External validation of the lumbosacral plexus-contouring protocol developed by Yi et al. (IJROBP 2012; 84: 376-82) for pelvic malignancies.

PurposeTo evaluate interobserver variability in contouring lumbosacral plexuses (LSP) using the protocol developed by Yi etal. (IJROBP 2012; 84: 376-82) and to review LSP dosimetries for conventional radiotherapy and intensity-modulated radiotherapy (IMRT) for pelvic cancers. Methods and materialsUsing the above-mentioned protocol, seven outliners independently contoured LSPs of 10 consecutive patients (five patients treated with conventional radiotherapy and five with IMRT). Interobserver variability was reviewed visually by using planning axial CT images and anteroposterior digitally reconstructed radiographs. Dosimetries of LSPs were also calculated and compared. ResultsThere was a notable learning curve for each outliner; duration to outline the first patient was 45-185 minutes, versus 15-50 minutes after six patients. We found significant interobserver variability among outliners below the level of the S2 nerve roots. The LSP volumes (mean volume range of 40.9-58.4cc) were smaller than those described in the atlas paper (71-138cc). The mean values of mean dose, maximum dose, V40Gy, V50Gy and V55Gy, respectively, for patients treated with conventional radiotherapy versus those treated with IMRT were 35.5Gy versus 33.6Gy, 52.2Gy versus 52.2Gy, 61.3% versus 54.4%, 14.9% versus 18.8% and 0% versus 2.5%. ConclusionWe conclude that the protocol developed by Yi etal. is a useful set of guidelines but suggest that additional at-risk components of the LSP also be contoured. We recommend that radiation oncologists practise nerve-sparing' radiotherapy by contouring LSPs, especially when using IMRT. We propose the term lumbosacral plexus regions' (LSPRs) to highlight the fact that LSPs are not always radiologically visible, only the regions where they are likely to be present.