Breathing interplay effects during Tomotherapy. A 3D gel dosimetry study

Abstract: Tomotherapy delivers intensity modulated radiation therapy, IMRT, which enables highly advanced treatment plans with homogenous absorbed dose coverage of the target with sharp dose gradients to spare surrounding healthy tissue. Helical tomotherapy consists of a linear accelerator mounted on a rotating gantry, the construction is similar to a CT although the energy is in the MV-range instead of kV and extra collimators and beam stopper are inserted [1-3]. During all kinds of radiation therapy patient motion is a concern since it can move the tumour outside the irradiated volume [4]. This will cause under or over dosed volumes which is an undesired effect, especially under dosage of the target or over dosage of organs at risk, OAR. Motion during radiation therapy is divided into two categories, intrafraction and interfraction motion. Interfraction motion is when the tumour moves between two different treatment sessions and intrafraction motion is when the tumour moves during a treatment session [5]. Tumours located in the abdomen often change location depending on the content of the stomach and intestine. This is an example of interfraction motion. Examples of intrafraction motion are movements due to the beating of the heart, coughing or breathing. For tumours in the thorax region breathing movement has the largest impact on the treatment [4, 6]. In conventional radiation therapy this is handled by creating a new volume including the clinical target volume, CTV, but also uncertainties in position and motion. There are two types of volumes that include these uncertainties, these are the internal target volume, ITV, and the planning target volume, PTV. The ITV includes uncertainties in positions within the body while the PTV also includes set-up deviations. For treatment planning the PTV is used and as a minimum the PTV has to be covered with the 95 % isodose volume of the absorbed dose that is prescribed. The aim of this study is to evaluate the delivered absorbed dose in 3D compared with the planned absorbed dose by the Tomotherapy treatment planning system and also to see how simulated breathing motion affects the delivered dose. To do this gel dosimetry is used. The idea of using a gel that changes properties when irradiated was first suggested in the 1950’s [7]. Since then, gel dosimetry has developed and is today a very useful tool for evaluating new equipment and treatment methods. The benefit of gel dosimetry is that it has high-resolution in three dimensions and that it is independent of the incident angle of the irradiation unlike many other detector systems [7]. Another advantage is that the gel is almost softtissue equivalent [8]. In decades a group of scientists in Malmö have used and developed the technique, and also applied it to IMRT- and VMAT-measurements [9-13]. To evaluate breathing interplay effects two gel phantoms are irradiated, one stationary and one in motion. If there is a difference between the two measurements that can not be explained by the simulated motion, breathing interplay effects have been found. Aside from the breathing interplay effect a comparison between the calculated dose distribution and measured dose distribution is done.