For a 60Co beam, what is the approximate increase in dose behind the lung due to the presence of lung in the beam path?

The correct answer is based on the understanding of how radiation interacts with different types of tissue, specifically the low-density lung tissue in comparison to other tissues. When considering a 60Co beam, the presence of lung tissue in the beam path leads to an increase in dose to the underlying tissues due to the lower attenuation characteristics of the lung.

As radiation passes through lung tissue, which has a lower density and atomic number relative to other tissues, it is less attenuated compared to passing through denser tissues like muscle or bone. This results in a higher dose being delivered to the tissues immediately behind the lung. The specific increase in dose for the scenario given is approximately 4%. This reflects the clinical observation and measurements taken in radiation therapy that indicate how lung tissue impacts dose distribution in treatment planning.

Referring to prior data and clinical experience, the approximate figures show that for most moderate energy photons like those produced by 60Co, an increase in dose of around 4% due to the presence of lung tissue is a consistent finding. This knowledge is crucial for accurate dosimetric planning and ensuring optimal treatment delivery while minimizing the risk to surrounding healthy tissues.