Critical review
Organ motion and its management

Presented in part at the PTCOG XXX/EHTG/ECHED Meeting, Cape Town, South Africa, April 1999.
https://doi.org/10.1016/S0360-3016(01)01453-5Get rights and content

Abstract

Purpose: To compile and review data on the topic of organ motion and its management.

Methods and Materials: Data were classified into three categories: (a) patient position-related organ motion, (b) interfraction organ motion, and (c) intrafraction organ motion. Data on interfraction motion of gynecological tumors, the prostate, bladder, and rectum are reviewed. Literature pertaining to the intrafraction movement of the liver, diaphragm, kidneys, pancreas, lung tumors, and prostate is compiled. Methods for managing interfraction and intrafraction organ motion in radiation therapy are also reviewed.

Introduction

The patient’s anatomy and position during the course of radiation therapy usually varies to some degree from those used for therapy planning purposes. This is mainly due to patient movement, inaccurate patient positioning, and organ motion. Consequently, the actual received absorbed dose distribution differs from the planned absorbed dose distribution. The two scenarios of relevance are an insufficient dose coverage of the targeted tumor volume and an overdosage of normal tissues. Both cases potentially compromise the clinical results. Variations in patient position and movement can be minimized with the help of precise patient positioning systems and rigid immobilization devices. For some anatomical sites, however, the internal motion of organs due to physiological processes presents a challenge.

Motion of the tumor volume is commonly accounted for by the use of margins that encompass the tumor volume. ICRU Reports 50 and 62 1, 2 define the relevant terminology. First, the gross tumor volume (GTV) is defined as the volume containing demonstrated tumor. Second, the clinical target volume (CTV) is defined to enclose the GTV plus a margin to account for suspected tumor involvement. The planning target volume (PTV) is defined by the CTV plus a margin to allow for geometrical variations such as patient movement, positioning uncertainties, and organ motion. In ICRU Report 62 (2), this margin is divided into two components: (a) internal margin (IM) to account for variations in size, shape, and position of the CTV; and (b) setup margin (SM) to account for uncertainties in patient position and beam alignment. The PTV is identical to the earlier definition of target volume, as defined in ICRU Report 29 (3).

Information about the amount and nature of the CTV motion is essential for the determination of the internal margin size. Data on various types of organ motion are available in the literature. In this report, they are compiled and presented for three different categories: organ motion related to patient position changes, motion that occurs in-between fractions, and that which occurs during fractions.

CTVs that are subject to significant movements require large margins that may include critical normal structures in the PTV. These structures may subsequently hamper the application of the intended therapeutic doses to the PTV. This issue is of particular importance in dose escalation studies. The use of smaller margins may, on the other hand, compromise the adequate absorbed dose coverage of the moving CTV. Efforts to manage and minimize the effect of CTV motion are described in the literature; these works are also reviewed in this report.

Section snippets

Position-related organ motion

A change in the patient position can be accompanied by internal organ motion, the magnitude of which is dependent on anatomical site. This type of organ motion arises when the patient position during the planning scan differs from the patient position during treatment.

Some hadron therapy facilities have fixed horizontal beam lines where patients are often treated in a seated or standing position. Ideally, these patients should be scanned in the treatment position using a vertical scanner, which

Interfraction organ motion

This motion occurs when the CTV position changes on a day-to-day level and is mainly associated with organs that are part of or adjacent to the digestive system. Changes in the patient’s condition, such as weight gain/loss, can also affect the relative position of the CTV.

Intrafraction organ motion

Organ motion that occurs while the patient is being irradiated is labeled intrafraction organ motion. Respiratory and cardiac motion are the main contributors to intrafraction motion, which effects mainly organs of the thorax and abdomen. Studies dealing with the quantification of intrafraction motion of several organs are detailed below.

Discussion

The amount of available data on internal organ motion varies greatly for different organs and types of motions. Whereas an understanding of the nature and magnitude of organ motion is necessary to determine the internal margin, there is no general agreement on the absolute relationship between the magnitude of motion and the size of the margin. If the effect of organ motion on the absorbed dose in the CTV is the criterion applied for margin size, the dose distribution in and around the CTV has

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