What happens when a black field is not a significant fraction of an unblocked field?

When a black field is not a significant fraction of an unblocked field, the effective primary radiation reaching the patient is negligible. In radiation therapy, a "black field" typically refers to an area blocked from receiving radiation or shielded from the radiation beam, whereas the "unblocked field" represents the area where the radiation is freely delivered. If the black field occupies a minimal portion of the unblocked field, it means that the primary radiation generated has a vast area it can impact, and thus the influence of the blocked area on the total dose delivered to the patient is minimal.

In this scenario, the majority of the radiation still travels through the unblocked fields, and therefore, while there might be some minor disruptions caused by the black field, it does not significantly alter the primary radiation that reaches the patient. As a result, the effective dose of primary radiation to the treated area is considered negligible, confirming that the vast majority of the radiation is still directed toward the intended treatment volume. The effectiveness of treatment remains largely intact since the therapy is still primarily delivered to the target areas, and the black field's impact does not impede the overall dose distribution significantly.