If the shutter timer error is negative, how should the treatment time be set on the unit?

When dealing with a negative shutter timer error, it indicates that the actual treatment time delivered is shorter than the intended treatment time calculated. To adjust for this discrepancy and ensure that the correct dose is delivered to the patient, the negative error must be subtracted from the computed beam on time. By doing so, you are effectively increasing the treatment time to compensate for the under-delivered dose caused by the negative error.

This method allows for accurate calibration of the treatment session, ensuring that the patient receives the full prescribed radiation dose. In this context, subtracting the negative timer error rectifies the situation, allowing for the necessary adjustments to the total treatment time.