Preclinical Biologic Assessment of Proton Beam Relative Biologic Effectiveness at Proton Therapy Center Houston

The clinical use of protons for the treatment of cancer is one of the most technologically advanced forms of therapy. The newly commissioned M.D. Anderson Cancer Center, Proton Therapy Center Houston is one of the newest facilities offering a complete range of advanced proton therapies designed specifically to minimize treatment-related normal tissue toxicity. As an integral part of the clinical commissioning of this proton beam, we performed an in vivo assessment of the relative biologic effectiveness of the 250-MeV protons relative to standard cobalt⁶⁰ using the jejunal crypt microcolony assay in mice. Our data are consistent with a generic relative biologic effectiveness of 1.1, and this relative biologic effectiveness is in agreement with that used to describe most other clinical proton therapy beams worldwide.

Introduction

Clinical proton therapy is one of the most advanced forms of radiotherapy available for many types of cancer. Proton therapy has the ability to deliver highly targeted radiation to tumors while sparing the surrounding normal tissue. The M.D. Anderson Proton Therapy Center is one of the newest facilities offering the complete range of advanced proton therapies. The clinical use of proton radiotherapy requires standardizing the physical dosimetry and defining the relative biologic efficiency of the clinical beam (1, 2). Prescriptions in proton therapy are made in terms of the “cobalt dose equivalent,” defined to be the absorbed dose in Gray multiplied by the relative biologic effectiveness (RBE). Conventionally, the RBE is the ratio of the dose of ⁶⁰Co γ-rays relative to that of the protons required to produce a defined biologic response. The value assigned to this RBE is crucial to any proton therapy. To define the RBE for the Proton Therapy Center Houston beam, we have determined radiation dose–survival curves for mouse jejunal crypt cells using 250-MeV protons produced by the Center’s high-energy synchrotron particle accelerator (Hitachi Ltd. Power Systems, Ibaraki-ken, Japan) and γ-rays from a conventional ⁶⁰Co irradiator.