To delineate the clinical topography of peripherally inserted central catheter (PICC)-related thrombosis in cancer patients.Most of the clinical features of PICC-related thrombosis are based on a single follow-up, which is insufficient to reflect the full topography of a thrombosis.This is an observational study conducted at West China Hospital, according to the STROBE guidelines.Cancer patients scheduled for PICC placement were potentially eligible; patients with contraindications to PICC placement or existing diseases affecting blood flow were excluded; and those who later withdrew or did not reply to our contact request during the follow-up period were eliminated from this study. Ultrasound was used to detect thrombosis from the insertion site, proximal insertion site, axillary vein to the subclavian vein once every two days for two weeks post insertion. The thrombosis and its involved venous segments, onset time and symptoms and signs were recorded.Among the 173 included patients, 126 (72.8 %) were identified as having thrombosis. Specifically, 113 and 126 patients were identified as having thrombosis within the first three days and the first week post insertion, respectively. In the 126 patients, thrombosis occurred at the insertion site (72.8 %) concurrently with thrombosis at the proximal insertion site (n = 120, 69.4 %), thrombosis in the axillary vein (n = 94, 54.3 %), and/or thrombosis in the subclavian vein (n = 41, 23.7 %). The log-rank test demonstrated that thrombosis in these four venous segments decreased significantly from the distal to the proximal central vein (log-rank test = 117.128, P < .001). Of 31 patients (17.9 %) who presented symptomatic thrombosis, only five patients experienced obvious swelling in the upper arm, and the other 26 patients exhibited atypical symptoms, such as soreness, tightness, numbness, tingling, or other discomforts in the palm, arm, armpit, and/or shoulder. In some thrombotic cases, ultrasonic assessment of PICC-related thrombosis did not parallel clinical symptoms and signs.PICC-related thrombosis is common and can occur very early post insertion in cancer patients, and most thromboses present atypical symptoms. More than half of the cases with thrombosis evaluated involve multiple venous segments, and the farther the venous segments are from the central vein, the higher the incidence of thrombosis tend to be and the earlier the onset time are.The results highlight the importance that medical staff pay particular attention to patients with catheters in the first week post insertion and be alert to thrombosis presenting atypical symptoms while keeping in mind that clinical symptoms and signs are not reliable for diagnosing thrombosis.Clinical Trials ChiCTR1900024890.Copyright © 2023. Published by Elsevier Ltd.