What mechanism primarily leads to damage when cells are exposed to photons or electrons?

The primary mechanism by which cells sustain damage when exposed to photons or electrons is indeed through indirect action. This occurs because high-energy radiation can interact with water molecules in the body—given that water constitutes a substantial portion of cellular composition—leading to the formation of free radicals.

Free radicals are highly reactive molecules that can diffuse through the cell and interact with various cellular structures, including DNA, proteins, and lipids. The damage is often more pervasive because it can affect multiple molecules and pathways, whereas direct ionization primarily targets specific molecules, such as DNA, which may limit the range of damage.

In the context of medical dosimetry, understanding this indirect action is crucial as it underscores the importance of cellular environment and the roles of radiolysis products in contributing to the overall biological effects of radiation exposure. This mechanism highlights why radiation therapy must be carefully planned, considering not only the tumor volume but also the surrounding healthy tissues that may be impacted by these indirect pathways.