Margin Reduction from IGRT for Soft-Tissue Sarcoma: Secondary Analysis of RTOG 0630 Results

Purpose— On RTOG 0630, a study of IGRT for primary soft tissue sarcomas of the extremity, six imaging modalities were used. We analyzed all daily patient-repositioning data collected in this trial to determine the impact of daily IGRT on CTV-to-PTV margin. Methods and Materials— Daily repositioning data, including shifts in right-left (RL), superior-inferior (SI), anterior-posterior (AP) directions and rotations, for 98 patients enrolled in RTOG 0630 from 18 institutions were analyzed. Patients were repositioned daily based on bony anatomy using the pretreatment images, including KV-orthogonal images (KVorth), MV-orthogonal images, KV fan-beam CT, KV cone-beam, MV fan-beam CT (MVCT), and MV cone-Corresponding beam CT. Mean and standard deviations (SD) for each shift and rotation were calculated for each patient and for each IGRT modality. The t-tests and F-tests were performed to analyze the differences in the means and SDs. Necessary CTV-to-PTV margins were estimated. Results— The repositioning shifts and day-to-day variations were large and generally similar for the 6 imaging modalities. Of the two most commonly used modalities, MVCT and KVorth, there were no statistically significant differences in the shifts and rotations (p=0.15 and 0.59 for RL and SI shifts, respectively, and p=0.22 for rotation), except for shifts in AP direction (p=0.002). The estimated CTV-to-PTV margins in RL, SI and AP directions would be 13.0, 10.4, and 11.7 mm from MVCT data, and 13.1, 8.6, and 10.8 mm from KVorth data, indicating that margins substantially larger than 5mm used with daily IGRT would be required in the absence of IGRT. Conclusion— The observed large daily repositioning errors as well as the large variations among institutions imply that the daily IGRT is necessary for this tumor site, particularly in multi-institutional trials. Otherwise, a CTV-to-PTV margin of 1.5 cm is required to account for daily setup variations.