen Fraktionen appliziert werden. Dagegen folgt die Anreicherung von Radiopharmaka im Organismus der Pharmakokinetik, und sie sind entsprechend ihrer effektiven Halbwertszeit über einen längeren Zeitraum kontinuierlich wirksam. Daraus resultieren Unterschiede in der Dosisleistung von 1 Gy/ min gegenüber maximal 1 Gy/h. Die Biokinetik der Radiopharmaka umfasst zelluläre Akkumulation und Retention mit stark variabler Affinität zu einzelnen Organen und kann moduliert werden. Im Randbereich von Mehranreicherungen fällt die Dosis steil ab. Die biologische Wirkung der kontinuierlichen Bestrahlung sinkender Intensität lässt sich mittels linear-quadratischem Modell in die Wirkung der fraktionierten Applikation in 2 Gy-Einheiten umrechnen, wobei jedoch bislang der Gültigkeitsbereich dieser Translation noch nicht bestimmt wurde. Die Sequenz wiederholter Applikation offener Radionuklide im Abstand von Monaten findet in der externen Radiotherapie keine Entsprechung. Für eine kombinierte externe – interne Radiotherapie sind Anstrengungen für eine gemeinsame Dosimetrie zu unternehmen, die in vitro und in vivo physikalisch und biologisch zu verifizieren ist.

Keywords

Radiotherapy - radionuclide therapy - dosimetry - BED - radiation tolerance

Schlüsselwörter

Radiotherapie - Radionuklidtherapie - Dosimetrie - BED - Normalgewebetoleranz

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