Determining Margins For Organs At Risk To Account For Anatomic Motion

The addition of a margin to an organ at risk (OAR) to ensure that the OAR will indeed be within its planning contour during the entire treatment course can reduce variation of OAR dosimetry. The purpose of this work is to determine such margins based on the data collected from image guided radiotherapy (IGRT). IGRT provides us with the necessary data and tools to adjust for the systematic setup errors, and mostly ensures that the dose is delivered to the planning target volume (PTV). However, such set-up adjustments may not fully address the anatomical motion of OARs. Therefore, both the systematic and random errors need to be considered in determination of OAR margins. Daily MVCTs collected with a Hi-Art helical Tomotherapy system were analyzed to obtain the anatomic motion data. The daily MVCT and the planning kVCT images were registered together based on alignment of bony anatomy. The difference in position of the center of mass of selected OARs, including kidneys, liver and bladder, between the planning kVCT and the daily MVCT was recorded. Overall, 29 kidney (left and right), 8 liver and 15 bladder cases were analyzed to determine systematic (Σ) and random (σ) variations due to their inter-fractional anatomic changes including respiratory motion (because each MVCT represented a random snapshot of respiration). The daily setup shifts corresponding to the MVCTs were also tracked. The one-dimensional margins are defined first by considering only the systematic contribution as 1.3Σ, which ensures that OAR is within the planning contour 90% of the time. Further, the random contribution can be accounted by addition or subtraction of 0.5σ. No statistically significant difference between left and right kidneys was observed and the two datasets were pooled together. The maximum and minimum average motion, and margins obtained with different recipes for anterior/posterior (A/P), left/right (L/R) and superior/inferior (S/I) directions are represented in the table below. It is seen that a margin of 10 mm for kidney and 20 mm for liver or bladder would be reasonable. We further explored the validity of using these recipes and the effect of the proposed margins on the treatment planning of abdominal irradiations (Table). The motion of OARs from the planning session and between fractions can be significant for various sites. This motion needs to be accounted in the evaluation of the dose delivered to OARs by introduction of appropriate margins. We have evaluated motion of kidneys, liver and bladder, and determined the required margins for these organs.Tabled 1Min/Max motion (mm)1.3Σ (mm)1.3Σ−0.5σ (mm)1.3Σ + 0.5σ (mm)OrganA/PL/RS/IA/PL/RS/IA/PL/RS/IA/PL/RS/IKidney−9.2−2.6−11.57562351091014.811.18.7Liver−6.2−13.2−3.8141994913926427.626.312.5Bladder−19.2−17.7−35.01518111414921131722.212.75.0 Open table in a new tab