Setup Uncertainty Analysis for Intensity-modulated Radiotherapy for Soft Tissue Sarcoma of the Thigh

Abstract

Use of the proper setup uncertainty margins in radiation therapy planning is important to ensure that the clinical target volume (CTV) is covered while preserving as much normal tissue as possible. Setup uncertainty has been extensively studied in the head and neck and pelvic regions, however, data is only now starting to emerge for extremity irradiation. Treatment of the thigh represents unique challenges in positioning and immobilization due to its ability to undergo many translations and rotations relative to the rest of the body. We evaluate three dimensional (3D) setup errors and report CTV to planning target volume (PTV) margins for soft tissue sarcoma patients undergoing intensity modulated radiation therapy (IMRT) to the thigh. Twenty-one patients immobilized in an Alpha cradle mold and treated via IMRT to the thigh between 2006 and 2008, a total of 114 pairs of orthogonal megavoltage portal images, were analyzed retrospectively using a template based matching technique. Individual setup errors, maximum and minimum displacements, and population based systematic (Σ) and random (σ) errors were calculated. Setup margins were then determined using published margin recipes. The mean translational setup errors (and range max to min) from all data were 1 (+21 to -15), 0 (+19 to -10), and 1 (+8 to -6) millimeters (mm) in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions respectively. The mean rotational errors from all data were -0.1° (+3.5° to -9°) and 0.2° (+6.5° to -4.5°) in the cranial and sagittal planes respectively. In the RL, AP, and SI directions, the standard deviation of the patient systematic errors was 4, 3, and 2 mm and the root mean square of the patient random errors was 4, 4, and 2 mm. This corresponds to a CTV to PTV margin of 12.5 (RL), 10.7 (AP), and 6.1 (SI) mm using the van Herk margin recipe formulation. Our institutional experience in IMRT for soft tissue sarcoma of the thigh provides useful data for the quantification of setup errors for this anatomical site. The observed population systematic and random errors for the thigh are comparable to reports in the literature for pelvic sites with similar immobilization methods. A CTV to PTV expansion of ≤ 12.5 mm should result in adequate CTV coverage for this patient population.