Session Item

The aim of treatment planning is to find the best possible balance between the chance of tumor control and the risk of development of toxicity. Treatment planning includes the entire process from target selection/delineation, definition of organs at risk (OAR), target dose prescription, setting dose planning aims, dose optimization and plan evaluation. This teaching lecture will focus on dose planning aims/constraints: how they are prioritized and used during dose optimization and plan evaluation. The objective of the lecture is to provide understanding of the role of dose planning aims/constraints for targets and organs at risk in achieving the best possible plan in pelvic radiotherapy.

Dose planning aims express goals for acceptable doses to target and organs at risk after dose planning. Dose planning aims are steered both by “risk of events” (disease failure or morbidity) and “achievability of dose”. “Risk of events” is related to dose effect and clinical priorities while “achievability of dose” is related to the physics possibilities/limitations of a given radiotherapy technique.

Dose planning aims should preferentially be based on clinical relevance and evidence such as dose effect relations between DVH parameters and outcome (disease control or morbidity). From such clinical evidence, predictions of TCP and NTCP can be made to help deciding how to balance between doses to target and different OARs during treatment planning. Eventually, TCP and NTCP predictions can also be used for consultation with the patient – in particular, in difficult cases where the risk of treatment related morbidity is high and compromises on target dose should be considered.

Dose planning aims also need to take “achievability of dose” into account. “Achievability” is understood as “what is physically feasible” under the condition of a given type of disease and a given technique. As an example: the cranial border of a pelvic nodal target volume has direct impact on bowel dose, and more ambitious bowel dose constraints can be used if the upper target border is at the promontorium as compared to upper target border being at aortic bifucation. As another example, availability of IMRT technique may allow for using more ambitious dose planning aims as compared to 3D conformal radiotherapy. Traditional methods of dose optimization and plan evaluation has been based on generic dose constraints. However, inter-planner variations have shown to be large when using generic dose constraints. In recent years, individualized dose constraints aided by knowledge based dose planning or automated treatment planning has shown a significant potential to improve the quality of treatment plans. Individualized dose constraints can take individual anatomy into account and secure that dose plans approach the potentially highest possible quality for the given patient.