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INTRODUCTION
Accurate pathologic staging is important for improving the long-term survival in patients with colorectal cancer because administration of adjuvant chemotherapy can improve the survival [1]. However, the benefit of adjuvant chemotherapy in patients with node-negative disease remains controversial [2,3], despite loco-regional recurrence and distant metastases in more than 30% of them. These data raise the question whether the current conventional nodal staging, which examines only one section of the lymph nodes by hematoxylin and eosin (H&E) staining, might be associated with a substantial rate of understaging.
It is recommended that a minimum of 12 lymph nodes be examined before the patient is considered free of lymph node metastases [4]. Actually, other reports recommended that more lymph nodes should be examined to ensure proper staging [5,6]. However, it is time-consuming and impractical to obtain more lymph nodes. Therefore, focusing on the lymph nodes that represent the nodal status is a more effective and reliable strateguy.
The sentinel lymph node (SLN) is defined as the first lymph node to receive lymphatic drainage from a primary tumor [7]. Therefore, the SLN is most likely to contain metastases and to have the potential to improve lymph node staging accuracy by thorough examination of the SLN using serial sectioning and immunohistochemical (IHC) staining. However, prognostic impact of micrometastasis detected by the IHC technique remains undetermined [8,9,10].
Previous studies investigating the feasibility and reliability of the SLN mapping in colorectal cancer have reported SLN detection rates from 85% to 100% [11,12,13], regardless of whether an in vivo or ex vivo technique was performed. There are several reports comparing the ex vivo technique with the in vivo technique in colon cancer surgery [14,15]. However, there are few reports that studied the SLN using serial sectioning [16]. The purposes of this study are to evaluate the detection rate, accuracy, and false negative rate of SLN technique using serial sectioning and to compare the results between the ex vivo and the in vivo technique in colon cancer.
METHODS
Patients
From February 2011 to October 2012, 34 colon cancer patients underwent SLN mapping during surgical resection. Exclusion criteria were synchronous colon carcinoma, recurrent or metastatic colon cancer, previous chemotherapy or radiation therapy, and emergency operation. Surgery was performed either via an open or laparoscopic approach. We planned to perform ex vivo mapping in the beginning and change to in vivo mapping. Exceptionally, ex vivo method was selected in case of difficulty in administering a subserosal injection and a poor medical status during in vivo period.
Eleven patients were analyzed with the in vivo method, and 23 patients with the ex vivo method. Patient characteristics and results of SLN mapping were evaluated. All of the procedures will be performed by one colorectal surgeon. The study was approved by the Institutional Review Board of Ajou University Hospital.
SLN detection processing
In the in vivo technique, 1-2 mL of 1% methylene blue was injected into the subserosa in four quadrants around the tumor after detection of tumor location. Laparoscopically, the colonic segment containing the lesion was carefully moved to the abdominal wall using atraumatic graspers, and a 22-gauge spinal needle was inserted into the abdominal wall and into subserosal layer of the colon in the tangential plane. The blue-stained lymph node was detected and marked with a suture within the first 10 minutes after injection. After completion of the surgical procedure, the marked lymph node was dissected from the mesentery and sent to the surgical pathology department for evaluation. In the ex vivo technique, 1-2 mL of 1% methylene blue was injected into the subserosal layer in four quadrants around the tumor. The injection sites were gently massaged for 2 to 3 minutes. The mesentery was inspected for blue LNs by palpation and visual examination. The identified blue lymph nodes were dissected and sent to the surgical pathology department for evaluation. Blue lymph nodes, partially blue lymph nodes, and lymph nodes with connecting blue vessels were regarded as SLNs.
Pathologic examination
All of the SLNs were cut in serial sections, stained with H&E, and evaluated for tumor involvement. Sentinel nodes were serially sectioned at 0.5-mm intervals and assessed using H&E staining. All of the non-SLNs were bivalved and examined with routine H&E evaluation. The SLN mapping procedure was tested in 10 patients before the study. All SLN mapping procedure were performed by the same surgeon. A positive sentinel node was defined as a blue-stained node containing single cells or cell aggregates demonstrating morphologic features consistent with colon carcinoma apparent on evaluation of H&E.
Detection and accuracy rates of SLN mapping
The identification rate was defined as the number of mapping procedures finding at least one blue node out of the total number of mapping procedures performed. Sensitivity of SLN mapping was defined as the proportion of patients with positive nodes found by routine H&E examination to have positive SLNs. False-negative cases were defined as those where the SLNs were negative but the NSLNs were positive. Skip metastases were defined as metastases in non-SLNs with negative SLNs metastasis. Accuracy of SLN mapping and biopsy was defined as the proportion of patients with successful lymphatic mapping having SLN examination.
Statistics
Pearson chi-square or Fisher exact test was used to assess differences in the clinicopathological features. Continuous data were compared by Student t-test. All statistical tests were two-sided and performed using SPSS ver. 15.0.0 (SPSS Inc., Chicago, IL, USA). A P-value ≤ 0.05 was considered to indicate a significant difference.
RESULTS
Clinicopathological characteristics
The SLN mapping procedure was performed in 34 patients. Mean age was 67.3 years (range, 44-81 years). Of these 34 patients, two had T1 lesions, 4 were classified as T2, 24 were classified as T3, and 4 were as T4. Regional LN involvement was identified in 17 patients (50.0%), and 17 patients (50.0%) were node-negative. Five patients were stage 1, 12 were stage 2, and 17 were stage 3. Primary tumors were located in the following sites: 13 in the right colon (38.2%) and 21 in the left colon (61.8%). Mean tumor size was 5.1 cm for the in vivo technique, and 5.3 cm for the ex vivo technique (Table 1).
Sentinel lymph node mapping
SLN mapping was successful in 90.9 % of the patients with the in vivo technique and 87.0% of the patients with the ex vivo technique (P = 1.000). Mean number of total lymph nodes retrieved was significantly different between two techniques (13.4 ± 6.0 vs. 24.2 ± 13.4, P = 0.003). Mean number of SLNs retrieved was not significantly different between the two techniques (1.2 ± 0.6 vs. 1.2 ± 0.9, P = 0.907). There was no significant difference between the two techniques with respect to sensitivity, negative predictive value, and skip metastasis. For the in vivo technique, the sensitivity was 50.0% (3/6), negative predictive value was 70.0% (7/10), and rate of skip metastasis was 50.0% (3/6). For the ex vivo method, the sensitivity was 55.6% (5/9), negative predictive value was 80.0% (16/20), and rate of skip metastasis was 44.4% (4/9) (Table 2).
DISCUSSION
The nodal analysis in patients with colorectal cancer is a significant determining factor for further oncological treatment and for the prediction of survival. Patients with nodal disease should undergo adjuvant chemotherapy because of recurrence potential. The optimal number of lymph nodes required to accurately predict lymph node negativity has been a point of debate. The Working Party Report to the World Congress of Gastroenterology recommended that a minimum of 12 lymph nodes be examined before the patient is considered free of lymph node metastases [4]. Baxter et al. [17] demonstrated an overall poor compliance rate with the guideline of harvesting more than 12 nodes in colorectal cancers, with only 37% of cases meeting the guideline.
The number of nodes recovered from a surgical specimen is mainly related to the extent of the surgical resection and the completeness of the pathologic examination [18,19]. The extent of the dissection performed by surgeons directly affects the survival. Therefore, surgeons should strictly adhere to the guidelines for colon cancer surgery. For complete cure of cancer, radical en bloc removal of lymph nodes should be performed. This is the reason why SLN mapping cannot contribute to conservative treatment of colorectal cancer. A recent report suggested that sentinel node mapping may be used to detect the aberrant drainage of sentinel nodes in colon cancer, leading to more extensive resection [20].
Pathologists should assess sufficient number of lymph nodes to detect tumor positive lymph nodes. Currently, the enlarged lymph nodes are removed manually, half-dissected, and examined to detect the tumor cell. To improve the nodal analysis, several methods have been introduced, such as SLN evaluation, IHC staining, and fat clearing technique. Fat clearing technique is time-consuming and difficult to perform in routine practice [21]. On the contrary, SLN mapping is a simple and inexpensive technique to improve the staging in a clinical setting. Furthermore, sentinel node mapping helps the pathologist to evaluate the nodal status using lesser nodes. Mainly the sentinel node mapping in colon cancer was used to assess the micrometastasis using serial sectioning and IHC staining. However, because its clinical significance remains unclear, we focused on the predictive ability of the SLN technique for nodal status.
Although the sentinel node mapping has a potential advantage to improve staging, there is controversy about the effectiveness of the sentinel node mapping in a clinical setting. Some papers reported that the sentinel node mapping does not improve staging accuracy in colon cancer because of its low sensitivity and high false-negative rate [22,23].
This study showed that the SLN mapping was successfully accomplished with an overall identification rate of 88.2%, with a detection rate of 90.9% for the in vivo technique and a detection rate of 87% for the ex vivo technique. Variable detection rates have been reported in the literature [11,12,13]. Our report suggested that there was no significant difference in the detection rate between the two procedures. Therefore, the ex vivo technique can be selected for SLN mapping although the in vivo technique is more physiological than the ex vivo technique.
The accuracy of the SLN mapping was 76.7% in our study. Previous studies reported accuracy between 78% and 100%. The reason that this study showed a little lower accuracy was that the number of the SLN s and the analysis technique used were different from those in previous studies. We removed one blue-stained node in most cases because we considered SLN as first node draining from a tumor. We analyzed the SLN by using the serial sectioning method because the clinical significance of lymph node micrometastases remains controversial [8,9,10].
Although this procedure was performed successfully with a good detection rate, it could not predict node positivity because the sensitivity was very low and the false negative rate was high in this study. This result is similar to those in some previous reports [13,24,25], although there are papers reporting contradictory results [11,12,26]. This can be explained by differences in inclusion criteria and analysis technique used. In our study, the high false negative rate may be due to high proportion of T3/T4 tumors. It is reported that skip metastases result from the obstruction of lymphatic channels due to bulky tumor or invasion and variations or connections in the lymphatic drainage pathway [22].
The number of SLNs was not different between the two groups. However, more number of lymph nodes were harvested after the ex vivo technique compared to the in vivo technique. This can be explained by the fact that the ex vivo mapping group included more cases of right colon cancers. The number of nodes recovered from a surgical specimen is related to the location of primary tumor [27]. Baxter et al. [17] reported that more nodes were examined in patients with right-sided colon cancer than in those with left-sided colon cancer. Chou et al. [28] reported that within the colon cohort, increased LN harvests were observed in tumors that were located in the ascending colon/hepatic flexure and transverse colon. This difference may be due to a longer resected specimen in right-sided colon cancer than in left-sided colon or rectal cancer. The limitations of this study are that it is not a randomized clinical trial with small sample size and inability to compare two methods with same injection technique. However, this study suggests that the SLN evaluation using serial sectioning in colon cancer is inappropriate in clinical practice.
In conclusion, this study showed that the in vivo technique is comparable to the ex vivo technique in detecting SLNs in colon cancer. Also, suggested that SLN evaluation using serial sectioning could not predict the nodal status with clinically acceptable accuracy despite a high detection rate. However, the clinical usefulness needs to be further investigated by performing large-scale trials in the future.
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