The impact of metallic cranial implants on proton-beam radiotherapy treatment plans for near implant located tumours : A phantom study on the physical effects and agreement between simulated treatment plans and the resulting treatment for near implant loc

Sammanfattning: Within the field of radiotherapy treatments of tumour diseases, the hunt for more accurate and effective treatment methods is a continuous process. For some years ion-beam based radiotherapy, especially the proton-beam based applications, has increased in popularity and availability. The main reason behind this is the fact that ion-beam based applications make it possible to modulate the dose after the planning target volume (PTV) defined by the radiation oncologist. This means that it becomes possible to spare tissue in another way, which might result in more effective treatments, especially in the vicinity of radio sensitive organs. Ion-beam based treatments are however more sensitive to uncertainties in PTV position and beam range as ion-beams have a fixed range depending on target media and initial energy, as opposed to the conventional x-ray beams that do not really have a defined range. Instead their intensity decreases exponentially at a rate dependent of the initial energy and target media. Therefore density heterogeneities result in uncertainties in the planned treatments. As the plans normally are created using a CT-images, for which metallic implants can yield increased heterogeneities both from the implants themselves and so called metal artifacts (distortions in the images caused by different processes as the X-rays used in image acquisition goes through metals). Metallic implants affects the accuracy of a treatment, and therefore also the related risks, so it is important to have an idea of the magnitude of the impact. Therefore the aim of this study is to estimate the impact on a proton-beam based treatment plan for six cranial implants. These were one Ti-mesh implant, one temporal plate implant, one burr-hole cover implant and three craniofix implants of different sizes, which all are commonly seen at the Skandion clinic. Also the ability of the treatment planning system (TPS), used at the clinic, to simulate the effects on the plans caused by the implants is to be studied. From this result it should be estimated if the margins and practices in place at the clinic, for when it is required to aim the beam through the implant, are sufficient or if they should be changed. This study consisted of one test on the range shift effects and one test on the lateral dose distribution changes, with one preparational test in the form of a calibration of Gafchromic EBT3 films. The range shift test was performed on three of the implants, excluding the three craniofix implants using a water phantom and a treatment plan created to represent a standard treatment in the cranial area. The lateral dose distribution change test was performed as a solid phantom study using radiochromic film, for two treatment plans (one where the PTV was located \SI{2}{\centi\metre} below surface, for all implants, and one where it was located at the surface, only for the Ti-mesh and the temporal plate). The results of both tests were compared to simulations performed in the Eclipse treatment planing system (TPS) available at Skandion. The result of the range shift test showed a maximum range shift of \SI{-1.03 +- 0.01}{\milli\metre}, for the burr-hole cover implant, and as the related Eclipse simulations showed a maximal shift of \SI{-0.17 +- 0.01}{\milli\metre} there was a clear problem with the simulation. However, this might not be because of the TPS but due to errors in the CT-image reconstruction, such as, for example, geometrical errors in the representation of the implants. As the margin applied for a similar situation at the Skandion clinic (in order to correct for several uncertainty factors) is \SI{4.2}{\milli\metre} there might be a need to increase this margin depending on the situation. For the lateral distribution effects no definite results were found as the change varied in magnitude, even if it tended to manifest as a decreasing dose for the first plan and a increasing dose for the second. It was therefore concluded that further studies are needed before anything clear can be said.