Does the presence of an implant including expander with internal port alter radiation dose? An ex vivo model.

Breast cancer is the most common cancer diagnosed among Canadian women, with 23,200 new cases reported in 2010 alone (1). Most of these women will undergo breast surgery as part of their treatment. Although breast-conserving surgery is considered to be the standard of care for most early breast cancers, a mastectomy is indicated in a variety of situations: where there is evidence of multifocal disease; where the cosmetic result of breast conserving surgery is expected to be suboptimal; where the primary tumour is large; and where there is evidence of locally advanced disease (2,3). Increasingly, women facing mastectomy are being offered immediate breast reconstruction. The surgeon may use a variety of techniques that either involve the patient’s own tissue, or involve placement of a saline or silicone implant on the chest wall with or without tissue expansion. In women for whom breast reconstruction is important, immediate reconstruction eliminates the need for a separate surgery. Unfortunately, information regarding the complete and final pathological diagnosis and staging of the cancer is not available to the surgeon or patient at the time of immediate reconstruction. As a result, patients undergoing this procedure may learn postoperatively that chest wall radiotherapy is recommended for further treatment of their cancer. With the publication of several positive randomized trials assessing the value and toxicity of locoregional radiotherapy (4,5), the indications for postmastectomy chest wall radiation now routinely include tumour size >5 cm, positive surgical resection margins, heavy node positivity, locally advanced breast cancer and recurrent cancer (6,7). In many jurisdictions, these indications have expanded to consider all node-positive breast cancers (7,8). As a result, unresolved issues surrounding the irradiation of the chest wall containing a post-mastectomy implant or expander require urgent attention as the number of patients undergoing this treatment increases. When reconstruction using an implant is planned, a tissue expander is most commonly placed subpectorally at the time of mastectomy. At a later date, it is replaced with a permanent saline or silicone implant. The expander is injected with saline at this time and subsequently at regular intervals to sufficiently expand the skin and soft tissues of the chest wall to accommodate the permanent implant. Tissue expansion can be performed via an external or internal port. The internal port of an expander has a circular metal base 3 cm in diameter that prevents the needle from passing through the port and puncturing the expander or passing into the chest wall. The port contains a rare-earth magnet that is used to localize the port from the surface of the overlying skin for each injection. Therefore, chest wall irradiation in which an implant is used for reconstruction will involve either irradiating the expander with the metallic port and magnet in situ or, if radiotherapy is delayed, irradiating the permanent implant. The presence of the port raises concern about dose pertubations around the port that could contribute to either underdosing with potential for loss of tumour control or overdosing with potential for increasing toxicity, including reconstruction failure. The rate of reconstruction failure increases following irradiation of breast implants, usually resulting from the formation of a fibrous capsule around the implant causing capsular contracture. Failure can also be caused by increases in infection, implant exposure and extrusion due to delayed wound healing. Whether this is partly a result of increased dose to the tissue adjacent to the implant is not clear and has not been studied. The purpose of the present study was to determine whether the radiation dose to the chest wall in the setting of postmastectomy radiotherapy is altered by the presence of a tissue expander or by a saline or silicone implant. Ultimately, we developed an ex vivo model to determine whether it is safe to irradiate a patient’s chest wall that contains an expander or implant.