New approach to anal cancer: Individualized therapy based on sentinel lymph node biopsy

Abstract

Oncological treatment is currently directed toward a tailored therapy concept. Squamous cell carcinoma of the anal canal could be considered a suitable platform to test new therapeutic strategies to minimize treatment morbidity. Standard of care for patients with anal canal cancer consists of a combination of radiotherapy and chemotherapy. This treatment has led to a high rate of local control and a 60% cure rate with preservation of the anal sphincter, thus replacing surgical abdominoperineal resection. Lymph node metastases represent a critical independent prognostic factor for local recurrence and survival. Mesorectal and iliac lymph nodes are usually included in the radiation field, whereas the inclusion of inguinal regions still remains controversial because of the subsequent adverse side effects. Sentinel lymph node biopsies could clearly identify inguinal node-positive patients eligible for therapeutic groin irradiation. A sentinel lymph node navigation procedure is reported here to be a feasible and effective method for establishing the true inguinal node status in patients suffering from anal canal cancer. Based on the results of sentinel node biopsies, a selective approach could be proposed where node-positive patients could be selected for inguinal node irradiation while node-negative patients could take advantage of inguinal sparing irradiation, thus avoiding toxic side effects.

Key Words: Anal carcinomal, Lymphnode metastasis, Sentinel lymphnode, Tumor staging

INTRODUCTION

In the past two decades, tangible efforts have been made to understand the natural course and behavior of anal canal carcinoma, and, especially, to improve the efficacy of multimodality chemo-radiation treatment. Currently, the main issues in the treatment of this neoplasm are the recognition of a reliable staging system and strategies to obtain long-term survival while reducing radiation related side effects.

Anal canal squamous cell carcinoma represents 1% to 2% of all gastrointestinal malignancies[1-3] and is associated with human papilloma virus infection[4]. Diagnostic procedures include clinical and rectal examination, endorectal ultrasound, magnetic resonance imaging (MRI), computed tomography (CT) scan, and positron emission tomography (PET) scan[1]. Clinical and pathological classification is based on tumor-node-metastasis staging developed by the American Joint Committee on Cancer[5].

The two most significant prognostic factors are tumor size and nodal status[6,7]. Inguinal lymph node involvement is considered a major independent prognostic factor for local recurrence and overall survival[2] with survival rates dropping from 70% in node-negative to 40% in node-positive patients. Syncronous inguinal metastases are strictly related to the tumor size and occur in 10% to 25% of patients[8,9]. Metachronous metastases are found, usually during follow-up, in 5% to 25% of patients who were groin lymph node-negative at the time of diagnosis[9].

Identification of the anal canal lymphatic drainage pattern is essential to predict secondary nodal involvement. Lymphatic drainage of anal canal cancer mostly depends on the tumor location. Based on anatomic and physiopathological studies, tumors located under the dentate line are more likely to drain to groin chains while tumors located above the dentate line are prone to drain to the internal iliac lymph node system, although the two drain systems are not separated from each other (Figure 1)[10-12]. A tumor located laterally in the anal canal is more likely to drain to the homolateral side. Moreover, tumors located in the midline have the tendency to drain bilaterally in the inguinal regions[12].

Figure 1 Anal canal lymphatic drainage pattern.

TREATMENT OPTIONS AND PROGNOSIS

Local excision with neoplasm-free margins is recommended for T1 well-differentiated tumors[13]. Abdominoperineal resection and permanent colostomy has been traditionally performed for anal canal cancer achieving 40% to 70% survival rate at five years[2,14]. Chemo-radiation treatment has replaced surgical resection since Nigro et al[15] introduced his protocol in 1974 and has raised survival and eradication of tumors from 70% to 90% in selected patients[16-19]. Predictably, prognosis is worsened by 50% with nodal involvement and tumor size larger than 5 cm[2]. Local recurrence shown by pathology or incomplete pathological response after multimodality chemo-radiotherapy treatment is an indication of subsequent abdominoperineal resection[13,20,21]. However, dose-related radiation side effects, such as anal ulcers, stenosis, and necrosis, can necessitate a subsequent colostomy in 6% to 12% of patients[18,22-25]. Irradiation fields involving the groin lymph nodes chain can cause inguinal fibrosis, external genitalia edema, epidermolysis with super-infection of skin, necrosis of the femoral head, femoral head fracture, and stenosis of iliac artery[18,22,24,26-30]. Death from radiation toxicity is reported in 2.0% to 2.7% of patients[9]. Moreover, risk of radiation side effects do not decrease over time and might pose a lifelong risk of developing late complications[24,29]. For these reasons, strategies to reduce the radiation field are advisable.

Metastatic involvement of inguinal nodes is a crucial point in the correct assessment of these patients. In fact, involvement of mesorectal nodes does not affect the therapeutic approach because they are generally included in the radiation fields, while routine inclusion of groin lymph node stations remains controversial. Inguinal lymph node involvement is difficult to establish. The diagnostic accuracy of clinical evaluation and imaging tools remains low. In the case of clinically palpable nodes, a biopsy could be performed. However, as in the majority of cases, a clinically negative groin does not imply absence of metastatic disease. A “wait and see” position is recommended by some institutions[31], while prophylactic groin irradiation is ordinarily practiced by other groups[32]. Less frequently, the decision to irradiate the inguinal region is based on primary anal tumor size[33] (Figure 2).

Figure 2 Radiation field of anal carcinoma with exclusion (green field) or inclusion of inguinal regions. A: Lateral view; B: Anterior view.

Prophylactic groin irradiation has been proposed by several authors with a reduction of inguinal metastatic recurrence as low as 2.5% to 3%[22,34-40]. Otherwise, Papillon et al[41] and Gerard et al[9] examined a large series of clinically node-negative patients after groin sparing irradiation and observed inguinal metachronous recurrence in 7.4% and 7.8% of patients, respectively.

Given this premise, it is clear that the majority of patients are over treated because effective nodal staging is not achieved. On the other hand, early T stage neoplasms with underrated nodal status may not receive the proper treatment by inguinal sparing.

RATIONALE AND IMPLICATIONS OF SENTINEL LYMPH NODE MAPPING

Because the standard treatment does not provide any specimens for pathological evaluation, the effective node status of these patients is not determined. Moreover, the wide lymphatic drainage pattern that characterizes the inguinal and the pelvic lymph node basin makes it difficult to predict synchronous and metachronous metastatic involvement[42]. Furthermore, it has been observed that the size of the nodes is not a predictable parameter for nodal involvement because 44% of positive inguinal lymph nodes at pathological examination are smaller than 5 mm[27]. Conversely, 50% of larger nodes appear to be inflammatory[14]. Of note, in our previous case series, smaller lymph nodes (4-7 mm) were more likely to harbor metastases than larger ones[43].

For these reasons, even more advanced imaging techniques, such as MRI, are not accurate in the detection of metastatic lymph nodes[44]. PET has been used as an aid to achieve better staging and it has been demonstrated to detect up to 20% of metastases not diagnosed by clinical or radiological examination[44]. Bannas et al[45] has demonstrated that PET/CT is superior to PET or CT alone for staging anal cancer, particularly in identifying local regional lymph node metastases. Recently, Mistrangelo et al[46] demonstrated that sentinel lymph node biopsy (SLNB) was superior to PET-CT for the staging of inguinal lymph nodes. In our experience, PET has a sensitivity of 33% and a specificity of 84%. Even though sensitivity of SLNB of the anal canal has not been yet addressed, the sensitivity of sentinel node biopsy detection in melanoma is reported to be up to 99%[47-49].

These findings advocate the need to find a more reliable technique to identify positive nodes. Currently, histological evaluation is the gold standard to assess the presence of metastases in lymph nodes. Standard surgical node dissection for suspicious inguinal nodes has been proposed[50,51]. However, this approach is also fraught with side effects[52,53]. In addition, elective inguinal dissection for clinically suspicious nodal involvement revealed metastasis only in 50% of cases at histology[14].

In this scenario, SLNB could help to accurately identify patients with inguinal metastatic spread and to avoid irradiation morbidity in node-negative subjects or, conversely, to enroll node-positive patients for inguinal irradiation. SLNB should not be performed in patients with clear evidence of clinically positive or suspicious inguinal nodes. Some reports have also suggested the exclusion of patients with locally advanced T4 cancer. Metastatic or even reactive nodes may alter the lymphatic drainage pattern, thus making the SLNB unreliable. For the same reason, patients who underwent prior surgical manipulation in the anal region should be excluded from the procedure[12,54,55].

PATIENT SELECTION AND FEATURES

Patients with histologically confirmed squamous cell carcinoma of the anal canal were eligible for the SLNB procedure. Previous reports have also included tumors of the anal margin, which should be considered separately due to different clinical behavior. For the reasons explained above, patients with clear or suspicious inguinal nodes were generally not enrolled[54].

From 2007 to 2012, 23 patients with proven squamous cell carcinoma of the anal canal and clinically negative inguinal nodes were enrolled in the prospective study. Pre-operative work-up included endoscopy and biopsy, pelvic RMI, endoanal ultrasound, and abdominal and lung CT scans. Tumor stage was classified as follows: T1, 3 pts; T2, 9 pts; T3, 7 pts; and T4, 4 pts.

In 2009, an inguinal sparing irradiation protocol was started. Fifteen patients were observed and three patients were excluded from the SLNB study for positive inguinal lymph nodes confirmed by cytology. Twelve patients with clinically negative inguinal regions were enrolled. After the SLNB procedure, patients with histologically positive inguinal nodes were treated with combined chemotherapy using 5-fluorouracil/mitomycin-C, and the standard radiotherapy field, including inguinal basins. Patients with tumor-free nodes did not undergo inguinal irradiation. Patients were then regularly followed-up every three months.

DESCRIPTION OF TECHNIQUES

Many authors have reported a standardized technique to isolate and retrieve sentinel lymph nodes for metastatic assessment[12,28,54-64]. The first procedural step was a lymphoscintigraphy to evaluate the main lymphatic drainage and the first node in which the tracer is captured. The radiotracer (0.2 mL 99mTC nanocolloid) injection was performed submucosally, directly around the anal lesion in the four cardinal points with the aid of an anoscope. As the injection can be painful, a needle-free injection system[56] could also be utilized to minimize patient discomfort. After injection, planar anterior and posterior images were taken with a Philips gamma camera, as previously described[43], to localize the sentinel lymph node. Typical drainage patterns displayed radiotracer accumulation in the inguinal area or in the iliac internal lymph node system (Figure 3). If the accumulation was observed in the perirectal or external iliac lymph nodes, then patients did not undergo the sentinel lymph node procedure. Only patients showing radio accumulation in the inguinal area were enrolled for sentinel node surgical retrieval, and the overlying skin was marked by a waterproof pen. Marking the skin assisted in the intraoperative identification of the nodes and minimized surgical incision.

Figure 3 Lymphoscintigraphy of a sentinel lymph node in anal carcinoma. Anterior view showing injection site (blue arrow) and sentinel lymph node (red arrow).

There was no consensus about the time gap between injection, lymphoscintigraphy acquisition, and surgery. Clearance of radioactive colloids by lymphatic drainage is related to the particle size; small particles are cleared first and large particles later[65]. Moreover, tracers with small particle size are washed out from true sentinel nodes and move to other nodes[66]. Therefore, a shorter period between injection and surgery is recommended with smaller particles. In our experience, 99mTC-nanocolloid with particles between 80 and 150 nm were employed. By virtue of their delayed wash-out, surgery could be performed even the day after injection. A delay of 12 to 16 h between injection and surgery allowed good intraoperative radio-localization with minimal interference by primary tumor radioactivity.

During surgery a second vital tracer, blue patent, was injected around the site of the anal tumor. In other reports, nearly all groups have used blue dye as a second vital tracer. The addition of intraoperative dye aids in the intraoperative search[67] and tends to result in higher rates of lymph node detection. There are no data regarding the usefulness of dye with SLNB in anal cancer; however, the importance of dual mapping to reduce false negative results has recently been demonstrated in breast cancer[68]. When accumulation was shown at lymphoscintigraphy on both groin regions, inguinal dissection was made bilaterally.

Local anesthesia was routinely employed in our institute, although the procedure can be performed under general or local regional anesthesia. During surgery, a hand-held gamma detection probe was used to identify the radioactive lymph node (NEOPROBE Neo2000 Gamma Detection System). A small incision was made under radio probe guidance and over the marked skin. The sentinel lymph node was retrieved by visualization through the blue dye and radio detection by the hand-held gamma probe. During radio-navigation the gamma probe was directed away from the anus to avoid signal detection from the primary site of injection[28]. Significant radio-colloid capture, compared to lymphatic basin, should be in a ratio of 5:1. After nodal excision, the surgical site was explored by the gamma probe to identify accessory nodes.

The isolated lymph node was sent for pathological examination and metastatic assessment. Hematoxylin and eosin (HE) staining was used to assess the presence of malignancy. Other reports have suggested using immunohistochemistry with pan-cytokeratin antibody markers for cases of negative HE[28,54,56]. In our study, the specimen was examined using a particularly accurate technique. Briefly, sentinel lymph nodes were step-sectioned into 50-micron slices and serial sections of three microns thick were cut at each level. This number of sections provided good accuracy without having to resort to the more costly immunohistochemical analysis. Among the other related reports, only Gretschel et al[54] have found micro-metastases or isolated tumor cells in lymph nodes examined after a negative HE.

Common complications related to lymph node mapping include wound infection, hematoma, and lymphorrea from lymphatic fistula with subsequent seroma in the surgical site. Post-procedural side effects are easily managed and rarely require reintervention.

RESULTS OF SENTINEL LYMPH NODE ASSESSMENT

Since the first report published by Keshtgar et al[56] in 2000, several reports have demonstrated the feasibility and effectiveness of the SLNB procedure in anal cancer[43,54,57-63,69,70]. The technical aspects and results of the published studies are shown in Table 1. In these studies, patients with T1-T4 anal tumors with clinical or imaging negative inguinal nodes were enrolled. Lymphoscintigraphy accuracy was generally high, rating 90% to 100% in examined reports, and inguinal capture, as explained above, depended on the localization of the primary tumor. Surgical gamma probe detection was nearly 100%[43,54,57-63,65,69,70]. Almost all groups suggested the double tracer technique to better visualize the sentinel lymph node intraoperatively. Metastatic node rate, among the sentinel nodes, varied between 0% and 33%; however, the case series were very heterogeneous and included advanced tumors (T stage ranging from T1 to T4) even though the majority of patients were clinically node-negative.

Table 1 Technical aspects and results of the published case series.

Ref.	Year	T stage	Groin clinical	Tot N cases	Inguinal detection rate (%)	Double tracer	N positive (%)	Complication	Bilateral detection	Surgical retrieval rate
Keshtgar et al[56]	2000	NA	Negative	1	1/1	Yes	1/1	NA	0/1	1/1
Perera et al[58]	2002	T1-T2	1/12	12	8/12 (66)	Yes	2/18 (11)	NA	0/8	8/8
Péley et al[57]	2002	NA	1/8	8	8/8 (100)	Yes	2/8 (25)	No complication	5/8	8/8
Rabbit et al[69]	2002	NA	Negative	4	3/4 (75)	Yes	0/3 (0)	NA	2/3	3/3
Bobin et al[62]	2003	NA	Negative	33	33/33 (100)	Yes	7/33 (21.2)	NA	NA	33/33
Ulmer et al[60]	2004	T2-T4	Negative	17	13/17 (76)	Yes	5/12 (41.6)	Lymphatic fistula (1 pt)	4/13	12/12
Gretschel et al[54]	2008	T1-T4	Negative	40	20/40 (50)	Yes	6/20 (30)	Wound infection (2 pts), lymphatic fistula (1 pt), hematoma (1 pt)	NA	20/20
Mistrangelo et al[64]	2009	T1-T4	NA	35	35/35 (100)	No	7/35 (20)	Lymphatic fistula (18 pts), lower limb lymphedema (1 pt)	22/35	34/35
Damin et al[55]	2010	T1-T3	Negative	15	15/15 (100)	Yes	4/15 (26.6)	Lymphatic fistula (1 pt)	13/15	15/15
de Jong et al[70]	2010	T1-T3	4/21	21	21/21 (100)	Yes	7/21 (33)	NA	14/21	21/21
De Nardi et al[43]	2011	T1-T3	Negative	11	9/11 (81)	Yes	3/9 (33)	Lymphatic fistula (1 pt)	2/11	9/9

NA: Not applicable.

Among 23 patients, 19 with inguinal capture at lymphoscintigraphy underwent the SLNB procedure. Inguinal dissection was made bilaterally in two patients where accumulation was shown at lymphoscintigraphy on both groin regions. Sentinel lymph node retrieval by gamma probe was possible in all patients. Histological examination of nodes showed the presence of metastases in five patients (26%).

Among the 12 patients enrolled in the inguinal sparing radiation protocol with a clinically negative inguinal region, 10 patients had negative pathological SLN and received an inguinal sparing irradiation. At a median follow-up of 20 mo, none of these patients had developed inguinal metastases.

Gretschel et al[54] reported that inguinal lymph node assessment was able to change the treatment plan recommended by national guidelines in 50% of patients. In the group of patients with inguinal sparing irradiation, inguinal recurrence was found in two out of 20 patients: one patient suffering from a T4 tumor, associated with disseminated disease, and one patient with T1 tumor that was previously treated by local excision. Mistrangelo et al[28] did not observe isolated inguinal recurrence in 28 node-negative patients at a median follow-up of 22 mo.

De Jong et al[70] reported that SNLB provided alteration of treatment in at least 11 of 21 patients. However, inguinal recurrence within 12 to 24 mo was observed in two out of 14 node-negative patients undergoing node-sparing irradiation. Figure 4 demonstrates a simple diagnostic-therapeutic algorithm to identify patients eligible for the SLNB procedure to individualize irradiation treatment based on inguinal node status.

Figure 4 Diagnostic-therapeutic algorithm for squamous cell anal carcinoma. SLNB: Sentinel lymph node biopsy.

CONCLUSION

In spite of the low incidence of anal canal carcinoma, noticeable advances have been achieved in the past 30 years in understanding its etiology, biological behavior and therapy, with the current therapeutic approach being primary radio-chemotherapy. The identification of lymph nodes metastases, especially in the inguinal area, is still the main issue that needs to be addressed.

The low incidence of metachronous metastases and the considerable side effects after inguinal node dissection and radiotherapy do not justify a prophylactic treatment[71]. A refined staging system with precise identification of disease extent could allow individualized therapy, ensuring the accurate coverage of disease while sparing disease-free organs.

In this context, SLNB, as a minimally invasive procedure, may improve disease staging and may be useful to select patients for inguinal radiation. Feasibility and efficacy of SLNB has been addressed by several reports and the clinical utility of this procedure in changing the therapeutic plan has also been outlined. However, further larger prospective studies are needed to confirm the clinical impact of this procedure. Continuous and stringent long-term follow-up is necessary to estimate the outcome in node-negative patients who did not undergo groin irradiation.