ABSTRACT
Time-dependent variation in the target location and the patient density distribution can occur on different timescales; i.e., over the entire treatment course, between subsequent treatment fractions, and within a treatment fraction. “Motion” describes gradual, but systematic, variations in the patient-density distribution. These can occur either due to a redistribution of density within the patient or due to the addition or disappearance of matter. Redistribution of density includes changes in the motion pattern such as breathing trajectory or changes in the baseline. Interfractional density variations can be as large as the gradual variations over the entire treatment course, but they have a more random character. For lung tumors, motion is typically largest for those tumors close to the diaphragm and typically largest in the superior–inferior direction. In proton therapy, the finite range and steep dose gradients make it much more complex to deliver an accurate treatment for moving tumors.
