Introduction {#s1}
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Cancer staging is an important factor in predicting patient survival and deciding the best treatment. In Japanese classifications of colorectal cancer (CRC), tumor deposits (TDs) are described as extramural discontinuous cancer spread without residual lymph node structures. Tumor nodules are incorporated into the category of lymph node metastasis^[@r1])^. In contrast, in the Tumor Node Metastasis (TNM) staging system of the International Union Against Cancer (UICC), the place of TDs in the staging system has changed several times. In the 5^th^ edition (1997), tumor nodules \> 3 mm in diameter were classified as regional lymph node metastases (RLNMs). Tumor nodules ≤ 3 mm were classified in the T category as discontinuous extensions. In the 6^th^ edition (2002), classification by size was abandoned. Tumor nodules with smooth contours were classified as RLNMs. Tumor nodules with irregular contours were classified in the T category and also coded as V1 (microscopic venous invasion) or V2 (if grossly evident). Nagtegaal and Quirke advocated that inclusion of TDs as lymph node involvement in the 6^th^ edition was not supported by the literature or recent studies on reproducibility^[@r2])^. In the 7^th^ edition (2009), classification based on shape was abandoned. TDs (satellites) were defined as macroscopic or microscopic nests or nodules of a primary carcinoma in the lymph drainage area of pericolorectal adipose tissue in the absence of histological evidence of residual lymph node in the nodule^[@r3])^. TDs were classified as N1c if they were seen in patients with no lymph node metastasis; the T classification was not changed. If a pathologist confirmed that they had totally replaced lymph nodes, they were recorded as positive lymph nodes. TDs were considered to be N1c in patients with no RLNM, but the significance of TDs in patients with RLNMs is unclear. Here, we investigated the impact of TDs on overall survival (OS) in CRC patients with RLNMs.

Patients and Methods {#s2}
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Between 2000 and 2008, a total of 551 patients underwent tumor resection for CRC at our medical center. We excluded 87 patients who had distant metastases or had received neoadjuvant chemotherapy or radiotherapy. The remaining 464 patients were analyzed statistically. We performed possible curative operations for them. Patients with RLNMs or TDs received adjuvant chemotherapy. Our adjuvant chemotherapy had three patterns: tegafur-uracil, tegafur-gimeracil-oteracil, or tegafur-uracil/calcium folinate. Our postoperative follow-up was similar till five years after surgery. The mean follow-up period was 63 months.

We performed autonomic nerve-serving surgery in the rectum, which was defined as the large intestine located between the inferior border of the second sacral vertebra and the superior border of the puborectalis muscle. Tumor depth of invasion was classified based on the UICC TNM 7^th^ edition. Lymph node metastasis was classified as follows: pN0, no evidence of lymph node metastasis; pN1, 1--3 lymph node metastases; and pN2, ≥ 4 lymph node metastases.

After resections, surgeons dissected fresh resected specimens and detected nodules in the fatty tissue macroscopically or by manipulation. Nodules were fixed in formalin solution and stained with hematoxylin and eosin. A pathologist verified the presence of lymph node metastases. When cancer nests were found in nodules with no evidence of residual lymph nodes, they were defined as TDs. They were recorded separately from lymph node metastases.

Relapse-free survival (RFS) and OS were considered based on the following 10 variables: gender, age at surgery, circumferential occupancy, tumor location, tumor depth, histology, lymph node metastasis, TDs, lymphatic invasion, and venous invasion. RFS was defined as time from primary surgery to first documented recurrence or death from any cause. OS was defined as time from primary surgery to death from any cause.

Univariate analysis of survival predictors was assessed by the Kaplan-Meier method. Differences for survival were evaluated by log-rank tests. Variables found to be significant in univariate analysis were tested by the stepwise Cox proportional hazards regression model. For remaining data, significant differences were determined by either chi-square test or Fisher's exact test, as appropriate. A value of *P* \< 0.05 was considered to be significant. Statistical calculations were performed using the StatView software (SAS Institute, Cary, NC, USA).

Results {#s3}
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The 464 patients included 280 men and 184 women (male/female ratio: 1.5/1). The mean age of the patients was 69 years (range: 34--99 years). Tumors were located at the colon in 371 patients (80.0%) and at the rectum in 93 patients (20.0%).

In the univariate analysis for RFS in all cases, significant variables were age at surgery, tumor depth, circumferential occupancy, tumor location, lymph node metastasis, TDs, lymphatic invasion, and venous invasion ([Table 1](#tbl_001){ref-type="table"}Table 1Univariate analysis for 5-year relapse-free survival and overall survival for all subjectsVariablesnRFS (%)*P* valueOS (%)*P* valueGender0.27360.6737Male28073.680.7Female18475.681.1Age at surgery**0.0005\<0.0001**≤ 6517683.589.2\> 6528868.675.6Circumferential occupancy**\<0.00010.0035**≤ 1/216284.186.0\> 1/230269.178.0Tumor location**0.0194**0.0639Colon37177.283.8Rectum9363.769.9Tumor depth**\<0.0001\<0.0001**≤ pT212788.791.2≥ pT333768.776.7Histology0.11160.2629Well differentiated42375.681.8Others4162.071.4Lymph node metastasis**\<0.0001\<0.0001**Negative29483.487.4Positive17058.769.4Tumor deposits**\<0.0001\<0.0001**Absence40379.284.8Presence6141.852.9Lymphatic invasion**\<0.0001\<0.0001**Negative32982.586.3Positive13554.666.9Venous invasion**\<0.00010.0085**Negative13084.484.9Positive33470.379.3RFS, relapse-free survival; OS, overall survival. Bold indicates statistical significance at *P*\<0.05.).

In the univariate analysis for OS in all the cases, significant variables were age at surgery, circumferential occupancy, tumor depth, lymph node metastasis, TDs, lymphatic invasion, and venous invasion ([Table 1](#tbl_001){ref-type="table"}). In the stepwise multivariate Cox proportional hazards analysis, TDs, age at surgery, lymphatic invasion, and tumor depth remained significant ([Table 2](#tbl_002){ref-type="table"}Table 2Stepwise multivariate Cox proportional hazards analysis for overall survival for all subjectsCovariates*P* valueHazard ratio (95% confidence interval)Tumor deposits (presence vs. absence)0.00022.493 (1.535--4.048)Age at surgery (\> 65 vs. ≤ 65)0.00072.229 (1.402--3.545 )Lymphatic invasion (positive vs. negative)0.00331.928 (1.245--2.986)Tumor depth (≥ pT3 vs. ≤ pT2)0.01582.127 (1.152--3.927)). In patients with RLNMs, the only significantly predictive variable was TDs ([Table 3](#tbl_003){ref-type="table"}Table 3Stepwise multivariate Cox proportional hazards analysis for overall survival in patients with regional lymph node metastasesCovariate*P* valueHazard ratio (95% confidence interval)Tumor deposits (presence vs. absence)0.00022.813 (1.644--4.812 )). The hazard ratio of TDs presence versus absence was 2.813 (*P* = 0.0002).

Of the 464 patients, 88 (19.0%) developed recurrence. Recurrence was observed in 49.2% of patients with TDs (30/61) compared with 14.4% of patients without them (58/403) (*P* \< 0.0001). In rectal cases, local recurrence occurred in 33.3% (4/12) of patients with TDs compared with 7.4% (6/81) of patients without TDs (*P* = 0.0226). In colon cases, it occurred in 6.1% (3/49) of patients with TDs compared with 2.8% (9/322) of patients without TDs (*P* = 0.2019).

TDs were detected in 13.1% of patients (61/464). They were located at the colon in 13.2% of patients (49/371) and at the rectum in 12.9% of patients (12/93). Sixty-one patients had 171 TDs. The mean number of TDs was 2.8 (2.5 at the colon, 4.1 at the rectum). The range was 1--16 (1--3, 48 patients; \>3, 13 patients). Depth of tumor invasion in patients with TDs was pT2 in 1 patient, pT3 in 3 patients, pT4a in 53 patients, and pT4b in 4 patients. Eighteen patients with TDs (18/61; 30.0%) had no lymph node metastasis (pN1c using the UICC TNM 7^th^ edition); their mean number of TDs was 1.2. Of 294 patients without lymph node metastasis, 6.1% (18/ 294) had TDs. OS curves are shown in [Figure 1](#fig_001){ref-type="fig"}Figure 1Overall survival curve according to presence of tumor deposits. *P*\<0.0001. and [Figure 2](#fig_002){ref-type="fig"}Figure 2Overall survival curve according to presence of tumor deposits in patients with regional lymph node metastases. N, lymph node metastases; TDs, tumor deposits. *P*\<0.0001. by presence of TDs. OS curves of pN(+)TDs(+)M(--) and M(+) are shown in [Figure 3](#fig_003){ref-type="fig"}Figure 3Overall survival curve of pN(+)TDs(+)M(-) and M(+). N, lymph node metastases; TDs, tumor deposits; M, distant metastases. *P*\<0.0001..

RLNMs, lymphatic invasion, venous invasion, tumor depth, and circumferential occupancy were significantly associated with the presence of TDs ([Table 4](#tbl_004){ref-type="table"}Table 4Correlation between variables and tumor deposits (TDs)VariablesNo. of TDs (--) patientsNo. of TDs (+) patients*P* valueGender0.4299Male24634Female15727Age at surgery0.7473≤ 6515422\> 6524939Circumferential occupancy**\<0.0001**\< 1/21557≥ 1/224854Tumor location0.9381Colon32249Rectum8112Tumor depth**\<0.0001**≤ pT21261≥ pT327760Histology0.5010Well differentiated36657Others374Regional lymph node metastasis**\<0.0001**Negative27618Positive12743Lymphatic invasion**\<0.0001**Negative30722Positive9639Venous invasion**\<0.0001**Negative1273Positive27658Bold indicates statistical significance at *P*\<0.05.). RLNMs were significantly associated with TDs, lymphatic invasion, venous invasion, tumor depth, and circumferential occupancy ([Table 5](#tbl_005){ref-type="table"}Table 5Correlation between variables and regional lymph node metastases (RLNMs)VariablesNo. of RLNMs (--) patientsNo. of RLNMs (+) patients*P* valueGender0.6105Male180100Female11470Age at surgery0.9182≤ 6511165\> 65183105Circumferential occupancy**\<0.0001**\< 1/212636≥ 1/2168134Tumor location0.2357Colon240131Rectum5439Tumor depth**\<0.0001**≤ pT211314≥ pT3181156Histology0.1768Well differentiated272151Others2219Tumor deposits**\<0.0001**Absence276127Presence1843Lymphatic invasion**\<0.0001**Negative27158Positive23112Venous invasion**\<0.0001**Negative11614Positive178156Bold indicates statistical significance at *P*\<0.05.).

Discussion {#s4}
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In the 7^th^ UICC TNM staging system, TDs were considered N1c in patients without RLNM, whereas the significance of TDs in patients with RLNMs remained unclear. Tateishi *et al.* showed that tumor nodules could justifiably be regarded as lymph node metastases with regard to OS^[@r4])^. Ueno *et al.* reported that N staging could predict survival outcome with the highest accuracy when both nodal involvement and nonvascular invasion-type extranodal cancer deposits were treated together as an N factor^[@r5])^. Nagtegaal *et al.* showed that the definitions in the 5^th^ edition UICC TMN were the most reproducible when tumor nodules \> 3 mm in diameter were considered as lymph node metastases and further noted that many questions remained about the definition, reproducibility, and use of this category in special situations, such as after neoadjuvant treatment^[@r6])^. On the other hand, Goldstein *et al.* reported that TDs were distinct from lymph node metastases and suggested that TDs of all sizes should be considered a single entity^[@r7])^. Puppa *et al.* advocated that TDs without lymphocytes and in close association with veins and/or nerves could be included in the M1a category for staging purposes^[@r8])^. Al Sahaf *et al.* suggested that extramural deposits be classified as metastases^[@r9])^. Wünsch *et al.* showed that TDs in which the origin remained unclear should be included in the M subcategory^[@r10])^. Ueno *et al.* reported that tumor nodules ≥ 5 mm growing with venous/perineural invasion had a considerable adverse prognostic effect^[@r11])^. Tong *et al.* suggested that disease categorized as T3N2bM0TDs (+) and T4N2bM0TDs(--/+) should be reclassified as stage IV^[@r12])^. Shimada *et al.* reported that extranodal cancer tissue had a more severe clinical impact than nodally involved tissue^[@r13])^. In our stepwise multivariate Cox proportional hazards analysis of patients with RLNMs, only TDs remained significant for OS. The prognosis of patients with RLNMs was not homogeneous according to presence of TDs ([Figure 2](#fig_002){ref-type="fig"}). The effect of TDs on OS was between that of lymph node metastasis and distant metastasis ([Figure 2](#fig_002){ref-type="fig"} and [Figure 3](#fig_003){ref-type="fig"}).

In Japan, the recurrence rate after curative resection was 17.3% between 1991 and 1996^[@r14])^. Of 464 patients, 88 (19.0%) developed recurrence in our study. Recurrence was observed in 49.2% of patients with TDs (30/61) compared with 14.4% of patients without TDs (58/403) (*P* \< 0.0001). Heald *et al.* suggested that minute foci in the mesorectum might lead to suture-line or pelvic recurrence^[@r15])^. Paty *et al.* reported that mesenteric implants were found in 6 of 13 patients with pelvic recurrence^[@r16])^. In our rectal cases, local recurrence occurred in 33.3% (4/12) of patients with TDs compared with 7.4% (6/81) of patients without TDs (*P* = 0.0226). TDs indicated a high risk of local recurrence in rectal cases. In our colon cases, local recurrence occurred in 6.1% (3/49) of patients with TDs compared with 2.8% (9/322) of patients without TDs (*P* = 0.2019). Local recurrence was not significantly associated with the presence of TDs in colon cases. The results regarding correlation between variables and TDs, shown in [Table 4](#tbl_004){ref-type="table"}, indicate that the presence of TDs was not significantly associated with the location of cancer. Anatomy or autonomic nerve-preserving surgery in the rectum might increase the risk of local recurrence in patients with TDs.

Many reports have suggested different groupings for TDs. In 1997, Ueno *et al.*reported that extrabowel skipped-cancer infiltration had four patterns: scattering, vessel invasion, neural invasion, and nodular^[@r17])^. In 2007, extranodal cancer deposits were classified as vascular and nonvascular invasion types^[@r5])^. In 2000, Goldstein *et al.* divided TDs into three groups: perineural, perivascular, or intravascular tumor extension^[@r7])^. In 2002 Ratto *et al.* divided mesorectal microfoci into four types: endovasal, endolymphatic, perineural, and isolated^[@r18])^. In 2003 Prabhudesai *et al.* reported that a significant proportion of extranodal deposits might represent blood-borne spread^[@r19])^. In 2007, Puppa *et al.* divided TDs into two patterns, with or without lymphocytes, and suggested that pericolonic TDs without lymphocytes were a destructive type of venous invasion and different from other types of vessel involvement^[@r20])^. In 2010, Shimada et al. reported that extranodal cancer tissue was classifiable into three types: a vascular and perineural invasion type, smooth nodule type, and irregular nodule type^[@r13])^. Wünsch *et al.* classified pericolonic TDs as venous invasions, lymphatic invasions, nerve sheath infiltrations, free pericolonic TDs, and continuous growth^[@r10])^. The previous reports suggested that TDs arose from various situations. In our study, TDs included vessel invasion, neural invasion, and nodules. Lymph node metastasis, lymphatic invasion, and venous invasion were significantly associated with the presence of TDs ([Table 4](#tbl_004){ref-type="table"}). TDs could not be incorporated into one category.

Conclusion {#s5}
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In stepwise multivariate Cox proportional hazards analysis of CRC patients with RLNMs, only TDs remained significant for OS (hazard ratio: 2.813; *P* = 0.0002). The prognosis of patients with RLNMs was not homogeneous according to presence of TDs. The effect of TDs on OS was between that of lymph node metastasis and distant metastasis.

We thank Dr. Yukinori Inadome for valuable comments concerning pathological aspects of the manuscript.
