When calculating the dose at 10 cm depth for a blocked field of 12 cm x 12 cm reduced to 9 cm x 9 cm, which statement is true?

When considering the effect of blocking on radiation fields, one important aspect to understand is how the effective primary dose behaves with changes in field size. In this scenario, when a field initially measuring 12 cm x 12 cm is reduced to 9 cm x 9 cm, the effective primary dose reaching the patient remains unchanged.

This is primarily because the primary dose that penetrates to a specific depth, such as 10 cm, is determined by factors like the treatment beam energy and the characteristics of the radiation source, rather than simply by the field size. The reduction in field size to 9 cm x 9 cm does not directly impact the primary dose as it penetrates, rather it will mainly affect the dose distribution and the surface exposure due to the shadowing effect from the blocking.

Other factors, such as how the dose is distributed within the blocked area, can change, but the effective primary dose at the specified depth does not vary solely based on field dimensions. Hence, the assertion that there will be no change in the effective primary dose reaching the patient at 10 cm depth is accurate in this context.