A Practical Method To Deal With Differential Motion Of Target And OAR During Image Guided Radiotherapy

Main Category: Radiology / Nuclear Medicine
Also Included In: Cancer / Oncology
Article Date: 22 Sep 2008 - 4:00 PDT

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Current image guided radiotherapy systems provide efficient on-line correction of target positioning. Significant motion, however, can occur between target and organs-at-risk (OARs), for instance for lung tumours (baseline shifts). If so, proper target positioning may lead to overdosage of OARs. Our aim is to devise and implement a practical image guidance system that can deal with such differential motion for peripheral lung tumours treated with hypofractionated radiotherapy.

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 is performed for both regions providing setup error data of cord and target. The software checks the proposed correction against predefined limits for both regions. The limit for the cord registration is set to the OAR expansion, while the limit for target registration is set to the target margin. When the correction would move the high dose region outside limits towards the OAR, a warning is 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. In10, the proximity of the tumour and OAR required re¬duced OAR expansions. In 8,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 showed that especially in hypofractionated radiotherapy of lung tumours, differential motion of tumour and OAR may lead to serious overdosage when OARs are not considered in the guidance procedure. We developed and implemented a practical system to deal with this problem.

Van Herk Marcel et al. The Netherlands Cancer Institute / Antoni van Leeuwenhoek hospital (Department of Radiation Oncology), Amsterdam, Netherlands

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ESTRO 27 (September 14th to 18th, 2008) offered an outstanding scientific programme combining lectures from eminent invited speakers, proffered papers and poster discussions, teaching lectures on a wide range of topics including clinical issues, brachytherapy, radiobiology, physics and technology as well as debates on controversial topics and clinical case discussions, a special poster reception, poster discussion sessions and electronic poster viewing.

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