Session Item

RT to soft tissue sarcoma of the extremities (STSE) improves local control rates, however, STSE survivors often develop significant toxicities (defined as ≥ grade 2 RTOG; grade 2+), which impact quality-of-life. Past clinical trials reported grade 2+ subcutaneous fibrosis (48.2% vs 31.5%), lymphoedema (23.2% vs 15.5%), bone fractures (11% vs 5%) and joint stiffness (23.2% vs 17.8%) associated with RT. Traditionally, RT plans are optimised to achieve maximum target coverage whilst avoiding high doses to weight-bearing bones and intermediate doses to the normal tissue (NT) limb corridor. Novel NT outlining guidelines and atlas were developed on the assumption that using these for RT planning may reduce RT toxicity. This study examines the interobserver variations in the lower limb NT outlining following the development of a novel approach.

Two STSE cases were selected as training cases. Case 1, myxoidfibrosarcoma of the thigh and knee, and case 2 a malignant peripheral nerve sheath tumour of the thigh. Target volumes were previously outlined as part of their treatment courses. A set of reference volumes was outlined on each case by one RTT and peer-reviewed by a consultant radiation oncologist, specializing in STSE.  NTs were then outlined by 3 additional observers (1 RTT and 2 clinicians) following the novel guidelines and atlas and re-outlined by the reference volume expert to test intra-observer variability. The novel NT for STSE completed as part of this work were the muscle compartments of the thigh (MCT), the neurovascular bundle (NVB), joints and bones (figure 1). Dice coefficients (DICE) were calculated for individual NT volumes against the gold standard. The Kruskal-Wallis test was performed to detect statistically significant differences in DICE between NT volumes.

Figure 2 shows a boxplot for case 1 DICE results. There was a good agreement for the anterior, posterior, gluteal and adductor MCT, femur, femoral NVB and knee joint showing DICE closer to 1. Significant statistical differences have been highlighted in the deep thigh and sciatic NVB and the lateral rotator MCT(p=0.008). Case 2 gave similar results for the same structures(p=0.08). Although there were differences in the DICE scores between RTTs and clinicians, these were not statistically significant(p=0.1). The intra-observer test was 6 months apart and showed no difference results in DICE.

Our results identify that the novel NT outlining for STSE are reproducible between the observers and in a multi-professional environment, RTT and clinician scores are consistent. This work has also identified areas of refinement and improvement required to improve the guidelines, particularly for the deep thigh NVB and lateral rotator MCT. These guidelines are currently under revision and a secondary assessment will be conducted to test for improvements.

The first author is clinical doctoral research fellow CDR-2018-04-ST2-004, funded by HEE/ NIHR.