Polygon method: A systematic margin assessment for breast conservation

Abstract

Background: Radiation therapy (RT) for women with ductal carcinoma in situ (DCIS) undergoing breast-conserving surgery (BCS) may be overtreatment for some, especially for those in which DCIS is eradicated, and ipsilateral breast tumor recurrence (IBTR) risk approaches the contralateral breast cancer (CBC) level. The aim of this study was to clarify whether the polygon method, a new systematic method of en face (tangential, shaved) margin assessment, can identify a subset of DCIS that can be safely treated by BCS alone.

Methods: A key tool of the polygon method is an adjustable mold that prevents the "pancake phenomenon" (flattening) of breast tissue after surgical removal so that the specimen is fixed in the shape of a polygonal prism. This preanalytical procedure enables us to command a panoramic view of entire en face margins 3-5-mm deep from the real peripheral cut surfaces. Competing risk analysis was used to quantify rates of IBTR and CBC and to evaluate risk factors.

Results: From 2000 to 2013, we identified 146 DCIS patients undergoing BCS with a contralateral breast at risk. In 100 DCIS patients whose margin was negative by the polygon method, 5 IBTR (3 DCIS and 2 invasive ductal carcinoma [IDC]) and 10 CBC (6 DCIS and 4 IDC) cases were identified during a median follow-up of 7.6 years (range, 0.9-17.4). Five- and 10-year cumulative incidence rates were 3.0% and 5.3% for IBTR, and 7.1% and 13.3% for CBC, respectively. Thus, patients with a negative margin consistently showed at least twofold lower IBTR than CBC despite omission of RT.

Conclusions: Japanese women classified with a negative margin by the polygon method show a very low risk of IBTR and account for approximately half of CBC cases. In this subset of DCIS patients, additional RT is not beneficial.

Keywords: contralateral breast cancer; ipsilateral local recurrence; new primary; pancake phenomenon; true recurrence.

Figure 1

An adjustable mold to maintain the polygonal prism shape of the specimen. The mold is composed of rectangular‐shaped plates that are hinged together to change the angle between adjacent plates to form a polygonal prism for storage of excised tissue. The plates are made of punching metal to enable fixative formalin to pass through them and the size of each plate was calculated to fit the cassette for histological specimen preparation. (For inquiries concerning an adjustable mold (polygon mold®), please refer to PLM Co., Ltd. Toyoyama‐cho, Aichi, Japan (http://plm‐co.jp/WP))

Figure 2

Flow chart illustrating the polygon method. To check the entire interphase between the resected and preserved breast tissue in wide local excision, we developed a precision margin assessment (the polygon method) incorporating the peripheral sectioning method originally designed for cutaneous neoplasms. The main tool for this method is an adjustable mold made of punching metal to prevent the pancake effect (flattening) of the specimen after surgical resection. After fixation in the adjustable mold, the specimen is in the shape of a polygonal prism. En face margins are cut to approximately 3‐4 mm in thickness. Histological sections are taken from the inner surface of the margins and observed by microscopy

Figure 3

Flow chart. Using the Breast Cancer Registry prospectively maintained by Nagoya Medical Center, we retrieved 146 patients of DCIS (Stage 0) for analysis in this study. According to the polygon method, the margin status was negative in 100 cases and positive in 46 cases. Of the margin‐negative group, 5 developed IBTR (3 DCIS and 2 IDC) and 10 developed CBC (6 DCIS and 4 IDC) during a median follow‐up period of 7.6 y (range, 0.9‐17.4). CBC, contralateral breast cancer; DCIS, ductal carcinoma in situ; IBTR, ipsilateral breast tumor recurrence; IDC, invasive ductal carcinoma

Figure 4

Examples of wide local excision specimens evaluated by the polygon method. In DCIS with positive margins, the distribution of DCIS (shown on the map as red bars) are well correlated with the positive sites (arrows) at the peripheral en face margins. 1. Low‐grade DCIS with a negative polygon margin. The polygon margin was negative and bread loaf slices also show a unifocal DCIS in the specimen. The patient developed invasive ipsilateral recurrence 8.3 y after surgery. 2. High‐grade DCIS with a negative polygon margin. The patient experienced ipsilateral DCIS one year after surgery. 3. Multifocal low‐grade DCIS with a negative polygon margin. The patient developed squamous carcinoma 0.8 y after surgery. 4. High‐grade DCIS with a positive margin in one block. The woman developed ipsilateral invasive recurrence 3.3 y after surgery. 5. High‐grade DCIS with a positive polygon margin at 2 blocks. The patient experienced ipsilateral in situ recurrence 2.1 y after surgery. 6. Intermediate‐grade DCIS with a positive polygon margin at 2 blocks. The woman had ipsilateral in situ recurrence 8 years after surgery. 7. High‐grade DCIS with a positive polygon margin in 2 blocks. This is the only case in this study in which the positive site was not the en face margin but the deep (pectoral muscle) margin. The patient developed ipsilateral in situ recurrence 3.2 y after surgery. DCIS, ductal carcinoma in situ

Figure 5

Cumulative incidence of ipsilateral and contralateral carcinoma. Competing risk analysis of 100 DCIS patients with negative margins demonstrated 5‐ and 10‐year IBTR rates of 3.0% and 5.3%, respectively, compared with 7.1% and 13.3%, respectively, for CBC. CBC, contralateral breast cancer; DCIS, ductal carcinoma in situ; IBTR, ipsilateral breast tumor recurrence