ABSTRACT
typically displayed as a set of coordinates representing the x, y and z positional displacements necessary in order to re-optimize the isocenter position. More complex sets of o set coordinates may include partial rotations (pitch and yaw) and other types of netuning related to the dosimetric in uences of surrounding structures. At an even higher level of optimization complexity, one can begin to consider change in target morphology, as one might imagine if target segments were growing or shrinking during the course of treatment. For instance, some recent data suggest that the contour of many adenocarcinoma targets may decrease by approximately 1% per day while under radiotherapy. is amount of target shrinkage, though small on a day by day basis, amounts to a signi cant shrinkage in tissue mass over the course of an entire 7-week period of radiotherapy. is may result in signi cant decreases in the beam attenuation pro le of the target under radiotherapy. For any of these types of changes in positional target registration compared to the baseline location, the possible need to re-plan the case is obvious. Minor registrational o sets may be appropriately corrected using simple isocenter shi s, but more complex perturbations in tissue location may result in dramatic dosimetric di erences for both target and nearby normal tissues. is kind of change may thus require complete re-planning of the case and the re-positioning of critical normal structures behind blocks. In some cases, a speci c template may be used to develop a family of plans during the initial treatment planning episode. Changes in positional coordinates may then be accommodated by choosing the plan within the treatment library that best accommodates the positional information detected by the day’s imaging studies.
