RMS is a malignant mesenchymal neoplasm that exhibits striated muscle differentiation, accounting for 5-10% of all solid tumors and 55-60% of soft tissue sarcomas in pediatric age group but relatively rare in adults. In this retrospective study, however, patients aged≥20 years accounted for 51%. The most common primary sites for RMS were head/neck, extremities and genitourinary tract, perianal and perineum area are rare and considered unfavorable RMS sites[1, 3, 23]. RMS in adults is more likely to occur in unfavorable sites than in adolescents, which may explains the majority of adults in this study. In addition, female predominance is noticed in Chinese literatures (71%), similar feature was reported in a Japanese study, indicating regional and ethnic differences in the occurrence of this disease.
The prognosis of primary RMS in perianal and perineal area is extremely poor, prognostic factors including age, pathological type, clinical group and staging. In this study, patients older than 20 years comprised 51% of all patients, 38% were classified as alveolar RMS and 52% were categorized into IRS group III-IV, which may contributed to the poor outcome. In addition, misdiagnose rate was 45.7% in this group, which was significantly related to poor prognosis (P=0.038).
PRMS is easy to be misdiagnosed as perianal abscess and may lead to poor prognosis, careful evaluation and differential diagnosis for suspected patients are crucial. Perianal mass is the the most common manifestation of both PRMS and perianal abscess. 97% patients presented as perianal mass in this study. Perianal abscess is almost always accompanied by pain. In this study, pain is found to be related with misdiagnosis (P = 0.010). When combined with infection, local redness, swelling and fever can be observed, which resembles perianal abscess and thus leads to high misdiagnosis rate. It is difficult to distinguish PRMS from perianal abscess by symptoms. For firstly-diagnosed patients with painful perianal mass, paying attention to the delay of seeking health care, predisposing factors for abscess formation (immune deficiency, HIV infection, diabetic ketoacidosis and Crohn's disease) and the increasing level of lactate dehydrogenase would be helpful for differential diagnosis. In addition, epidemiological characteristics can provide clues, that perianal abscesses mainly affect adults and male infants younger than 1 year[28-30], which is different from the population features of PRMS reported in literatures[25, 31, 32]. Therefore, solid or even malignant tumor, instead of perianal abscess, should be considered when the patient is a girl presented with perianal mass. Theoretically, misdiagnosis would delay the correct assessment of this disease. However, correlation between misdiagnosis and the average time interval from symptom onset to pathological diagnosis was found insignificant in this study (P = 0.712). On the contrary, the average diagnosis time of misdiagnosed cases is shorter than that of cases without misdiagnosis (2.6 months vs. 3.4 months). The possible reason is that patients’ complaints about pain prompted clinicians to carry out emergency abscess incision/resection, rendering an earlier acquirement of postoperative pathological results than those without pain.
Doppler ultrasound is a noninvasive, accessible and radiation-free method for preliminary examination of patients with perianal painful masses suspected as PRMS. It has unique advantages in distinguishing solid, cystic or lacunar masses, providing high specificity and sensitivity of the diagnosis of perianal abscess. Endoanal ultrasound can clarify tumor involvement of the anal canal, and thus provide clues for accurate diagnosis and following treatment. 5 cases of PRMS treated in our center showed solid masses with uneven echo and rich intratumoral blood flow signal with or withoud clear boundary, which was consistent with the relevant reports[34-36]. 1 case was indicated perianal abscess by Doppler ultrasound, but pathological result following concomitant ultrasound-guided biopsy corrected the diagnosis, suggesting a method to further lower the risk of misdiagnosis. Endoscopic ultrasound-guided fine needle aspiration is recommend for its accuracy and minimal invasion when conducting biopsy.
The sonographic features of RMS is variable and nonspecific and it has certain limitation on the assessment of deep-tissue lymph nodes and distant metastasis, hence the necessity of using CT or MRI for further evaluation. MRI has clearer soft tissue imaging compared with CT and can better reveal the invasion of RMS to the adjacent pelvic organs. It has become the first choice for pelvic RMS[39-41] but few reports are available for MRI description of PRMS. By observing the sagittal, coronal, and axial T2 weighted high-resolution images of the pelvis, we found a significant feature of PRMS that the hyposignal of the external anal sphincter (EAS) is replaced by the hypersignal of the tumor, causing a discontinuity of the anal sphincter complex. In some cases, the muscle signal of EAS can be observed cutting into the tumor; a pseudocapsule like structure was formed by the compressed EAS surrounding the tumor (Figure 1). According to these features, we suggest that PRMS probably originates from EAS. Compression rather than direct invasion of rectal wall and anal canal is observed (n=4), which is different from the characteristics of anal canal cancer[42, 43] and thus valuable for differential diagnosis.
Among the 15 patients treated in our center, 5 underwent abscess incision or drainage previously in other hospitals and 4 of them had poor prognosis. For the 2 patients survived for more than 5 years, one was initially diagnosed as perianal abscess but only treated with antibiotics without surgery; both of them received radical operation (R0) in our center and no evidence of recurrence was found after 5 year’s follow up. Wrong surgical treatment can destroy the integrity of tumor and leads to poor prognosis. If PRMS is diagnosed on time and distant metastasis is excluded, surgical treatment should be carried out as soon as possible. Taking into account the tumor is likely to be originated from EAS, ELAPE (extra-levator abdominalperineal excision) could be the choice for most patients to achieve R0 excision. If the tumor was relatively confined and only a small portion of EAS was involved, LRR with reserved anus can be performed. In fact, 5 patients received LRR in our center and 3 of them survived for more than 5 years; 1 recurred due to failure to achieve R0 resection and is undergoing further treatment untill now; 1 has no evidence of recurrence after 2 years of follow-up but course of disease was prolonged due to rectovaginal fistula caused by surgical damage of perineum. LRR can achieve long-term survival without compromising anal function, but R0 resection must be guaranteed and perineum shall be protected during the operation to avoid rectovaginal/rectourethral fistula.
Optimal treatment of PRMS is controversial. Comprehensive therapy is recommended by relative literatures but most of them are single-center retrospective analysis of few cases. Recently, a multicenter study showed a 64-86% 5-year survival rate of patients receiving chemotherapy combined with surgery or radiotherapy reached. Among the 15 patients treated in our center, 9 received radical operation and chemotherapy and the 5-year OS of them is 57%; the other 6 patients received chemotherapy alone and had poor prognosis. 4 patients received chemoradiotherapy with or without surgery in our center have survived up to now with no evidence of recurrence, but long-term efficacy of radiotherapy remains to be evaluated. Inguinal lymph node metastasis is one of the main factor of poor outcome. In this group, 11 patients were complicated with lymph node metastasis. 5-year survival rate was only 26% for the 9 patients with preoperative inguinal lymph nodes involvement. One patient had ilium lymph node metastasis in 5 months and died in 11 months postoperatively. Concerning the high incidence of regional lymph node metastasis in PRMS, it is suggested that inguinal lymph nodes resection or irradiation should be performed prophylactically, but such aggressive approach may be avoided with the application of PET-CT which can effectively improve the detection rate of lymph node metastasis[44, 45]. Breast metastasis was found in 2 cases in this study, which was also reported in a Japanese literature, providing a clue for the specific metastatic pathway and histological characteristic of PRMS.