Abstract
The aim of this study is to provide guidelines for the selection of external‐beam radiation therapy target margins to compensate for target motion in the lung during treatment planning. A convolution model was employed to predict the effect of target motion on the delivered dose distribution. The accuracy of the model was confirmed with radiochromic film measurements in both static and dynamic phantom modes. 502 unique patient breathing traces were recorded and used to simulate the effect of target motion on a dose distribution. A 1D probability density function (PDF) representing the position of the target throughout the breathing cycle was generated from each breathing trace obtained during 4D CT. Changes in the target D95 (the minimum dose received by 95% of the treatment target) due to target motion were analyzed and shown to correlate with the standard deviation of the PDF. Furthermore, the amount of target D95 recovered per millimeter of increased field width was also shown to correlate with the standard deviation of the PDF. The sensitivity of changes in dose coverage with respect to target size was also determined. Margin selection recommendations that can be used to compensate for loss of target D95 were generated based on the simulation results. These results are discussed in the context of clinical plans. We conclude that, for PDF standard deviations less than 0.4 cm with target sizes greater than 5 cm, little or no additional margins are required. Targets which are smaller than 5 cm with PDF standard deviations larger than 0.4 cm are most susceptible to loss of coverage. The largest additional required margin in this study was determined to be 8 mm.PACS numbers: 87.53.Bn, 87.53.Kn, 87.55.D‐, 87.55.Gh
Highlights
The management of target motion during external-beam radiation therapy (EBRT) is crucial for ensuring agreement between prescribed and delivered dose to the patient and, successful treatment outcomes
Intrafraction motion can originate from a variety of sources such as cardiac motion, gastrointestinal peristalses or respiratory motion.[1]. The works of other authors have described the motion of targets in the lung due to respiration.[2,3,4] It is generally noted that target motion in the lung is largest in the superior–inferior (SI) direction, each patient presents with unique motion
Accepted motion encompassing techniques include margin expansion,(6-8) the use of an internal target volume (ITV) determined by the range of target locations,(9,10) and probabilistic approaches to volume definition.[11]. Because the expansion of treatment volumes is accompanied by an increase in normal tissue complication probability (NTCP), a balance must be struck between improving target dose coverage with larger treatment volumes and potential complications to healthy tissues
Summary
The management of target motion during external-beam radiation therapy (EBRT) is crucial for ensuring agreement between prescribed and delivered dose to the patient and, successful treatment outcomes. A report by the American Association of Physicists in Medicine (AAPM) Task Group TG-76(1) highlights several different methods for managing respiratory motion. These include motion encompassing techniques (margin and target volume definitions), various breath-hold techniques, gated treatment delivery, and target tracking methods. Each of these methods presents tradeoffs between dose conformality, technical feasibility, demand on clinical resources, and patient condition. The least technically demanding (and most widely used) of these approaches is to define treatment volumes that account for the expected motion of the target Within this approach to motion management there are several different methods which can be used to define treatment volumes. Because the expansion of treatment volumes is accompanied by an increase in normal tissue complication probability (NTCP), a balance must be struck between improving target dose coverage with larger treatment volumes and potential complications to healthy tissues
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