Inaccuracies in using the lumpectomy scar for planning electron boosts in primary breast carcinoma

To determine the accuracy of using the lumpectomy scar, specifically the midpoint or center of the scar, to define the tumor bed in the electron beam boost for the treatment of early stage breast carcinoma.

Electron boost simulation films from 316 cases of early breast carcinoma treated with lumpectomy and radiotherapy were reviewed. For each case which had surgically placed lumpectomy bed clips (N = 316), four clinical set-up methods ("hypothetical fields") of several field sizes were compared to the actual location of the tumor bed (as defined by the surgical clips). Each method was based on using the center of the scar as the center of the field and is described as follows: Method 1 uses a standard circular cone of a given diameter, method 2 also uses circular cones, but the diameter is based on the scar length; method 3 uses an oval field in which a constant margin is kept around the scar; method 4 results in an oblong field in which a 2 cm margin is placed on the lateral edge of the scar, but a larger margin around the center of the scar. The adequacy of each of these popular clinical set-up techniques was then analyzed for the population as a whole. "Inadequate" coverage was defined as any portion of the field edge coming within 1 cm of at least one surgical clip.

(1) Method 1: Inadequate coverage was found in 43%, 26%, and 17% of cases, using 7, 8, and 9 cm cones, respectively. (2) Method 2: Inadequate coverage was found in 88%, 61%, 36% and 20% of cases, with field size = scar length + 0, 2, 3, and 4 cm, respectively. (3) Method 3: Inadequate coverage was found in 34%, 17%, and 10% of cases, using 3, 3.5, and 4 cm margins, respectively. (4) Method 4: Inadequate coverage was found in 36% and 24% of cases using 3.5 and 4 cm margins around the scar center, respectively. Inadequate coverage was found in 51% and 42% of cases using margins equal to one-half the scar length or one-half the scar length + 1 cm, respectively.

We conclude that the lumpectomy scar is often a poor indicator of the location of the underlying tumor bed as defined by surgical clips. We recommend the use of clip placement and simulation of the electron boost to maximize target definition.