Strahlentherapie und Onkologie

, Volume 186, Issue 8, pp 444–451

Radiobiological Comparison of Hypofractionated Accelerated Partial-Breast Irradiation (APBI) and Single-Dose Intraoperative Radiotherapy (IORT) with 50-kV X-Rays

Original Article

Abstract

Background and Purpose:

Intraoperative radiotherapy (IORT) of the tumor bed in early breast cancer is presently performed with a single dose of 50-kV X-rays from a miniaturized X-ray machine using spherical applicators. The purpose was to model the biological effect of hypofractionated accelerated partial-breast irradiation (APBI) with ten fractions.

Material and Methods:

The relative biologic effectiveness (RBE) was estimated from the linear-quadratic (L-Q) formalism including repair of sublethal damage or assuming a constant RBE = 1.2–1.5. The radial distribution of biological effect was assessed from clinical dose-response curves. In accordance with clinical convention, the dose for APBI was prescribed at 1 cm depth in the tumor bed, whereas for IORT it was prescribed at the applicator surface.

Results:

The fraction size was fitted to give the same risk of late normal-tissue reaction (fibrosis) as single-dose IORT with a maximum dose of 20 Gy. The isoeffective fraction size at 1 cm depth varied between 1.01 Gy for RBE estimated from the L-Q model and 1.64 Gy for constant RBE. The applicator size and dose prescription point influenced the radial dose distribution. The “sphere of equivalence” within which the risk for local recurrence is the same for whole-breast radiotherapy was predicted to extend to 11–15 mm distance from the applicator for α/β = 10 Gy and 9–13 mm for α/β = 4 Gy for hypofractionated APBI, representing an increase of the sphere of equivalence by 2.5–6 mm relative to single-dose IORT.

Conclusion:

An increase of the therapeutic window with hypofractionated APBI relative to single-dose IORT should be feasible.

Key Words

Breast Radiotherapy Fractionation APBI Modeling 

Strahlenbiologischer Vergleich hypofraktionierter, akzelerierter Teilbrustbestrahlung (APBI) und einzeitiger intraoperativer Radiotherapie (IORT) mit 50-kV-Rontgenstrahlen

Zusammenfassung

Hintergrund und Ziel:

Die intraoperative Radiotherapie (IORT) des Tumorbetts bei frühen Mammatumoren mit 50-kV-Rontgenstrahlen von einem miniaturisierten Röntgengerät wird derzeit als Einzeitbestrahlung mit sphärischen Applikatoren durchgeführt. Das Ziel war, die biologische Wirkung einer hypofraktionierten, akzelerierten Teilbrustbestrahlung (APBI) mit zehn Fraktionen zu modellieren.

Material und Methodik:

Die relative biologische Wirksamkeit (RBE) wurde mit dem linear-quadratischen Formalismus einschließlich Reparatur subletaler Schäden veranschlagt, alternativ wurde ein konstanter Wert von RBE = 1,2–1,5 angenommen. Die radiale Risikoverteilung biologischer Wirkung wurde mit Hilfe von klinischen Dosis-Wirkungs-Kurven bestimmt (Abbildung 1). Die Dosis für APBI wurde anhand der klinischen Verschreibungskonventionen in 1 cm Tiefe des Tumorbetts festgelegt, während die Dosis für IORT an der Oberfläche verschrieben wurde.

Ergebnisse:

Die Fraktionsgröße wurde angepasst, bis das gleiche Risiko später Normalgewebsreaktion (Fibrose) wie nach Einzeit-IORT mit einer maximalen Dosis von 20 Gy erreicht wurde (Abbildung 2). Die isoeffektive Fraktionsgröße in 1 cm Tiefe war 1,01 Gy, wenn RBE mit dem linear-quadratischen Modell bestimmt wurde, und 1,64 Gy für konstante RBE (Tabelle 1). Die Applikatorgröße und der Dosisverschreibungspunkt beeinflussten die radiale Dosisverteilung (Abbildung 3). Die Äquivalenzkugel, innerhalb welcher das Rezidivrisiko gleich groß wie bei Ganzbrustbestrahlung ist, erstreckte sich bis 11–15 mm Abstand von der Applikatoroberfläche für α/β = 10 Gy und bis 9–13 mm für α/β = 4 Gy (Abbildung 4). Dies entspricht einer Vergrößerung um 2,5–6 mm im Vergleich zur Einzeit-IORT.

Schlussfolgerung:

Eine Vergrößerung des therapeutischen Fensters durch hypofraktionierte APBI gegenüber Einzeit-IORT sollte möglich sein.

Schlüsselwörter

Teilbrustbestrahlung Strahlentherapie Fraktionierung Modellierung 

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Copyright information

© Urban & Vogel, Muenchen 2010

Authors and Affiliations

  1. 1.Department of Radiation OncologyUniversity Medical Center MannheimMannheimGermany
  2. 2.Klinik für Strahlentherapie und RadioonkologieUniversitätsmedizin MannheimMannheimGermany

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