Canadian consensus on TRK-inhibitor therapy for NTRK fusion-positive sarcoma.

Malignant sarcomas are rare accounting for <1% of all adult solid malignancies and approximately 11% to 13% of all pediatric malignancies. TRK-inhibitors have demonstrated robust and long-lasting responses in patients with NTRK fusion-positive solid tumors, including sarcoma. Access to these agents in many jurisdictions such as Canada remains limited. We undertook a modified Delphi consensus to articulate and convey the clinical importance of these agents for the Canadian sarcoma community. A systematic search of published and presented literature was conducted to identify clinical trials reporting outcomes on the use of TRK-inhibitors in relapsed/refractory NTRK fusion-positive sarcoma. Three main consensus questions were identified: (a) is there currently an unmet clinical need for systemic therapy options in relapsed/refractory sarcoma? (b) do TRK-inhibitors confer a clinical benefit to patients with NTRK fusion-positive sarcoma? (c) do phase I/II basket trials provide sufficient evidence to justify funding of TRK-inhibitors in NTRK fusion-positive sarcoma? Response rates to the first and second surveys were 57% (n = 30) and 42% (n = 22), respectively. There was strong agreement among the Canadian sarcoma community that there was unmet clinical need for effective systemic therapy options in relapsed/refractory sarcoma, that TRK-inhibitors are a safe and effective treatment option for patients with NTRK fusion-positive sarcoma, and that available phase I/II basket trials provide sufficient evidence to support funding of these agents in relapsed/refractory NTRK fusion-positive sarcoma. TRK-inhibitors are a safe and effective systemic therapy option for patients with relapsed/refractory NTRK fusion-positive sarcoma.

Solitary fibrous tumor: molecular hallmarks and treatment for a rare sarcoma.

Solitary fibrous tumor (SFT) is a rare soft tissue sarcoma subtype which mainly affects adults in the fifth and sixth decades of life. Originally part of a spectrum of tumors called hemangiopericytomas, classification has been refined such that SFTs now represent a distinct subtype. The identification of NAB2-STAT6 fusion in virtually all SFTs has further aided to define this rare subgroup. SFTs have a spectrum of behavior from benign to malignant, with evidence suggesting risk of metastases related to age at diagnosis, extent of necrosis, mitotic rate and tumor size. The standard treatment for localized disease is surgical excision with or without radiotherapy. Retrospective and prospective evidence suggests antiangiogenic treatment is effective for unresectable disease. Further translational work is required to understand the biology driving the differential behavior and identify more effective treatments for patients with metastatic disease.

A Timely Oral Option: Single-Agent Vinorelbine in Desmoid Tumors.

INTRODUCTION: Desmoid tumors (DT) are rare collagen-forming tumors that can exhibit locally aggressive patterns of behavior. The aim of this study was to evaluate the efficacy and safety of treatment of DT with single-agent oral vinorelbine. MATERIALS AND METHODS: A retrospective review of patients treated with vinorelbine 90 mg orally on days 1, 8, and 15 of a 28-day cycle from January 2004 to July 2019 was performed. Response was assessed using RECIST version 1.1. Descriptive statistics were employed. RESULTS: A total of 29 patients were included. Response rate was 20.7% (6/29), and clinical benefit rate (response by RECIST 1.1 and/or clinical symptom improvement) was 65.5% (19/29). No patient experienced grade 3 or above toxicity. Common toxicities were grade 1-2 nausea (14/26, 48.3%), fatigue (9/26, 31.0%), and diarrhea (4/26, 13.8%). CONCLUSION: Single-agent oral vinorelbine is an effective, safe, and well-tolerated treatment for DT. It represents a new oral alternative for management of DT.

Avapritinib in the treatment of PDGFRA exon 18 mutated gastrointestinal stromal tumors.

Gastrointestinal stromal tumors (GIST) can be molecularly classified based on different subtypes including mutations in KIT and PDGFRA. Patients with PDGFRA mutations are an important subgroup that commonly arise in the stomach and are associated with a more indolent disease course. Importantly, the most common PDGFRA molecular subtype, the D842V mutation in exon 18 of the gene which alters the activation loop, is imatinib insensitive in in vitro studies. Poor responses to imatinib have been seen clinically compared with PDGFRA exon 18 non-D842V-mutated GIST. Avapritinib (BLU-285) is a potent KIT and PDGFRA-specific tyrosine kinase inhibitor which has shown >90% response rates in patients with PDGFRA exon 18 D842V-mutated GIST. Results from the Phase I trial of avapritinib have indicated that this drug should be the standard of care for patients with PDGFRA exon 18 D842V-mutated GIST.

The Role of Imaging in the Diagnosis of Primary and Secondary Breast Angiosarcoma: Twenty-Five-Year Experience of a Provincial Cancer Institution.

BACKGROUND: Breast angiosarcoma may arise spontaneously (primary breast angiosarcoma (PBA)) or may arise secondary to a biological insult, such as radiation therapy (secondary breast angiosarcoma (SBA)). We evaluated the imaging findings of patients diagnosed with PBA and SBA within the province of British Columbia, Canada. MATERIALS AND METHODS: This was a multi-center, retrospective study of patients diagnosed with PBA and SBA over a 25-year period. Patients were identified via a provincial database which registers all cases of sarcoma. Patients diagnosed with histologically proven PBA and SBA were eligible for inclusion. Multimodal breast imaging reviewed included mammography, ultrasound, magnetic resonance imaging, and computed tomography. RESULTS: Thirteen patients were diagnosed with PBA and 22 patients were diagnosed with SBA. The median (interquartile range (IQR)) age of patients diagnosed with PBA (45.5 years (19.7 years)) was less than that of patients diagnosed with SBA (75.8 years (13.8 years), P < .001). Patients diagnosed with PBA (90.9%) were more likely to present with a parenchymal mass clinically and radiographically than those with SBA (28.6%, P < .002). Patients diagnosed with SBA (71.4%) were more likely to present with cutaneous findings than patients diagnosed with PBA (0.0%, P < .05). Without specific clinical context, the imaging findings of PBA and SBA were observed to be non-specific. CONCLUSION: This is the only study which evaluated the imaging findings of patients diagnosed with PBA and SBA within a large, defined geographical area. Given non-specific imaging findings, awareness of the disease and clear and timely communication between radiologists and clinicians is required to ensure appropriate diagnosis and management.

Survival Outcomes of Ewing Sarcoma and Rhabdomyosarcoma by High- versus Low-Volume Cancer Centres in British Columbia, Canada.

Due to the rarity and complexity of treatment for Ewing sarcoma and rhabdomyosarcoma, studies demonstrate improved patient outcomes when managed by a multidisciplinary team at high-volume centres (HVCs). Our study explores the difference in outcomes of Ewing sarcoma and rhabdomyosarcoma patients based on the centre of initial consultation in British Columbia, Canada. This retrospective study assessed adults diagnosed with Ewing sarcoma and rhabdomyosarcoma between 1 January 2000 and 31 December 2020 undergoing curative intent therapy in one of five cancer centres across the province. Seventy-seven patients were included, 46 seen at HVCs and 31 at low-volume centres (LVCs). Patients at HVCs were younger (32.1 vs. 40.8 years, p = 0.020) and more likely to receive curative intent radiation (88% vs. 67%, p = 0.047). The time from diagnosis to first chemotherapy was 24 days shorter at HVCs (26 vs. 50 days, p = 0.120). There was no significant difference in overall survival by treatment centre (HR 0.850, 95% CI 0.448-1.614). Variations in care exist amongst patients treated at HVCs vs. LVCs, which may reflect differences in access to resources, clinical specialists, and varying practice patterns across centres. This study can be used to inform decisions regarding triaging and centralization of Ewing sarcoma and rhabdomyosarcoma patient treatment.

Real-world experience of tyrosine kinase inhibitors in children, adolescents and adults with relapsed or refractory bone tumours: A Canadian Sarcoma Research and Clinical Collaboration (CanSaRCC) study.

OBJECTIVES: We conducted a retrospective multi-centre study to assess the real-world outcome of regorafenib (REGO) and cabozantinib (CABO) in recurrent/refractory bone tumours (BTs) including osteosarcoma (OST), Ewing sarcoma (EWS) and chondrosarcoma (CS)/extra-skeletal mesenchymal CS (ESMC). METHODS: After regulatory approval, data from patients with recurrent BT (11 institutions) were extracted from CanSaRCC (Canadian Sarcoma Research and Clinical Collaboration) database. Patient characteristics, treatment and outcomes were collected. Progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan-Meier method. RESULTS: From July 2018 to May 2022, 66 patients received REGO or CABO; 39 OST, 18 EWS, 4 CS and 5 ESMC. Median age was 27.8 years (range 12-76); median starting dose was 60 mg for CABO (n = 37, range 40-60) and 120 mg for REGO (n = 29, range 40-160). Twenty-eight (42.4%) patients required dose reduction: hand-foot syndrome 7 (10.6%), nausea/vomiting 1 (1.5%), diarrhoea 1 (1.5%), 2 elevated LFTs (3%), elevated bilirubin 1 (1.5%) and mucositis 1 (1.5%). The median OS for patients with OST, EWS, CS and ESMC was 8.5 months (n = 39, 95% CI 7-13.1); 13.4 months (n = 18, 95% CI 3.4-27.2), 8.1 (n = 4, 95% CI 4.1-9.3) and 18.2 (n = 5, 95% CI (10.4-na), respectively. Median PFS for OST, EWS, CS and ECMS was 3.5 (n = 39, 95% CI 2.8-5), 3.9 (n = 18, 95% CI 2.1-5.9), 5.53 (n = 4. 95% CI 2.13-NA) and 11.4 (n = 5, 95% CI 1.83-14.7), respectively. Age, line of therapy, REGO versus CABO, or time from diagnosis to initiation of TKI were not associated with PFS on univariable analysis. CONCLUSION: Our real-world data show that TKIs have meaningful activity in recurrent BT with acceptable toxicities when started at modified dosing. Inclusion of TKIs in earlier lines of treatment and/or maintenance therapy could be questions for future research.

Fibroadenoma in vulval ectopic breast tissue in a patient with PTEN Hamartoma Tumour Syndrome.

PTEN is a tumour suppressor gene involved in regulating cell division. Pathogenic germline variants in PTEN predispose to benign and malignant growths of numerous organs, including of the breast. In the following report, we describe the first documented case of a fibroadenoma developing in ectopic breast tissue of the vulva in a patient with a germline pathogenic variant in PTEN. This highlights the risk of hyperplasia developing in any breast tissue, including rare ectopic sites, particularly in patients with underlying germline variants in cancer susceptibility genes.

The perplexing role of immuno-oncology drugs in osteosarcoma.

Osteosarcoma is a rare, primary tumour of bone. Curative treatment consists of multi-agent chemotherapy and complete surgical resection. Despite the use of multi-agent chemotherapy, the risk of recurrence is high. Survival outcomes for patients with osteosarcoma have not changed since the 1980's. Based on biologic rationale, there has been interest in adding immunotherapies to upfront curative intent chemotherapy, including mifamurtide (a macrophage activator) and interferon. However, results to date have been disappointing. In the metastatic setting, checkpoint inhibitors alone have not proven effective. Ongoing translational work is needed to further understand which patients may benefit from immune-oncology approaches with standard cytotoxic chemotherapy.

Future Directions in the Treatment of Osteosarcoma.

Osteosarcoma is the most common primary bone sarcoma and is often diagnosed in the 2nd-3rd decades of life. Response to the aggressive and highly toxic neoadjuvant methotrexate-doxorubicin-cisplatin (MAP) chemotherapy schedule is strongly predictive of outcome. Outcomes for patients with osteosarcoma have not significantly changed for over thirty years. There is a need for more effective treatment for patients with high risk features but also reduced treatment-related toxicity for all patients. Predictive biomarkers are needed to help inform clinicians to de-escalate or add therapy, including immune therapies, and to contribute to future clinical trial designs. Here, we review a variety of approaches to improve outcomes and quality of life for patients with osteosarcoma with a focus on incorporating toxicity reduction, immune therapy and molecular analysis to provide the most effective and least toxic osteosarcoma therapy.

What Matters to Us: Impact of Telemedicine During the Pandemic in the Care of Patients With Sarcoma Across Scotland.

PURPOSE: In Scotland, approximately 350 sarcoma cases are diagnosed per year and treated in one of the five specialist centers. Many patients are required to travel long distances to access specialist care. The COVID-19 pandemic brought a number of rapid changes into the care for patients with cancer, with increasing utilization of telemedicine. We aimed to evaluate how the utilization of telemedicine affects professionals and patients across Scotland and care delivery, at the Beatson West of Scotland Cancer Centre Sarcoma Unit. METHODS: Between June 8 and August 25, 2020, we invited patients and professional sarcoma multidisciplinary team members to participate in separate online anonymous survey questionnaires, to assess their attitudes toward telemedicine. Data were extracted, and descriptive statistics were performed. RESULTS: Patient satisfaction (n = 64) with telemedicine was high (mean = 9.4/10) and comparable with traditional face-to-face appointments (mean = 9.5/10). Patients were receptive to the use of telemedicine in certain situations, with patients strongly opposed to being told bad news via telemedicine (88%). Providers recommended the use of telemedicine in certain patient populations and reported largely equivalent workloads when compared with traditional consultations. Providers reported that telemedicine should be integrated into regular practice (66%), with patients echoing this indicating a preference for a majority of telemedicine appointments (57%). CONCLUSION: Telemedicine in sarcoma care is favorable from both clinician and patient perspectives. Utilization of telemedicine for patients with rare cancers such as sarcomas is an innovative approach to the delivery of care, especially considering the time and financial pressures on patients who often live a distance away from specialist centers. Patients and providers are keen to move toward a more flexible, mixed system of care.

Gastrointestinal leiomyosarcoma demonstrate a predilection for distant recurrence and poor response to systemic treatments.

BACKGROUND: Primary leiomyosarcoma (LMS) of the gastrointestinal (GI) tract is rare. Limited literature exists regarding the clinical characteristics and outcome for patients with localised and metastatic disease. METHODS: A retrospective chart review was performed for patients greater than 18 years of age diagnosed with GI LMS at The Royal Marsden Hospital between 1 January 2000-1 May 2020. Descriptive statistics were performed. Patients were censored at data cut-off date of 27 June 2020. RESULTS: Forty-six patients with a median age at diagnosis of 54 years (range 25-85) were identified. Fifteen percent (n = 7) of patients previously received abdominal radiation for an unrelated cancer. All patients with localised disease (n = 36) had resection with oncological margins. For patients who underwent potentially curative surgery, median recurrence-free survival (mRFS) was 13 months (0.4-183 months), and half of these patients (n = 18) developed recurrent disease post resection (distant n = 16, local n = 2). Median overall survival (mOS) was 27 months for patients with distant recurrence. Twenty-one percent (n = 10) of patients presented with synchronous metastatic disease and their mOS was 19 months. Median progression-free survival (mPFS) for patients treated with conventional chemotherapy ranged from 2.0 to 8.0 months. CONCLUSION: The risk of recurrence is significant, and recurrence-free survival was short even with complete oncologic resection. The relationship of prior abdominal radiotherapy to the development of GI LMS warrants further investigation. Outcomes with systemic therapy for metastatic disease were poor and there is a need for the development of more effective systemic therapies.

Systemic therapies in advanced epithelioid haemangioendothelioma: A retrospective international case series from the World Sarcoma Network and a review of literature.

BACKGROUND: This observational, retrospective effort across Europe, US, Australia, and Asia aimed to assess the activity of systemic therapies in EHE, an ultra-rare sarcoma, marked by WWTR1-CAMTA1 or YAP1-TFE3 fusions. METHODS: Twenty sarcoma reference centres contributed data. Patients with advanced EHE diagnosed from 2000 onwards and treated with systemic therapies, were selected. Local pathologic review and molecular confirmation were required. Radiological response was retrospectively assessed by local investigators according to RECIST. Progression free survival (PFS) and overall survival (OS) were estimated by Kaplan-Meier method. RESULTS: Overall, 73 patients were included; 21 had more than one treatment. Thirty-three patients received anthracyclines regimens, achieving 1 (3%) partial response (PR), 25 (76%) stable disease (SD), 7 (21%) progressive disease (PD). The median (m-) PFS and m-OS were 5.5 and 14.3 months respectively. Eleven patients received paclitaxel, achieving 1 (9%) PR, 6 (55%) SD, 4 (36%) PD. The m-PFS and m-OS were 2.9 and 18.6 months, respectively. Twelve patients received pazopanib, achieving 3 (25%) SD, 9 (75%) PD. The m-PFS and m-OS were.2.9 and 8.5 months, respectively. Fifteen patients received INF-α 2b, achieving 1 (7%) PR, 11 (73%) SD, 3 (20%) PD. The m-PFS and m-OS were 8.9 months and 64.3, respectively. Among 27 patients treated with other regimens, 1 PR (ifosfamide) and 9 SD (5 gemcitabine +docetaxel, 2 oral cyclophosphamide, 2 others) were reported. CONCLUSION: Systemic therapies available for advanced sarcomas have limited activity in EHE. The identification of new active compounds, especially for rapidly progressive cases, is acutely needed.

A retrospective review of 145 patients with angiosarcoma: Radiation therapy, extent of resection and chemotherapy are important predictors of survival.

Angiosarcoma is a subset of soft-tissue sarcomas with poor 5-year survival rate. Given its rarity, limited large cohort data is available for this disease. Therefore, the present study evaluated data from patients with angiosarcoma treated at a provincial Institution (BC Cancer) to determine potential modifiable predictors of survival. A retrospective review of patients across British Columbia (Canada) was conducted at the Sarcoma Outcome Unit of BC Cancer from January 1, 1969 to September 19, 2017. Cox proportional hazard models were used to calculate hazard ratios (HR) for the overall survival (OS) and progression free survival (PFS) of patients. A total of 145 patients with angiosarcoma were identified, of which 68 were metastatic/unresectable at presentation. Of the 145 patients included, 38 received chemotherapy, with 15 receiving taxane. A single patient received chemotherapy in a neoadjuvant setting. Of the resectable patients, 71 had first line surgery and 38 had curative-intent radiation during their treatment. Of the study cohort, 38 patients received prior radiation for an unrelated cancer and 4 patients had pre-existing chronic lymphedema. Resectable disease (HR, 0.22; P<0.01), first treatment with either surgery (HR, 0.08; P<0.01), radiation (HR, 0.19; P<0.01) or chemotherapy (HR, 0.22; P<0.01) were predictors of improved OS. First line surgery resulted in improved OS (HR, 0.36; P<0.01) and PFS (HR, 0.48; P<0.01). In addition, OS was positively impacted by the extent of surgery [complete (R0) vs. microscopic residual tumor (R1); HR, 0.26; P<0.01; R0 vs. macroscopic residual tumor (R2); HR, 0.08; P<0.01) resection. Extent of surgery and any radiation treatment were determined to be important predictors of OS. The results also revealed that patient outcome was improved following any treatment compared with supportive care alone. In conclusion, multidisciplinary care is critical for the treatment of patients diagnosed with angiosarcoma.

Distinct Features of Psychosocial Distress of Adolescents and Young Adults with Cancer Compared to Adults at Diagnosis: Patient-Reported Domains of Concern.

Purpose: Adolescents and young adults (AYA) (18-40) are a population of patients with cancer, who have distinctive developmental and psychosocial pressures. Using validated distress screening tools, we investigated psychosocial needs of AYA compared to older adults with cancer at diagnosis. Methods: AYA and older adult patients from British Columbia, Canada, between 2011 and 2016, who completed the Canadian Problem Checklist (CPC) and the PsychoSocial Screen for Cancer-Revised (PSSCAN-R) within 6 months of their cancer diagnosis were included in the study. Emotional, informational, physical, practical, social, and spiritual domain concerns are identified using the CPC. Psychosocial needs and distress are evaluated using the PSSCAN-R. Baseline demographics were obtained from the cancer registry. Based on gender, primary tumor site, and presence of metastasis, a 3:1 case match was performed with older adults (>40 years old). Statistical analyses included Chi square and Fisher's exact tests. Results: Two thousand and forty five AYA were case matched with 6050 older adults. Majority of patients were female (61.9%), and at diagnosis, 12.1% had metastatic disease. Top three tumor types were breast (20.4%), lymphoma (11.5%), and gastrointestinal (10.8%). The top five concerns for AYA (% AYA, % adults) were fear/worry (56.6, 42.9), understanding of illness (47.6, 41.4), sleep (35.2, 28.9), sadness (34.1, 20.0), and finances (33.8, 15.0). AYA reported higher symptoms of anxiety at baseline (% AYA, % older adults), both moderate (26.0, 19.9) and severe (26.6, 17.1) p < 0.01. Conclusion: Significant differences in psychosocial needs for AYA were seen at diagnosis across multiple domains, specifically, higher emotional, informational, physical, and financial distress. Development of supportive programming geared toward these domains early at diagnosis could benefit this distinct population.

Paraneoplastic dermatomyositis associated with metastatic leiomyosarcoma of unknown primary.

BACKGROUND: Sarcomas are rare and heterogeneous tumours of mesenchymal origin, with over 100 histological subtypes. Paraneoplastic dermatomyositis has rarely been described in sarcoma. This is the first documented case of paraneoplastic dermatomyositis in a patient with metastatic leiomyosarcoma. CASE PRESENTATION: A 43-year-old female diagnosed with metastatic leiomyosarcoma of unknown primary presented with a mild rash in sun-exposed areas of her face and upper chest, with no other neuromuscular symptoms. This rash resolved with systemic treatment with doxorubicin for metastatic leiomyosarcoma. Imaging assessment confirmed overall stable disease after chemotherapy completion. She presented acutely 2 months later with new onset rash in a shawl-like distribution, periorbital oedema and proximal muscle weakness. Based on the characteristic cutaneous signs and symmetrical proximal muscle weakness, abnormal electromyography and raised skeletal muscle enzymes with a positive anti-transcription intermediary factor-1 gamma antibody result, a diagnosis of paraneoplastic dermatomyositis was made. Re-evaluation of her metastatic leiomyosarcoma revealed disease progression. Second-line chemotherapy was commenced once the dermatomyositis was controlled on steroid therapy. Systemic anti-cancer therapy was again associated with mild improvement in dermatomyositis symptoms. DISCUSSION: Paraneoplastic dermatomyositis heralded disease progression after first-line chemotherapy; however, in hindsight, subtle cutaneous features were present at sarcoma diagnosis. The temporal relationship between paraneoplastic dermatomyositis and metastatic leiomyosarcoma is key in this case, as fluctuations in dermatomyositis severity correlated with growth of metastatic disease. Understanding this relationship may provide clues for tumour progression and prompt timely initiation of anti-cancer therapy. It is important to recognise that in addition to the more common cancers associated with paraneoplastic dermatomyositis, it can also occur in rarer tumours such as leiomyosarcoma.

Telemedicine During the COVID-19 Pandemic: Impact on Care for Rare Cancers.

PURPOSE: Many patients with cancer, often those with rare cancers such as sarcomas, travel long distances to access expert care. The COVID-19 pandemic necessitated widespread changes in delivery of cancer care, including rapid adoption of telemedicine-based care. We aimed to evaluate the impact of telemedicine on patients, clinicians, and care delivery at the Royal Marsden Hospital (RMH) Sarcoma Unit during the pandemic. METHODS: Data were extracted from patient records for all planned outpatient appointments at the RMH Sarcoma Unit from March 23 to April 24, 2020. Patients and clinicians completed separate questionnaires to understand their experiences. RESULTS: Of 379 planned face-to-face appointments, 283 (75%) were converted to telemedicine. Face-to-face appointments remained for patients who needed urgent start of therapy or performance status assessment. Patients lived on average > 1.5 hours from RMH. Patient satisfaction (n = 108) with telemedicine was high (mean, 9/10), and only 48% (n = 52/108) would not want to hear bad news using telemedicine. Clinicians found telemedicine efficient, with no associated increased workload, compared with face-to-face appointments. Clinicians indicated lack of physical examination did not often affect care provision when using telemedicine. Most clinicians (n = 17; 94%) believed telemedicine use was practice changing; congruently, 80% (n = 86/108) of patients desired some telemedicine as part of their future care, citing reduced cost and travel time. CONCLUSION: Telemedicine can revolutionize delivery of cancer care, particularly for patients with rare cancers who often live far away from expert centers. Our study demonstrates important patient and clinician benefits; assessment of longer-term impact on patient outcomes and health care systems is needed.

FAM46C/TENT5C functions as a tumor suppressor through inhibition of Plk4 activity.

Polo like kinase 4 (Plk4) is a tightly regulated serine threonine kinase that governs centriole duplication. Increased Plk4 expression, which is a feature of many common human cancers, causes centriole overduplication, mitotic irregularities, and chromosomal instability. Plk4 can also promote cancer invasion and metastasis through regulation of the actin cytoskeleton. Herein we demonstrate physical interaction of Plk4 with FAM46C/TENT5C, a conserved protein of unknown function until recently. FAM46C localizes to centrioles, inhibits Plk4 kinase activity, and suppresses Plk4-induced centriole duplication. Interference with Plk4 function by FAM46C was independent of the latter's nucleotidyl transferase activity. In addition, FAM46C restrained cancer cell invasion and suppressed MDA MB-435 cancer growth in a xenograft model, opposing the effect of Plk4. We demonstrate loss of FAM46C in patient-derived colorectal cancer tumor tissue that becomes more profound with advanced clinical stage. These results implicate FAM46C as a tumor suppressor that acts by inhibiting Plk4 activity.