Differences between current and historical breast cancer axillary lymph node irradiation based on arm position: implications for radiation oncologists.

PURPOSE: To identify differences in regional node irradiation using historical treatment planning techniques between 2 arm positions.

MATERIALS AND METHODS: Sixteen breast cancer patients were scanned using a wide-bore computed tomography (CT) scanner. The patients were scanned in 2 arm positions: historical position (HP), in which the ipsilateral arm is at 90 degrees to the body axis; and standard-bore position (CT-P), in which the arms are above the head. The locations of the axillary lymph nodes were compared between the 2 positions. The dose distribution to the axillary lymph nodes was compared between the HP and the CT-P using fields designed based on bony landmarks.

RESULTS: When the arm position changed from the HP to the CT-P, level I lymph nodes moved anteriorly and medially. Level II and III axillary nodes moved posteriorly and medially. If historical treatment planning techniques are used to treat the axillary lymph nodes with the patient in the CT-P, level I nodes could receive a higher dose of radiation and levels II and III could be significantly underdosed as compared with treatment in the HP. The dose distribution for the CT-P was more homogeneous compared with that of the HP.

CONCLUSION: Coverage of the axillary lymph nodes varies significantly with arm position when using historical treatment planning techniques. Physicians should accurately contour the lymph node levels on the treatment planning CT and not rely on bony landmarks to design the axillary fields. CT-based treatment planning should be used to ensure adequate coverage of these nodes.