Prone versus supine positioning for whole and partial-breast radiotherapy: a comparison of non-target tissue dosimetry.

PURPOSE To compare non-target tissue (including left-anterior-descending coronary-artery (LAD)) dosimetry of prone versus supine whole (WBI) and partial-breast irradiation (PBI). METHODS AND MATERIALS Sixty-five post-lumpectomy breast cancer patients underwent CT-imaging supine and prone. On each dataset, the whole-breast clinical-target-volume (WB-CTV), partial-breast CTV (tumour-bed + 15 mm), ipsilateral-lung and chest-wall were outlined. Heart and LAD were outlined in left-sided cases (n=30). Tangential-field WBI and PBI plans were generated for each position. Mean LAD, heart, and ipsilateral-lung doses (x(mean)), maximum LAD (LAD(max)) doses, and the volume of chest-wall receiving 50 Gy (V(50Gy)) were compared. RESULTS Two-hundred and sixty plans were generated. Prone positioning reduced heart and LAD doses in 19/30 WBI cases (median reduction in LAD(mean)=6.2 Gy) and 7/30 PBI cases (median reduction in LAD(max)=29.3 Gy) (no difference in 4/30 cases). However, prone positioning increased cardiac doses in 8/30 WBI (median increase in LAD(mean)=9.5 Gy) and 19/30 PBI cases (median increase in LAD(max)=22.9 Gy) (no difference in 3/30 cases). WB-CTV>1000cm(3) was associated with improved cardiac dosimetry in the prone position for WBI (p=0.04) and PBI (p=0.04). Prone positioning reduced ipsilateral-lung(mean) in 65/65 WBI and 61/65 PBI cases, and chest-wall V(50Gy) in all WBI cases. PBI reduced normal-tissue doses compared to WBI in all cases, regardless of the treatment position. CONCLUSIONS In the context of tangential-field WBI and PBI, prone positioning is likely to benefit left-breast-affected women of larger breast volume, but to be detrimental in left-breast-affected women of smaller breast volume. Right-breast-affected women are likely to benefit from prone positioning regardless of breast volume.