EXTERNAL BEAM RADIOTHERAPY

Techniques used to reduce safety margins used in External Beam Radiotherapy Treatment Planning

Techniques used to reduce safety margins used in External Beam Radiotherapy Treatment Planning

Introduction

Radiotherapy is one of the major treatment options in cancer management. According to best available practice, 52% of patients should receive radiotherapy at least once during the treatment of their cancer. Together with other modalities such as surgery and chemotherapy it plays an important role in the treatment of 40% of those patients who are cured of their cancer (Haken & Thornton, 1992: 121). Radiotherapy is also a highly effective treatment option for palliation and symptom control in cases of advanced or recurrent cancer. The process of radiotherapy is complex and involves understanding of the principles of medical physics, radiobiology, radiation safety, dosimetry, radiotherapy planning, simulation and interaction of radiation therapy with other treatment modalities.

Safety Margins and Radiotherapy

Clinical radiotherapy procedures aim at high accuracy. However, there are many error sources that act during treatment preparation and execution that limit the accuracy. As a consequence, a safety margin is required to ensure that the planned dose is actually delivered to the target for (almost) all patients. Before treatment planning, a planning computed tomography scan is made. In particular, motion of skin with respect to the internal anatomy limits the reproducibility of this step, introducing a systematic setup error. The second important error source is organ motion (Fiorino & Reni, 1998: 285). The tumor is imaged in an arbitrary position, leading to a systematic organ motion error. The image may also be distorted because of the interference of the scanning process and organ motion. A further systematic error introduced during treatment planning is caused by the delineation process. During treatment, the most important errors are setup error and organ motion leading to day-to-day variations. There are many ways to define the margins required for these errors. In this article, an overview is given of errors in radiotherapy and margin recipes, based on physical and biological considerations. Respiration motion is treated separately.

The treatment process of external beam radiotherapy of solid tumors inherently introduces geometrical uncertainties. The main sources of uncertainty are tumor delineation inaccuracies of the gross tumor volume (GTV), unknown extent of microscopic tumor, organ positional variation within the patient, and setup variations. Recently, much attention has been paid to measurement and reduction of geometrical errors (Dubois & Prestidge, 1998: 785). However, residual uncertainties always remain. In conformal radiotherapy, these uncertainties are generally handled by a safety margin. The purpose of this article is to summarize error sources in radiotherapy, describe methods for quantifying the errors, and describe the impact that these errors have on the dose delivered to the clinical target volume (CTV). Next, we will discuss treatment margins based on physical and biological considerations. Finally, a number of controversies concerning errors and margins will be discussed.

Several geometrical uncertainties are involved in the delineation process of the GTV. First, the imaging modalities have a limited resolution, in particular perpendicular to the slice planes causing ...