Inter-clinician delineation variation for a new highly-conformal flank target volume in children with renal tumors: A SIOP-Renal Tumor Study Group international multicenter exercise

•Recently, highly-conformal target volumes for flank delineation were defined.•Delineation variation of this target volume was assessed in an international setting.•Ten radiation oncologist delineated the GTV and CTV of six individual cases.•‘Unacceptable’ delineation variation was found in a large number of participants.•This indicates the need for central target volume review before radiotherapy onset. Recently, the SIOP-RTSG developed a highly-conformal flank target volume definition for children with renal tumors. The aims of this study were to evaluate the inter-clinician delineation variation of this new target volume definition in an international multicenter setting and to explore the necessity of quality assurance. Six pediatric renal cancer cases were transferred to ten radiation oncologists from seven European countries (‘participants’). These participants delineated the pre- and postoperative Gross Tumor Volume (GTVpre/post), and Clinical Target Volume (CTV) during two test phases (case 1–2 and 3–4), followed by guideline refinement and a quality assurance phase (case 5–6). Reference target volumes (TVref) were established by three experienced radiation oncologists. The Dice Similarity Coefficient between the reference and participants (DSCref/part) was calculated per case. Delineations of case 5–6 were graded by four independent reviewers as ‘per protocol’ (0–4 mm), ‘minor deviation’ (5–9 mm) or ‘major deviation’ (≥10 mm) from the delineation guideline using 18 standardized criteria. Also, a major deviation resulting in underestimation of the CTVref was regarded as an unacceptable variation. A total of 57/60 delineation sets were completed. The median DSCref/part for the CTV was 0.55 without improvement after sequential cases (case 3–4 vs. case 5–6: p = 0.15). For case 5–6, a major deviation was found for 5/18, 12/17, 18/18 and 4/9 collected delineations of the GTVpre, GTVpost, CTV-T and CTV-N, respectively. An unacceptable variation from the CTVref was found for 7/9 participants for case 5 and 6/9 participants for case 6. This international multicenter delineation exercise demonstrates that the new consensus for highly-conformal postoperative flank target volume delineation leads to geometrical variation among participants. Moreover, standardized review showed an unacceptable delineation variation in the majority of the participants. These findings strongly suggest the need for additional training and centralized pre-treatment review when thi