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Relative biological effectiveness (RBE) values for proton beam therapy
Harald Paganetti1, Andrzej Niemierko, Marek Ancukiewicz
1Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA 02114, USA. hpaganetti@partners.org
Summary
A generic relative biological effectiveness (RBE) of 1.1 is currently reasonable for clinical proton therapy. However, further research is needed to determine tissue- and dose-specific RBEs to optimize treatment planning and patient outcomes.
Area of Science:
- Radiation Oncology
- Medical Physics
- Radiobiology
Background:
- Clinical proton beam therapy traditionally uses a generic relative biological effectiveness (RBE) of 1.0 or 1.1.
- This practice is based on the interpretation that RBE variations are minor compared to measurement capabilities.
Purpose of the Study:
- To reassess the rationale for using a generic RBE in clinical proton therapy.
- To evaluate the accumulated clinical experience and experimental data on RBE.
Main Methods:
- Examined experimental RBE determinations from in vitro and in vivo systems.
- Assessed considerations for moving to tissue-, dose/fraction-, and LET-specific RBE values.
- Reviewed patient morbidity data and analyzed experimental efforts to determine RBE accuracy.
Main Results:
- In vitro studies show RBE values ranging from 0.9 to 2.1 (average ~1.2), while in vivo studies range from 0.7 to 1.6 (average ~1.1).
- Both in vitro and in vivo data suggest a statistically significant increase in RBE at lower doses per fraction, more pronounced in vitro.
- A measurable increase in RBE is noted at the terminal few millimeters of the Spread Out Bragg Peak (SOBP), extending the bioeffective range by 1-2 mm.
Conclusions:
- Current uncertainties in RBE for human tissues preclude tissue-specific values.
- A generic RBE of 1.1 remains reasonable, but the "hot region" at the SOBP's end requires consideration in treatment planning.
- Prospective studies on normal tissue reactions and RBE determinations for late-responding tissues are needed, particularly concerning dose per fraction.