Clarkson's formula is used to determine the scatter produced in which type of fields?

Clarkson's formula is specifically designed to calculate the scatter produced in irregular fields, making it suitable for scenarios where the radiation treatment area does not have a uniform shape. This formula takes into account the complex geometry and varying dimensions of irregular fields, allowing for a more accurate assessment of dose distribution.

In radiation therapy, accurate dose calculations are crucial, especially in areas where the field shape can lead to differing levels of scatter radiation. Irregular fields are common in clinical practice, as tumors may not conform neatly to standard shapes. Clarkson's approach allows clinicians to adjust for the additional complexity that arises from these non-uniform treatment areas, ensuring that the delivered dose meets the therapeutic goals while minimizing exposure to surrounding healthy tissue.

Other types of fields like rectangular, square, or extended fields generally have more straightforward geometry, making them amenable to simpler scatter calculations or different methodologies. As such, the application of Clarkson's formula is most relevant and beneficial in the context of irregularly shaped fields, which is why it is the right answer.