Stereotactic body radiotherapy: current strategies and future development

Abstract

Stereotactic body radiotherapy (SBRT) has emerged as the standard treatment for medically inoperable early-staged non-small cell lung cancer (NSCLC). The local control rate after SBRT is over 90%. Some forms of tumour motion management and image-guided radiation delivery techniques are the prerequisites for fulfilment of its goal to deliver a high radiation dose to the tumour target without overdosing surrounding normal tissues. In this review, the current strategies of tumour motion management will be discussed, followed by an overview of various image-guided radiotherapy (RT) systems and devices available for clinical practice. Besides medically inoperable stage I NSCLC, SBRT has also been widely adopted for treatment of oligometastasis involving the lungs. Its possible applications in various other cancer illnesses are under extensive exploration. The progress of SBRT is critically technology-dependent. With advancement of technology, the ideal of personalised, effective and yet safe SBRT is already on the horizon.

Keywords: Stereotactic body radiotherapy (SBRT); image-guided radiation therapy (IGRT); non-small cell lung cancer (NSCLC); personalized medicine.

Figure 1

Intrinsic physical properties of SBRT. (A) The MLCs (rectangles in yellow, shields to shape the radiation field) in touch with the target; (B) isodose curve. The steepest part of the curve is at 60–90% isodose level (shown in red). SBRT, stereotactic body radiation therapy; MLC, multi-leaf collimators

Figure 2

Breathing waveform to show that ABC is activated at deep inspiration.

Figure 3

Abdominal compressor applied to abdominal wall to limit patient’s breathing effort. In this way, the tumour movement with respiration will be restricted.

Figure 4

4-dimensional CT setup. Movement of the infrared reflective block placed on patient’s abdominal skin will be tracked by the infrared camera.

Figure 5

RMP respiratory gating: radiation beam is on only at end expiration. RPM, real-time patient management.

Figure 6

ExacTrac^® Adaptive Gating system. The two X-rays tubes are embedded to the room floor, and the two amorphous silicon flat panel X-ray detectors and the infrared tracking camera are mounted on the ceiling.

Figure 7

Onboard Imager^®. The X-ray tube and X-ray detector are mounted to the linear accelerator so that the X-ray tube-detector pair is always 90º to the treatment head of the linear accelerator.

Figure 8

ABC pre-treatment verification of tumour position. The small circle is the area that the fiducial marker is expected to stay during radiotherapy. (A) No tumour displacement; (B) tumour displaced from its expected position, as indicated by displacement of the fiducial markers outside the small circles. The patient has to be repositioned for correction of tumour displacement before start of radiotherapy. ABC, active breathing control.

Figure 9

RT-RT system. (A) Four X-ray tube-image intensifier pairs for IGRT; (B) if the fiducial markers are displaced outside its allowed region (area marked by the parallelogram), radiation beam will be tuned off automatically. RT-RT, real-time tumour-tracking radiation therapy; IGRT, image-guided radiation therapy.