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Original Research |
1 All authors: Department of Radiology, Breast Imaging Section, Memorial Sloan-Kettering Cancer Center, H118, 1275 York Ave., New York, NY 10021.
Received November 16, 2004;
accepted after revision February 1, 2005.
Address correspondence to L. Bartella
(bartelll{at}mskcc.org).
Abstract
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MATERIALS AND METHODS. A retrospective review was undertaken of 1,336 breast MRI examinations performed during a 2-year period. Among these, 68 nonpalpable mammographically occult invasive cancers were identified in 57 women with a median age of 50 years (range, 30–72 years). MRI findings were classified according to the breast MRI lexicon. Medical records were reviewed to determine the histology.
RESULTS. Indications for performing MRI were extent of disease assessment in 72% (41/57), high-risk screening in 25% (14/57), and problem solving in 3% (2/57). MRI lesion types in these 68 invasive cancers were nonmass in 57% (39/68) and mass in 43% (29/68). Kinetics were plateau in 59% (40/68), washout in 38% (26/68), and persistent in 3% (2/68). Histology was invasive ductal cancer in 65% (44/68), mixed invasive ductal and lobular cancer in 19% (13/68), and invasive lobular cancer in 16% (11/68). The cancer stage was I in 61% (34/56), II in 32% (18/56), and more advanced in 7% (4/56). Sixty-three percent (43/68) of lesions were minimal cancers, defined as invasive cancers measuring under 1 cm.
CONCLUSION. In this study of mammographically and clinically occult cancers detected by MRI, 57% (39/68) of invasive breast cancers were evident as nonmass enhancement, and 63% were minimal breast cancers.
Keywords: breast cancer • MRI
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Sixty-eight nonpalpable mammographically occult invasive breast cancers were identified in 57 women. The median age was 50 years (range, 30–72 years). The indication for breast MRI was assessment of disease extent in women with known biopsy-proven ipsilateral breast cancer in 72% (41/57), screening of high-risk women in 25% (14/57), and problem solving in 3% (2/57). In the 41 women with biopsy-proven cancer, the primary known breast cancer was not examined in this study. The protocol for this review was approved by our institutional review board.
Directed sonography using the Acuson Sequoia 512 and an 8–15 MHz probe was performed in 24 patients, and a sonographic correlate was identified in five. In the remaining 33 women, directed sonography was not performed at the discretion of the interpreting radiologist and treating clinician. The mammographic parenchymal density [5] was class 4 (dense) in 15 patients, class 3 (heterogeneously dense) in 32, class 2 (scattered fibroglandular densities) in nine, and class 1 (fatty) in one. The average interval between the mammography and breast MRI was 6 weeks (range, 0–24 weeks).
Histologic sampling of these lesions was performed using MRI-guided needle localization and surgical excision in 46% (26/57) of patients, mastectomy in 26% (15/57), lumpectomy or reexcision in 23% (13/57), and sonogram-guided biopsy in 5% (3/57).
Breast MRI Technique and Interpretation
Diagnostic breast MRI examinations were performed with the patient prone in
a 1.5-T commercially available system (Signa, Lightening, or Excite; GE
Healthcare) using a dedicated surface breast coil. Our imaging protocol
includes a localizing sequence followed by a sagittal fat-suppressed
T2-weighted sequence (TR/TE, 4,000/85). A T1-weighted 3D fat-suppressed fast
spoiled gradient-echo sequence (17/2.4; flip angle, 35°; bandwidth, 31.25
MHz) was then performed before and three times after a rapid bolus injection
of 0.1 mmol/L of gadopentetate dimeglumine (Magnevist, Berlex Laboratories)
per kilogram of body weight and delivered through an in-dwelling IV
catheter.
Image acquisition started after injection of contrast material and saline bolus. Images were obtained sagittally for an acquisition time per volumetric acquisition of less than 3 min each. Total imaging time per breast, including three contrast-enhanced acquisitions, was approximately 20 min. Section thickness was between 2 and 3 mm without a gap using a matrix of 256 x 192 and a field of view of 18–22 cm. Frequency was in the anteroposterior direction. After the examination, the unenhanced images were subtracted from the first contrast-enhanced images on a pixel-by-pixel basis.
Breast MRI examinations were interpreted by breast imaging specialists in conjunction with clinical history and other breast imaging studies, including mammograms and sonograms when available.
Data Collection and Analysis
The institution at which the study was performed is an academic center
where more than 600 breast MRI examinations, 16,000 mammograms, and 1,000 new
breast cancer cases were evaluated annually during the study period.
Mammograms were reviewed by one of nine attendings specializing in breast
radiology. Diagnostic breast MRI for 68 consecutive, mammographically occult
nonpalpable invasive breast cancers were reviewed by one radiologist with 4
years of experience as a breast imaging specialist.
T2-weighted, T1-weighted unenhanced, and T1-weighted images obtained within the first 2 min after IV contrast injection in all cases were reviewed on the PACSTM workstation (Centricity PACS, GE Healthcare) for review by the radiologist. The radiologist could page back and forth through sequential slices and adjust the window and level settings at the workstation.
Data recorded included lesion size, signal intensity on T2-weighted images (hyperintense vs not hyperintense), morphologic features, and final assessment categories according to the Breast BI-RADS MRI Lexicon [5]. Visual analyses of the time course of enhancement in these three enhanced sequences were performed and categorized as "persistent" (increasing signal intensity throughout the dynamic period), "plateau" (stabilized enhancement without change in signal intensity between the initial and subsequent enhanced images), or "washout" (abrupt decline in signal intensity after the initial enhanced images) [6–9].
After these data were recorded, histologic findings were reviewed and correlated with MRI interpretations. Medical records were reviewed to determine breast cancer stage and treatment. Minimally invasive breast cancers were defined as invasive breast cancers smaller than 1 cm [10]. Data were entered into a computerized spreadsheet (Excel, Microsoft).
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Nonmass Enhancement
MRI features of invasive cancer in 39 nonmass lesions are shown in
Table 3. The most common MRI
features of these lesions were focal clumped enhancement in 35% (14/39),
plateau kinetics in 67% (26/39), no T2 signal in 65% (25/39), and no T1 signal
in 82% (32/39).
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Sonographic Correlation
Sonographic correlates were found in only five of 24 lesions that were
scanned. In this small group, there were two invasive ductal carcinomas, two
invasive lobular carcinomas, and one invasive mammary cancer. Four lesions
were masses and one was a nonmass lesion on MRI. MRI lesions ranged in size
from 1.2–2.9 cm. Histologic lesion size, known in three of the five
lesions, ranged from 0.7–2.5 cm.
Histology Subtypes and MRI Characteristics
MRI lesion features as a function of invasive cancer histology are shown in
Table 4. Fifty-seven percent
(25/44) of invasive ductal and 73% (8/11) of invasive lobular carcinomas were
evident as nonmass lesions; 54% (7/13) of invasive carcinomas with mixed
ductal and lobular features were evident as masses. No T2 signal was present
in 61% (27/44) of invasive ductal carcinomas, 73% (8/11) of invasive lobular
carcinomas, and 69% (9/13) of mixed invasive ductal and lobular cancer.
Forty-five of 68 invasive cancers had associated DCIS. Of these, 44% (20/45)
were masses and 56% (25/45) were nonmass lesions. Twenty-two percent (10/45)
had a high T2 signal, 13% (6/45) had an intermediate T2 signal, and 65%
(29/45) had no associated T2 signal.
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The MRI lesion features did not differ as a function of the indication for the initial MRI scan.
Staging
Staging information was available for 56 of 57 patients. Thirty-four (61%)
women had stage I disease and 18 (32%) had stage II disease. Four (7%)
patients had more advanced disease; one had bone metastases and three had
metastatic skin nodules at the time of diagnosis. Eighty-two percent (47/57)
of the women had a known biopsy-proven breast cancer at the time of the
MRI-detected disease. In 11% (6/57), the known cancer was in the contralateral
breast. In 47% (32/68) of the lesions, pathology yielded multifocal invasive
malignancy.
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Most invasive cancers in our study were evident on MRI as nonmass (57%) rather than mass (43%) lesions and had plateau (59%) versus washout (38%) kinetics (Figs. 5A and 5B). Among masses, the most common enhancement pattern was heterogeneous (59%) rather than rim (14%) enhancement. A substantial percentage (24%) of these invasive cancers had high T2 signal (Fig. 6), contrary to the findings of Kuhl et al. [12], who reported that high T2 signal is a sign of benign disease; an additional 12% had an intermediate signal on T2 imaging. It is important to know that invasive cancers may have high or intermediate T2 signal, so as not to dismiss such lesions as benign.
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Our findings show that a high percentage of invasive lobular cancers (73%) manifested as nonmass rather than mass lesions; this finding is concordant with a previous report from Nunes et al. [14]. Otherwise, the results from this study do not show a definite correlation of the MRI characteristics of invasive breast cancers with the histology subtypes, although the number of lesions in specific subgroups is relatively small. Further work with larger numbers of invasive cancers with various histologies is needed for detailed subgroup analysis.
The relative lack of sonographic correlates for these invasive cancers is an important finding. Only 21% of the lesions scanned in our series had a sonographic correlate. Few published data address the frequency of sonographic correlates specifically for invasive breast cancers. In previous reports, the frequency of sonographic correlates for MRI-detected lesions warranting biopsy has ranged from 23–100% [15–17]. LaTrenta et al. [17] found that in the cancers with a sonographic correlate, 78% were invasive and 22% were DCIS; in the cancers without a sonographic correlate, 50% were invasive and 50% were DCIS. The absence of a sonographic correlate for MRI-detected invasive breast cancers in our series stresses the need for facilities performing breast MRI to have the capacity for MRI-guided localization and biopsy.
Our study has several limitations. This is a retrospective study. Eighty-two percent of the patients had a concurrent biopsy-proven cancer and had undergone breast MRI for extent of disease assessment. These MRI-detected cancers were nonpalpable and mammographically occult, but 83% of the women had dense or heterogeneously dense breasts on mammography (classes 3 and 4). Because the study included only invasive cancers, the data address the prevalence of specific MRI features in invasive cancers but do not address the positive predictive value of different features. Issues of cost effectiveness, interobserver variability, and long-term outcome are not addressed.
In conclusion, among our patients, most mammographically occult, nonpalpable invasive breast cancers identified on MRI were nonmass lesions (57%) and were minimal breast cancers (63%). A high T2 signal, lack of a sonographic correlate, lack of T1 precontrast signal, and heterogeneous, plateau enhancement are features that should not be dismissed when encountered. Use of BI-RADS lexicon descriptors should facilitate comparison of our results with future studies. Further work is necessary to evaluate the positive predictive value of MRI features, to determine if MRI features are predictive of cancer type or histology, and to assess the long-term outcome in women with breast cancers detected by MRI.
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