Peritumoral edema may potentially harbor sarcoma cells. The extent of suspicious edema (SE) included in the treatment volume is subject to clinical judgment, balancing the risk of missing tumor cells with excess toxicity. Our goal was to determine variability in SE delineation by sarcoma radiation oncologists (RO).

Twelve expert ROs were provided with T1 Gadolinium and T2-weighted MR images of 10 patients with high-grade extremity soft tissue sarcoma. Gross tumor volume, CTV3cm (3cm longitudinal and 1.5cm radial margin) and CTV2cm (2cm longitudinal and 1cm radial margin) were contoured by a single observer. Suspicious peritumoral edema, defined as abnormal signal on T2 images, was independently delineated by all 12 ROs. Contouring agreement was analyzed using the Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm and kappa statistics.

The mean volumes of GTV, CTV2cm and CTV3cm were respectively 130 cm³ (7–413 cm³), 280cm³ and 360cm³. The mean consensus volume computed using the STAPLE algorithm at 95% confidence interval was 188cm³ (24–565cm³) with a substantial overall agreement corrected for chance (mean kappa =0.71; range: 0.32–0.87). The minimum, maximum and mean volume of suspicious edema (excluding the GTV) were 4cm³, 182cm³ and 58 cm³ (representing a median of 29% of the GTV volume). The median volume of suspicious edema not included in the CTV2cm and in the CTV3cm was 5 cm³ and 0.3cm³ respectively. There were 3 large tumors with >30cm³ of suspicious edema not included in the CTV3cm volume.

Despite the fact that SE would empirically seem to be a more subjective volume, a substantial or near-perfect inter-observer agreement was observed in SE delineation in most cases with high-grade STS of the extremity. A median of 97% of the consensus SE is within the CTV2cm (99.8% within the CTV3cm). In a minority of cases, however, significant expansion of the CTVs is required to cover SE.