In this thesis two topics in biological physics are explored. These are presented as manuscripts which have been submitted to scientific journals.

The first topic deals with administering radiation therapy to cancer patients using a miniature x-ray source that can be inserted through a catheter into a body cavity at the tumor site. The manuscript investigates how ionizing radiation deposits its energy for the special case where there is additional attenuating material in the path between the radiation source and patient. It was previously speculated that a global correction factor can be applied to account for the attenuating substance. However, it will be shown that using a global correction factor leads to incorrect dose calculations and thereby incorrectly predicting the dose delivered to the patient.

The second topic explores the use of semiconductor nanocrystals (quantum dots) for imaging biological systems. The manuscript studies the fluorescence anisotropy of quantum dots to obtain information on their orientation so they can be used as fluorescence markers when studying biological systems. The manuscript also determines how the phenomenon of quantum dot blinking affects the anisotropy. Finally, it demonstrates how actin filaments can be used as scaffolds to build nanowires from quantum dots.