| Document and Entity Information | Apr. 09, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Document Period End Date | Apr. 09, 2024 |
| Entity Registrant Name | Fusion Pharmaceuticals Inc. |
| Entity Incorporation State Country Code | Z4 |
| Entity File Number | 001-39344 |
| Entity Tax Identification Number | 00-0000000 |
| Amendment Flag | true |
| Entity Central Index Key | 0001805890 |
| Entity Address, Address Line One | 270 Longwood Rd., S. |
| Entity Address, City or Town | Hamilton |
| Entity Address, State or Province | ON |
| Entity Address, Country | CA |
| Entity Address, Postal Zip Code | L8P 0A6 |
| City Area Code | 289 |
| Local Phone Number | 799-0891 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common shares, no par value per share |
| Trading Symbol | FUSN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Amendment Description | On April 9, 2024, Fusion Pharmaceuticals Inc. (the “Company”) presented a poster at the American Association for Cancer Research Annual Meeting 2024 (the “Original AACR Poster”), which showed interim efficacy and safety data from the Phase 2 TATCIST open-label clinical trial evaluating FPI-2265, an actinium-225 based PSMA-I&T targeting radioconjugate (“RC”) for the treatment of metastatic castration-resistant prostate cancer. The Original AACR Poster noted that a ≥50% decline in prostate-specific antigen by 12 weeks after first treatment (“PSA50”) was achieved in 61% of lutetium-naïve participants and 42% of lutetium-treated participants. PSA50 was actually achieved in 54% of lutetium-naïve participants and 43% of lutetium-treated participants. All other data contained in the Original AACR Poster remains unchanged. |
