CRC Daily: Costa Rica's 1996 radiotherapy tragedy

A medical mistake resulted in deaths, and important lessons for healthcare

On August 22, 1996, a worker at San Juan de Dios Hospital in San José, Costa Rica, conducted a routine change of the Cobalt-60 source in the Alcyon II radiotherapy unit.

As he calibrated the machine to the new source, the radiophysicist made an error when calculating the dose rate. The mistake went unnoticed as the Alcyon II resumed operations with cancer patients on August 26.

It would take more than a month for the grave error to be caught. Over those 32 days, 115 patients received treatment on the mis-calibrated machine. At least a dozen deaths and many more permanent injuries are linked to the mistake.

It was — and remains — the worst medical tragedy in Costa Rica’s history.


It took the sharp eye of a radiation oncologist at Calderón Guardia Hospital, also in San José, to recognize there might be a problem. He noticed unusually severe effects in some of his patients who had been treated with the Alcyon II unit and followed up on the observation.

The error was tragically simple. 0.3 units on the Alcyon II’s control panel typically correspond to 0.3 minutes, or 18 seconds. But the August 22 (mis)calculation instead equivocated 0.3 units with 30 seconds.

The exposure time had been overestimated by a factor of 30/18 = 1.66. As a result, the dose rate would have been underestimated by the same factor, and therefore the dose to patients would consequently have been higher than that intended.

On September 27, the person in charge of dosimetry at San Juan de Dios realized the problem and contacted the Ministry of Health, which immediately shut down the Alcyon II unit. But by then, it had caused irreparable damage.

Many of the overexposed patients were already displaying acute reactions such as skin ulcerations, severe mucositis, nausea, vomiting and diarrhea. Over the coming months, many of the more chronic illnesses had begun to manifest, including permanent damage to the CNS, the skin, the GI system, and the cardiovascular system

According to an International Atomic Energy Agency (IAEA) report from that year:

  • A number of the patients examined will be at risk of brain necrosis or loss of hearing (and in at least one case, blindness) for years to come.

  • About 10% of the total number of patients are at very high risk of spinal cord effects; some are already paralysed. 

  • Radiation induced changes in the heart have been reported in patients.

In 2001, the radiophysicist whose mistake caused the radiation overdoses was charged with 16 culpable homicides and sentenced to six years in prison.


Costa Rica’s 1996 radiotherapy incident was a tragedy, but it provided the country — and the world — with valuable lessons.

On Costa Rica’s request, the IAEA conducted an expert assessment of the events, and their recommendations have been “applied to accident prevention in radiotherapy worldwide.”

Those recommendations included additional educational programs for radiotherapy staff; the implementation of an ongoing quality-assurance programs; and, perhaps most importantly, redundancy.

As the report noted, “human error is the most common cause of radiation accidents” — as it was in Costa Rica, where poor record-keeping and relying on a single person’s calculations elevated risks.

Because there was no redundant and independent calibration of the Alcyon II unit, it opened the door “for a mistake to remain undiscovered until it resulted in an accident.”

While no system is perfect, and civilian radiation accidents still occur, Costa Rica’s radiotherapy tragedy has hopefully made medical care safer for patients worldwide.

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