By: Dr. John Cho and Dr. Louis Lao
CANCER
Figure 1
STEREOTACTIC BODY RADIATION THERAPY IN EARLY STAGE NON-SMALL CELL LUNG CANCER
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Radiation therapy is the medical discipline that treats malignant diseases with ionizing radiation. The practice of treating cancer with radiation has been around for more than a century. Shortly following Willhelm Conrad Roentgen’s discovery of x-rays in late 1895, the very first (breast) cancer patient was treated with x-rays in early 1896.
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adiation, as a therapeutic modality, is a local treatment. Its efficacy in killing tumour cells depends, amongst others, on dose. However, higher doses also incidentally damage the normal surrounding tissue, increasing treatment toxicity. Empirically, less radiation toxicity was observed if the total dose was divided into smaller doses (i.e. fractions) over multiple days. Fractionation exploits radiobiological differences in cellular repair mechanism between the tumour and normal tissue and is an important means of reducing toxicity and sparing normal tissue. Further normal tissue sparing can be achieved by conforming the radiation dose around the tumour target, reducing the dose to adjacent normal tissue. The degree of conformality is, in part, limited by the technical constraints and associated treatment uncertainty (such as random positioning errors, patient motion, etc.). Radiation oncology is a technical discipline. In the last decade, the field has witnessed tremendous advances both in treatment planning and treatment delivery. This is largely due to more powerful
computers and better imaging devices. These advances allow targeting of tumours in a precise and accurate manner and, consequently, make possible the sparing of adjacent normal tissue.
WHAT IS SBRT? SBRT stands for stereotactic body radiotherapy (also known as SABR, stereotactic ablative radiotherapy). SBRT is a method of delivering ablative doses of radiation safely. Typically, very large doses per fraction (i.e. 7.5-20 Gy) are given over a few fractions (i.e. 1 to 8 fractions over 1 to 2 weeks). Fractionation effects to spare normal tissue become less important with larger fraction sizes. By necessity, the dose gradients must be very sharp and tightly conformed around the tumour target in order to limit the dose spilling into the surrounding normal tissue (Figure 1). SBRT represents the culmination of several important technological developments in radiation therapy. These technologies constitute essential components of a SBRT treatment and allow clinicians the ability to target the tumour precisely and accurately so that each treat-
DECEMBER 2011 / JANUARY 2012 BIOTECHNOLOGY FOCUS 11
