Estimating the value of surgical clips for target volume delineation in external beam partial breast radiotherapy

Aims: To investigate the role of surgical clips in defining the clinical target volume (CTV) for three-dimensional conformal external beam radiotherapy-partial breast irradiation (3D-CRT-PBI) using preoperative computed tomography scans. Materials and methods: A group of patients with early breast cancer underwent conservative surgery with placement of surgical titanium clips (at least three clips required). All patients had a treatment planning computed tomography simulation before (CT1) and after surgery (CT2). The two sets of images were co-registered with a match point registration. The relationship between the clips-based CTV for PBI delineated on CT2 and the initial tumour location on CT1 was studied, evaluating the percentage of intersection volume. Results: Twenty-eight patients participated in this study. In total, 13 patients (46.4%) had an intersection volume ≥ 50% and 10 patients (35.7%) had complete intersection (intersection volume = 100%). An increased median intersection volume was observed in patients with more than six clips (. P = 0.007) and in patients with a larger portion of breast volume covered by the PBI-CTV (CTV/BV; P = 0.010). Intersection volume increased with the number of clips, after adjustment for CTV/BV (linear coefficient = 5.1693; P = 0.043). Also, a maximum distance from the chest wall ≤0.7 cm and CTV/BV > 9.5% were found to be predictors of an intersection volume ≥50% (area under the curve 0.841; confidence interval 0.649-0.952; P < 0.0001; area under the curve 0.800; confidence interval 0.607-0.926; P = 0.0004) and of an intersection volume of 100% (area under the curve 0.776, confidence interval 0.573-0.916, P = 0.046; area under the curve 0.752, confidence interval 0.536-0.935; P = 0.032). Conclusions: Titanium clips are essential and six or more increase the accuracy of tumour bed delineation for PBI; also the primary tumour location as well as the percentage of volume of breast covered by PBI-CTV may influence the correct delineation of PBI-CTV.