Radioterapia

Editors: Khan, Faiz M. Title: Treatment Planning in Radiation Oncology, 2nd Edition Copyright ©2007 Lippincott Williams & Wilkins
> Tab le o f Conte nts > Chapter 4 - Patie nt and O rgan Mo ve me nt

Chapter 4 Patient and Organ Movement
James Michael Balter

Introduction
The driving tenet of conformal radiotherapy is the precise delivery of focal radiation doses to the target, so that aneffective dose can be delivered while limiting concomitant normal tissue irradiation and related toxicity risk. Technical advancements, such as three-dimensional (3-D) treatment planning and IMRT (intensity-modulated radiation therapy) have provided significant gains in specifying means to provide such dose distributions. Accurate delivery, so that intended and actual doses agree, is a morecomplicated matter. The problems of patient positioning and motion have been studied extensively. Although there are currently areas that need further exploration, it is possible to consider the magnitude of various uncertainties in dose delivery due to patient position variation and organ movement, and to discuss rational strategies for dealing with these uncertainties in the context of precisionradiotherapy.

Description of the Problem of Geometric Variation
The International Congress on Radiological Units (ICRU) has addressed the relative problem of geometric variations. In reports 50 and 62, concepts are evolved to attempt to standardize means of reporting doses. Some of the concepts presented in these reports have served as the basis for numerous investigations over the past few years,and have been adopted as standards for clinical trials. A brief discussion of the key concepts as they apply to geometric variation follows. The key structures that are delineated are the gross tumor volume (GTV) and organs at risk (OARs). The GTV is generally defined as the “visible†target, that is, that can be delineated from imaging or related information. The OARs are tissue structuresthat are dose limiting due to risk of radiation-induced toxicity. The next volume of interest is the clinical target volume (CTV). This target volume ideally expands about the GTV to include a reasonable expectation of the true target extent on a (static) patient model. The basis for CTV expansions includes intraobserver as well as interobserver variations in tumor delineation, as well as areasonable expectation of the extent of disease below the sensitive range of the imaging modality. Geometric uncertainty influences both the target volume as well as OARs. To ensure adequate geometric coverage of the target, the CTV is expanded. Internal organ movement is encompassed by an internal margin (IM) about the CTV to make the internal target volume (ITV), and setup error influences a setup margin(SM) about the ITV to yield the planning target volume (PTV). When the patient is imaged to define the CTV and critical structures, the position is sampled. In general, this sample occurs once, specifically during the computed tomography (CT) scan for treatment planning. To obtain this sample, the patient

is immobilized and positioned, with typical reference marks placed on the skin and/orimmobilization device at the principal axes of the CT scanner for verification of position and orientation. The sample of the patient serves as the model for treatment planning, and all subsequent targeting and density modeling is based on the information obtained during this session. Suppose the patient has the same treatment planning scan repeated several times over the course of a typicalfractionated treatment regimen (4–8 weeks). With no prior reference to positioning (i.e., no attempts to match the reference marker positions from one CT scan to the next), each CT scan will yield a patient model that differs in both position and configuration. These variations may be slight (indicating good reproducibility of patient position) or severe. Let us now suppose that attempts are made to...