Epidemiological characteristics of 1385 sacral tumors and tumor-like lesions not including metastatic cancers in one institution in China

Background: Sacral tumors and tumor-like lesions are a rare group of lesions that can affect children and adults of all ages. Little is known about the epidemiological characteristics of them in China. Methods: 1385 patients with sacral tumors and tumor-like lesions, which had the clinical record at our bone tumor center from 2000 to November 2018 were analyzed. The metastatic cancers were not included in the present study. Results: 51.7% (716 cases) were malignant and 48.3 % (669 cases) were benign tumors or tumor-like lesions. Of malignant tumors, chordoma was the most common malignant tumor (316 cases, 22.8% of all tumors), followed by chondrosarcoma, myeloma and other histologic types. The most common histological type of benign tumors was giant cell tumor accounting for 14.8% (205 cases) of all tumors, followed by neurobroma, schwannoma and other types. The most common age group affected by malignant bone tumors was the 51- to 60-year-old group, followed by the 41- to 50-year-old group. The most commonly affected age group for benign tumors and tumor-like lesions was the 31- to 50-year-old group, followed by the 21- to 30-year old group. Furthermore, the following histologic types had the gender predilection. Chordoma, chondrosarcoma, myeloma and osteosarcoma affected more frequently males than females. Malignant peripheral nerve sheath tumor, lymphoma, giant cell tumor, neurobroma, tuberculosis, teratoma and epidermoid cyst more frequently affected females than males. Conclusions: The large cohort of sacral tumors and tumor-like lesions in our database may reveal their epidemiologic characteristics in China and epidemiological feature of sacral tumors and tumor-like lesions is fairly distinct from the mobile spine and extremities.


Introduction
Sacral tumors and tumor-like lesions are a rare group of tumors, that can affect children and adults of all ages. A great number of speci c histologic subtypes of them have been delineated, each of them with unique appearance and biological potential [1,2]. A barrier is presented to etiologic study due to the rarity and heterogeneity and it poses a great challenge in the understanding of epidemiologic features [3]. Little is known about their epidemiologic features in China. China is a relatively big country with a steady population and the number of patients with sacral tumors and tumor-like lesions is huge and our hospital is a biggest specialized musculoskeletal tumor cencer in China. This provide us a bene cial condition to study the epidemiological characteristics of sacral tumors and tumor-like lesions. The present study is aiming to provide the rst epidemiological analysis of sacral tumors and tumor-like lesions not including metastatic cancers in Chinese population between 2000 and 2018, speci cally to describe epidemiologic characteristics of them by gender, age and subtype.

Patients And Methods
We retrospectively reviewed 1385 patients with sacral tumors and tumor-like lesions, which had the clinical record at our bone tumor center from 2000 to November 2018. There were 709 male and 676 female patients with a mean age of 43.9±17.0 years (range, 2-86 years). Histologic diagnosis was con rmed by biopsy or operative specimen. The inclusion criteria for the present study were as follows: (1) Patients had the de nitive histological diagnosis; (2) Diagnostic time was from the year of 2000 to 2018. The exclusion criteria were as follows: (1) Without con rmed histological diagnosis; (2) Diagnostic time was beyond the range of 2000 to 2018; (3) Metastatic cancers at the sacrum. All patients which were included in the present study were gave written informed consent for their data to be included in this study during the follow-up. All data were obtained from the clinical and radiograph records. This study was approved by the Institutional Review Board/Ethics Committee of the authors' institution. Following data were collected in the present study: age, gender, affected sacrum level, pathological diagnosis.
For the initial clinical evaluation and diagnosis, all patients received plain radiographs, CT, MRI and bone scanning. After imaging, we performed the needle biopsy to clarify the diagnosis. For the malignant tumors, the routine follow-up including clinical examination, radiographs of the extremity and chest were performed every 3 months for the rst 6 months, every 6 months for the rst 3 years, and then annually. The chest CT scanning was performed every 6 months for the rst 3 years, and then annually. For the benign tumors, the routine follow-up were performed every 6 months for the rst year and then annually.

Statistical analysis
Continuous variables were summarized with means and ranges; categorical variables were summarized with frequency counts and percentages. The Student's t test was used to compare the age in different histologic types of male and female in Table-5&6. In the present study, other statistical methods were not applied into the analysis. The SPSS software (version 19.0; SPSS Inc, Chicago, IL, USA) was used for all statistical analyses. P≦ 0.05 indicated a statistically signi cant difference.

Incidence of histological subtypes
The histological types of sacral tumors and tumor-like lesions were listed in Table- The incidences were presented separately by gender (males and females) and age at diagnosis was grouped into nine subgroup (from 0 to 10 years cohort to ≥80 years cohort). The most common age group affected by malignant bone tumors was the 51-to 60-year-old group (21.1%, 151cases), followed by the 41-to 50-year-old group (19.4%, 139 cases) (Table-3). The most commonly affected age group for benign tumors and tumorlike lesions were the 31-to 40-year-old and 41-to 50-year-old groups (21.1%, 141 cases and 21.1%, 141 cases), followed by the 21-to 30-year old group (19.6%, 131 cases) ( Table-4).
We analyzed the age and gender related epidemiologic characteristics of the top seven primary sacral malignant tumors (Figure-3 (Table-5 The epidemiologic characteristics of patients with sacral tumor and tumor-like lesions could help us to perform the differential diagnosis. Chordoma, chondrosarcomas, myeloma and lymphoma more frequently occurs at the older age, otherwise osteosarcoma and Ewing sarcoma are more frequent at the young age.
Chordoma and teratoma are commonly located at the lower level. For malignant tumors, osteosarcoma, myeloma and chordoma at the sacrum have the mele predilection while MPNST is more frequent in femele population. For benign tumors and tumor-like lesions, epidermoid cyst and teratoma have the obvious femele predilection.
Our results also showed the male predominance was most pronounced among 306 chordomas, accounting for 68.6% and age of 51-70 yrs was the most common of age interval, which was concordant with the previous studies. The peak age in our cohort was the range of 51 to 60 and it was distinctly uncommon in patients younger than 30 years, only accounting for 5.9% (18/306). Furthermore, the peak age of chordoma was higher than that of chondrosarcomas, MPNST and lymphoma.
Chondrosarcomas (CS) rank the second in the incidence of all primary malignant bone tumors, with reporting about 25% incidence and following after osteosarcoma. Regarding age, it is more common in adults between 40-80 years old, and is slightly more common in men [8]. CS constituted over 20.4% of the malignant tumors in Mayo Clinic series and more than two-thirds of chondrosarcomas were in the trunk and the upper ends of the femur and humerus. CS was a relatively rare and the incidence at the sacrum was 1.9% (24/1293) among all chondrosarcomas in Mayo Clinic series [9]. However in our cohort, the analysis of epidemiological feature of CS showed an obvious predilection of male (M: F=1.24:1) and the peak age ranged from 41 to 50 years.
Moreover, chondrosarcoma was the second common histological type at the region of the sacrum for the primary sacral malignant tumors and the incidence of CS among the malignant tumors was 10.3% (74/716).
Although myeloma and lymphoma are not the primary sacral tumors, they are more common lesions at the sacrum in the clinical practice. It is necessary to analyze their epidemiologic features to distinguish them from primary malignant sacral tumors. Myeloma, a tumor of hematopoietic derivation, is the most common primary neoplasm of bone. There were more than 5000 patients with myeloma documented in the Mayo Clinic les. They reported 67.7% were males and the largest concentration of age was in the sixth and seventh decades of life. The well-known rarity of myeloma in patients who were younger than 40 years was shown in the Mayo Clinic series [9]. Likewise, myeloma at the sacrum had the gender predilection (M: F=2.12:1) and 67.9% of all myelomas were males in our series. The median age of myeloma was 54.1±13.2 yrs and the peak age ranged from 41 to 50 years. Only two patients in our cohort were younger than 30 years and no patient was in the rst and second decades of life. Generally, the epidemiological characteristics of sacral myeloma were similar to other boney myeloma. Bone lymphoma is a rare disease. It is estimated that bone lymphoma is accounting for about 5% of extranodal lymphomas and 3-7% of all malignant bone tumors [15].
Parker et al. rstly described the malignant lymphoma of bone and separated it from Ewing sarcoma [16]. The Mayo Clinic database showed the 905 cases of malignant lymphoma comprised as 12.7% of the malignant bone tumors in their series and males predominated at a ratio of 4 to 3 in their cohort. In our cohort, approximately 51.4% of lymphoma occurred in patients between 41-60 years, with a peak incidence in the fth and sixth decade of life. Only 5.7% of the patients with lymphoma were younger than 20 years and no one was younger than 10 years old. However, our series revealed lymphoma at the sacrum had the female predilection (M: F=0.75:1), which was not concordant with the Mayo Clinic experience.
Malignant peripheral nerve sheath tumors (MPNST) include malignant schwannoma (malignant peripheral schwannoma) and neuro brosarcoma. MPNST represents a relatively common subtype of soft tissue sarcoma and is particularly likely to occur in individuals with type 1 neuro bromatosis (NF1) [10][11][12][13]. A SEER database analysis showed 64 MPNST in the spinal location. Their mean age at diagnosis was 50.9 years with more patients in the higher age group and fty-six percent of patients were male. However, it had no description of the incidence at the sacrum [14]. In our cohort, the analysis of epidemiological feature of MPNST showed an obvious predilection of female (M: F=0.68:1) and the peak age ranged from 41 to 50 years. Our result showed the epidemiologic characteristic of age predilection was in concordance with the previous study in SEER database, but gender predilection of MPNST at the sacrum was not similar to the result of SEER database [14].
Ewing sarcoma (ES) and osteosarcoma (OS) are more frequent for young patients. The incidence of ES is a little higher than that of OS at the sacrum in the present study. ES is a distinctive, small, round cell sarcoma that is considered one of the most lethal of all bone tumors. In Mayo Clinic database, ES comprised 8.6% of the total malignant tumors and had a distinct predilection for males (62%). Furthermore, appromimately 75% were in the rst two decades of life and the incidence of ES at the sacrum accounted for 5.9% (36/611) of all patients with ES [9]. The sacrum is not the frequent site for Ewing sarcoma. In our PKUPH database, Ewing sarcoma located at the sixth top of all sacral malignant tumors. The peak incidence was in the second decade (40.4% of all 47 Ewing sarcomas), followed by the third and fourth decades (38.2%) and it had no predilection of gender, which was not concordant with the Mayo Clinic experience. Osteosarcoma is the most common malignant bone tumor. Mayo Clinic series les recorded 1952 osteosarcomas, accounting for 27.5% of all malignant tumors and 19.2% of all bone tumors and approximately 58% of patients with OS were male. In their database, the incidence of OS at the sacrum accounted for 2.0% of all patients with OS and the anatomic site around the knee was the most common site. The second decade was the most common age distribution and among 1952 osteosarcomas, 192 patients were older than 60 years who had the preexisting condition such as Paget disease, previous radiation, infarct, chronic osteomyelitis and cyst of degenerative joint disease as the second peak [9]. According to our results, osteosarcoma at the sacrum was not frequent as the site around the knee and placed as the sixth top of all primary sacral malignant tumors. Although only one patient with the sacral OS was in the rst decade of life, the peak incidence was in the second decade (37.5% of all 40 osteosarcomas), followed by the third decade (32.5%). We noticed no one patient aged more than 60 years. This result demonstrated that the age distribution of sacral osteosarcoma had the peak age of 11-20 and didn't show another age peak of more than 60 years. Moreover, osteosarcoma was not common at the region of sacrum. Thus, when an adolescent patient presents with a malignant tumor, we should take these two histologic types into consideration.
Our PKUPH database showed that giant cell tumor (30.6%, 205/669) was the most frequent in the cohort of benign tumors and tumor-like lesions, followed by neuro broma (23.2%, 155/669) and schwannoma (17.8%, 119/669). The epidemiological characteristics of primary sacral benign tumors were distinct from the one of spinal benign tumors. Some rare histologic types should be considered when the differential diagnosis [19][20][21][22][23][24][25]. It has been reported that hemangioma was the most frequent benign primary spine tumor in one hospital of China, which was different from the region of sacrum and our database showed giant cell tumor was the most common histological type at the sacrum [1]. Hemangioma at the sacrum accounted for 2.1% (14/669) in the cohort of benign tumor and tumor-like lesions. It illustrated that hemangioma was not a frequent tumor at the sacrum, which was different from the epidemiologic characteristics of the mobile spine. It is more important to distinguish some benign histologic types such as giant cell tumor, hemangioma and aneurysmal bone cyst, due to their higher vascularity and intraoperative hemorrhage. Thus, although these lesions are benign, more attention should be paid to the problem of intraoperative blood loss.  [17]. Their epidemiological feature of extremities was different from the characteristics of our sacral GCT and this illustrated that predominate gender may depend on the anatomical site. Our cohort also had 110 cases of female, accounting for 53.7% of all GCTs in our series. Approximately 79.5% of GCT occurred in patients between 21-50 years, with a peak incidence in the third and fourth decade of life. Only 8.8% of the patients with GCT were older than 50 years and only 11.7% of GCT occurred in patients before 20 years old. Furthermore, no one was younger than 10 years old. Thus, it illustrated that giant cell tumors are not common in the adolescent population.
Benign peripheral neurogenic tumors include neuro broma and peripheral schwannoma. Neurogenic tumors arising from the sacrum are rare, with only about 7% of intraspinal neurogenic tumors involving the sacrum.
Neuro bromas can occur in any site, both deep soft sissue and super cial cutaneous lesions. They are usually painless and they are often excised for cosmetic purpose. Individuals with NF1 may have multiple neuro bromas that need excision for functional purpose as well as to exclude the possibility of malignant transformation. The analysis of epidemiological feature of neuro broma in our series showed a little predilection of female (M: F=0.80:1) and the peak age ranged from 41 to 60 years. Meanwhile, schwannomas are relatively common benign lesions of the peripheral nerves, which are thought to derive from Schwann cell.
They affect all age groups and are usually solitary sporadic tumors. Schwannomas can also develop along the spine adjacent to the neural foramina and frequently show a "target sign" on imaging that can be very suggestive of this speci c diagnosis. They are usually slow growing and often discovered incidentally. It has been reported that sacral schwannomas typically occur in middle-aged patients without any predilection for gender. Pennington et al. performed a systematic review and reported presacral schwannoma can reasonably be treated with either en bloc or piecemeal GTR and recurrence was infrequent. In their review, the patients showed no gender predilection [18]. Our database analysis revealed epidemiological feature of schwannoma also had no gender predilection and the peak age ranged from 31 to 60 years. This result was concordant with the result of previous study in the literature. Neuro broma and schwannoma are always located at the higher level of sacrum, which are distinct from the terotoma and we can consider this epidemiologic feature when the differential diagnosis is performed.
There were several limitations in the present study. Firstly, although the present cohort in our tumor center is large, the data drawn from the present analysis may not necessarily re ect the epidemiologic characteristics of the whole nation. Secondly, the present data lacked the information of prognosis for all patients. Thus, in the next step we will collect more information about the oncological prognosis of all patients.
In conclusion, the large cohort of sacral tumors and tumor-like lesions in our database may reveal their epidemiologic characteristics in China and epidemiological feature of sacral tumors and tumor-like lesions is fairly distinct from the mobile spine and extremities.

Con ict of interest statement and ethical approval
The authors declare that they have no con ict of interest. This article does not contain any studies with human participants or animals performed by any of the authors and informed consent was obtained from all individual participants included in the study.

Funding
This work was supported by the Natural Science Foundation of China (NO:81702657).