A novel technique for post-mastectomy breast irradiation utilising non-coplanar intensity-modulated radiation therapy

Abstract

The aim of this study was to investigate if non-coplanar intensity-modulated radiation therapy (IMRT) in the post-mastectomy setting can reduce the dose to normal structures and improve target coverage. We compared this IMRT technique with a standard partial wide tangential (PWT) plan and a five-field (5F) photon-electron plan. 10 patients who underwent left-sided mastectomy were planned to 50.4 Gy using either (1) PWT to cover the internal mammary (IM) nodes and supraclavicular fields, (2) 5F comprising standard tangents, supraclavicular fields and an electron field for the IM nodes or (3) IMRT. The planning target volume (PTV) included the left chest wall, supraclavicular, axillary and IM lymph nodes. No beams were directed at the right lung, right breast or heart. Mean dose-volume histograms were constructed by combining the dose-volume histogram data from all 10 patients. The mean PTV to receive 95% of the dose (V95%) was improved with the IMRT plan to 94.2% from 91.4% (p = 0.04) with the PWT plan and from 87.7% (p = 0.012) with the 5F plan. The mean V110% of the PTV was improved to 3.6% for the IMRT plan from 16.8% (p = 0.038) for the PWT plan and from 51.8% (p = 0.001) for the 5F plan. The mean fraction volume receiving 30 Gy (v30Gy) of the heart was improved with the IMRT plan to 2.3% from 7.5% (p = 0.01) for the PWT plan and 4.9% (p = 0.02) for the 5F plan. In conclusion, non-coplanar IMRT results in improved coverage of the PTV and a lower heart dose when compared with a 5F or PWT plan.

Figure 1

Non-coplanar intensity-modulated radiation therapy beam arrangement as defined according to IEC 61217 (Gantry Angle, Table Kick) [34].

Figure 2

Depiction of partially wide tangent field arrangement. (a) Axial CT slice: red, medial tangent; blue, lateral tangent. (b) Three-dimensional depiction: red, surface path of medial and lateral tangents; yellow, surface path of anteroposterior (AP) and posteroanterior (PA) supraclavicular beam.

Figure 3

Depiction of five-field arrangement. (a) Axial CT slice: red, medial tangent; orange, lateral tangent; green, angled IM (internal mammary) electron beam. (b) Three-dimensional depiction: red, surface path of medial and lateral tangents; yellow, surface path of anteroposterior (AP) and posteroanterior (PA) supraclavicular beam; green, surface path of angled IM electron beam.

Figure 4

Isodose distribution of patient number 8. (a) Partially wide tangent (PWT), (b) five-field (5F) and (c) non-coplanar intensity-modulated radiation therapy (IMRT) in axial plane. Isodose lines are as follows: 100%–5040 cGy (red), 95%–4788 cGy (green), 80%–4032 cGy (yellow), 60%–3024 cGy (purple), 50%–2520 cGy (light blue). PTV shaded purple and bolus shaded green.

Figure 5

Isodose distribution of patient number 8. (a) Partially wide tangent (PWT), (b) five-field (5F) and (c) non-coplanar intensity-modulated radiation therapy (IMRT) in sagittal plane. Isodose lines are as follows: 100%–5040 cGy (red), 95%–4788 cGy (green), 80%–4032 cGy (yellow), 60%–3024 cGy (purple), 50%–2520 cGy (light blue). PTV shaded purple and bolus shaded green.

Figure 6

Combined dose–volume histograms (n = 10 plans) for partially wide tangents (PWT), five-field and non-coplanar intensity-modulated radiation therapy (IMRT) for (a) planning tumour volume (PTV), (b) left lung, (c) heart, (d) right breast and (e) right lung.