First case of osteosarcoma in a dinosaur: a multimodal diagnosis

Wednesday, August 5th, 2020  Vol 21 August 2020

Seper Ekhtiari, Kentaro Chiba, Snezana Popovic, Rhianne Crowther, Gregory Wohl, Andy Kin On Wong, Darren H Tanke, Danielle M Dufault, Olivia D Geen, Naveen Parasu, Mark A Crowther, *David C Evans

Osteosarcoma is a primary bone malignancy with a worldwide incidence of 3·4 cases per million people per year. In humans, the incidence of osteosarcoma peaks in the second decade of life, which is thought to be related to the rapid growth velocity of bone at this age. To date, the cause, genetic alterations, oncogenic events, and evolutionary history of osteosarcoma are poorly understood, thus limiting further advancements in genome-informed targeted therapy for osteosarcoma. Paleopathology is limited in dinosaurs because of a loss of soft tissues, and varying states of fossil preservation and postmortem damage. Diagnosis has been further hindered by a hesitation to destructively analyse dinosaur bones given their rarity and uniqueness. Unambiguous diagnosis of malignant cancer in a dinosaur has been elusive. There is a single case of reported metastatic malignancy in a dinosaur based only on radiological and gross morphological examination. The lesion was a suspected Ewing’s sarcoma-like reaction in a hadrosaur (duck-billed dinosaur). The authors did not confirm the diagnosis histologically, nor could they confirm the source of the lesion. Benign neoplasms have been described in a sauropod, with the lesions confirmed macroscopically, radiologically, and by histological analysis. There has also been a case of osteosarcoma reported in a Triassic turtle species based only on gross and radiographic examination.

To date, to our knowledge, no cases of malignant cancer have been reported in dinosaurs with sufficient reliability to be considered confirmed, at least by modern medical standards, which require biopsy and examination at the cellular level. Here we report the first confirmed finding of an osteosarcoma in a dinosaur: a specimen of Centrosaurus apertus, a herbivorous ceratopsian (horned) dinosaur from the Dinosaur Park Formation (AB, Canada) which dates from approximately 77·0–75·5 million years ago. This diagnosis is confirmed through gross, radiographic, and histological analysis (appendix p 1) by experts in musculoskeletal oncology and human pathology, as well as by direct comparison to a confirmed case of human osteosarcoma and a normal Centrosaurus fibula. Given that birds (as living theropod dinosaurs) and reptiles are much more closely related to non-avian dinosaurs than humans, this comparison is only moderately important from a phylogenetic and physiological standpoint. However, given the advanced diagnostics already available for human patients and the highly specialised expertise in human medicine, the comparison is of very high value. Both the pathological (figure; specimen Royal Tyrrell Museum of Palaeontology [TMP] 1989.018.0108) and the normal (specimen TMP 2014.050.0192) fibulae were collected from monodominant Centrosaurus apertus bonebeds within the Dinosaur Park and Oldman formations, respectively, in southern Alberta, Canada. The human specimen (figure) was obtained, with institutional review board approval, from an above-knee amputation surgical specimen. The patient was male, aged 19 years, with previously diagnosed high-grade conventional osteosarcoma on a core biopsy obtained from a radiographically identified lesion in the proximal fibula.

The patient received chemotherapy before the amputation. Gross pathological examination of the human fibula showed a tumour centred within the medullary cavity associated with cortical destruction and a large soft tissue mass (appendix p 2). Radiographic (x-ray, MRI, and CT) examination of the human fibula revealed a Codman triangle surrounding the proximal meta-diaphysis of the fibula, with a large, circumferential soft tissue mass. There was cortical destruction, with ossific spicules extending into the soft tissue mass. Consistent with other human osteosarcomas, the tumour was not fully calcified and was marked with dispersed islands of bone throughout the tumour body. The majority of bone found within the tumour was dispersed as these smaller unconnected islands, mostly exhibiting circular morphology. This finding is in contrast to trabeculae, which are known to show clearer directionality and regular separation patterns aligning with the axis of force application (appendix p 3). Microscopic examination showed neoplastic osteoid and immature bone formation with a permeative pattern replacing the intramedullary cavity and entrapped remnants of host lamellar bone. Although neoplastic cells were not present because of chemotherapy treatment response, the architecture of abnormal neoplastic bone formation and permeation were features sufficient to make a diagnosis of osteosarcoma (figure). The pathological dinosaur specimen (TMP 1989.018.0108) is the distal half to two-thirds of an adult Centrosaurus fibula (appendix p 2) and is a rare case of pathology of a major appendicular bone in this well sampled genus. The bone would have been divided into two pieces during the animal’s life, potentially indicating the presence of a pseudarthrosis, although this diagnosis cannot be confirmed in the absence of the proximal segment. A large morphological abnormality, previously described and attributed to a healing fracture callus, is located in the proximal half of the specimen. Gross morphology, radiological examination, and histological examination by our interdisciplinary team reinterpreted this pathology as an osteosarcoma, with similar confidence as would be required to confirm the diagnosis in a human sample.

Gross examination of the dinosaur specimen showed a mass that takes up the proximal half of the specimen (appendix p 2). The compact bone covering the tumour is very thin in some areas, suggestive of neocortex formation (appendix p 4). In addition, multiple large foramina are visible, consistent with the large and permeable vessels formed during tumour angiogenesis (appendix p 2). The high-resolution x-ray CT scan of the dinosaur fibula shows a Codman triangle, which is indicative of permeation through the cortex with periosteal reaction from the tumour. Evidence of gross cortical and medullary involvement with complete disruption of the cortex is reminiscent of high-grade osteosarcomas. The morphology of the primary tumour appears semiporous like trabecular bone, but its organisation is highly irregular with a lack of anisotropy, suggestive of a pathology that violates patterns of bone growth dictated by Wolff’s law. Sclerotic regions within the lesion are reminiscent of abnormal new bone formation. These findings are consistent with, and similar to, the findings in the human specimen. Histological examination similarly showed a lack of zonation with bone maturation that would be seen if the lesion were a fracture callus and provided evidence that the tumour extends and permeates proximodistally throughout the cortex of the
bone. Again, this bidirectional and distant invasion is not consistent with a fracture callus. The gross, radiographic, and histological appearance of the dinosaur fibula we describe are very similar to the known human osteosarcoma, despite the lack of preserved soft tissue structures in the dinosaur fibula. Other differential diagnoses were rejected. Specifically, these diagnoses included (1) fracture, which was rejected because of the extension of abnormal bone very distal to the lesion despite more normal cortex in this location and the lack of a proximal fracture fragment; (2) osteomyelitis, which was# rejected because of a lack of characteristic pockmarking and the bone-forming nature of the lesion; (3) nonossifying fibroma, which was rejected because of the highly disorganised bone and no well defined zone of transition; and (4) osteochondroma, which was rejected given that the lesion clearly does not arise from or near a physis. Thus, our findings indicate the presence of an osteosarcoma, meeting all the criteria discussed in terms of the morphological,
radiological, and histological confirmation of this diagnosis. The extensive invasion of the cancer throughout the bone suggests that it persisted for a substantial period of the animal’s life and might have invaded other body systems.

A similarly advanced osteosarcoma in a human patient, left untreated, would certainly be fatal. In conclusion, to our knowledge, we provide the first confirmed case of malignant bone cancer in a dinosaur. Our findings highlight the need for multimodal analysis of unclear or uncharacteristic lesions in extinct vertebrae. This approach is not necessary for all simple and characteristic lesions. However, unusual or uncharacteristic lesions should be considered for further diagnostic using this multimodal approach, including those previously described before the availability and accessibility of modern diagnostic techniques. We anticipate that this approach of multimodal analysis at all available levels (gross morphology, radiological examination, and histological examination) will facilitate the identification of additional illnesses and injuries in other ancient species. Evidence suggests that malignancies, including bone cancers, are rooted quite deeply in the evolutionary history
of organisms.