Evaluation of axillary dose coverage following whole breast radiotherapy: Variation with the breast volume and shape

Even though this is not the first investigation addressing axillary dose coverage in patients submitted to tridimensional whole breast radiotherapy, this particular study aimed at evaluating the impact of the breast volume and shape in the dose reaching all three axillary levels, in early stage breast cancer patients.

Total breast irradiation with 50Gy in 2Gy daily fractions using 2 tangent fields is currently the standard adjuvant treatment option. There is a major conviction, so far still awaiting scientific validation, that this technique provides therapeutic doses to the three axillary levels, particularly among patients with voluminous and pendulous breasts.

This group of investigators aimed at clarifying this pre-assumed concept, by estimating dose levels reaching the axilla within different breast morphotypes. By creating a method based in Computed Tomography images in three planes, to quantify the breast volume and shape in the supine position, it was possible to report on the axillary dose variation, in different settings of patients.

Despite observing higher dose levels with the higher breast volumes and most pendulous breasts to the point where near-therapeutic doses were documented, the results showed no curative dose levels to the axilla. This way, this investigation confirmed the need to individualize axillary treatment whenever there is clinical indication, even in patients with voluminous and pendulous breasts. It also revealed the possibility that lower doses, regarding current treatment setting, might be effective to  treat undetected or micrometastatic disease in the axilla.

Authors and Affiliations:

Artur Aguiar ^a, Helena Gomes Pereira ^a, Isabel Azevedo ^a, Luciano Gomes ^b

^a Serviço de Radioterapia Externa e ^b Serviço de Física Médica do Instituto Português de Oncologia do Porto Francisco Gentil, E.P.E.

Abstract:

Objective: To evaluate the axillary dose coverage in patients treated with tridimensional whole breast radiotherapy (3D-WBRT), according to the breast volume and shape in treatment position.

Background: Several studies have demonstrated an insufficient dose contribution to the axillary levels, using 3D-WBRT, remaining unclear whether the breast volume and shape can influence it.

Materials and methods: We retrospectively delineated the axillary levels on planning CT-images of 100 patients, treated with 3D-WBRT along 2012 in our institution. To estimate the shape we established an anatomic CT-based interval, defined as the Thoracic Extent (TE). The breast volume matched its CTV. Mean dose levels and V95 (volume receiving at least 95% of the prescribed dose) were evaluated.

Results: Mean axillary level I (A1), II (A2) and III (A3) volume was 56.1 cc, 16.5 cc and 18.9 cc, respectively, and mean doses were 43.9 Gy, 38.6 Gy and 19.5 Gy. For breast volumes of <800 cc, 800–999 cc, 1000–1199 cc and >1200 cc, mean A1 V95 was 38%, 51%, 61.2% and 57.2% whereas median A2 V95 was 8.3%, 13.4%, 19.4% and 28% respectively. Regarding shape, where the breast relative position to the TE was categorized in intervals between 31% and 40%, 41% and 50%, 51% and 60%, and 61% and 70%, mean A1 V95 was 38.7%, 43.1%, 51.1% and 77.3% whereas mean A2 V95 was 6.1%, 11.2%, 17.1% and 37% respectively.

Conclusions: We observed inadequate dose coverage to all axillary levels, even after applying a sub-analysis accounting for different breast volumes and shapes. Although higher doses were associated with the more voluminous and pendulous breasts, axillary coverage with 3D-WBRT seems to be inefficient, regardless of the breast morphology.

Journal: Radiotherapy and Oncology

Link: http://www.thegreenjournal.com/article/S0167-8140%2814%2900413-7/abstract