This report, compiled by a team of senior experts, contains the assessment of a radiological accident which led to serious overexposure of radiotherapy patients in Panama. From August 2000 to February 2001, due to a calculation error in the data entry of the treatment plan, the patients were treated with a dose 100% higher than the regular dose. The report evaluates the doses incurred, undertakes a medical evaluation of the affected patients’ prognosis and treatment, and closes with a number of findings, conclusions and lessons to be learned. Contents: Executive summary; 1. Introduction; 2. Background information; 3. The accidental exposure; 4. The response to the accidental exposure; 5. Dose assessment; 6. Test of the computer software using different approaches for data entry; 7. Medical assessment; 8. Findings, conclusions and lessons to be learned; Annex I: Termination report to the contact points; Annex II: Literature review of radiation effects in pertinent tissues in this accidental exposure; Annex III: Data on individual patients involved in this accidental exposure.