Method Devised To Tackle Differential Motion of Target During Imaging

Current image-guided radiotherapy systems provide effective online correction of target positioning. Significant motion, however, can occur between target and organs-at-risk (OARs), for instance, for lung tumors (baseline shifts). Therefore, accurate target positioning may lead to overdosage of OARs. The goal of a new study was to devise and implement a practical image guidance system that can deal with such differential motion for peripheral lung tumors treated with hypofractionated radiotherapy.

In this study, during planning, the cord was mostly expanded by 1 cm, but sometimes less expansion was required to reach an acceptable plan. The guidance system has two regions of interest for registration, placed on spinal column and target. Image registration was performed for both regions providing setup error data of cord and target. The software checked the proposed correction against predefined limits for both regions. The limit for the cord registration was set to the OAR expansion, while the limit for target registration was set to the target margin. When the correction would move the high dose region outside limits towards the OAR, a warning was given. In a separate review procedure, the operator "fades” between correct target and OAR positioning to reach a compromise with both regions within limits.

The system was applied in 86 patients. In 10, the proximity of the tumor and OAR required reduced OAR expansions. In eight, a compromise had to be made because the baseline shift exceeded the OAR margin during one or more fractions. In all cases, it was possible to find a correction that satisfied both target and OAR constraints.

This study demonstrated that particularly in hypofractionated radiotherapy of lung tumors, differential motion of tumor and OAR might lead to serious overdosage when OARs are not considered in the guidance procedure. The researchers developed and implemented a practical system to deal with this problem.

Dr. Van Herk Marcel and colleagues, from the department of radiation oncology at the Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital (Amsterdam, The Netherlands), presented the study's findings September 2008, at ESTRO 27 (European Society for Therapeutic Radiology and Oncology), held in Gothenburg [Göteborg], Sweden

Related Links:
Netherlands Cancer Institute