Challenges in radiotherapy planning : dose verification in the vicinity of the border of tissue-prosthesis medium

The success or failure of radiotherapy largely depends on the accuracy with which the dose will be delivered to a specific volume in the patient’s body. One of the problems associated with radiotherapy planning for patients with endoprostheses is the inaccuracy of the algorithm calculating the dose distribution in the treatment planning system for the area in the vicinity of the border of tissue–prosthesis medium. The aim of this study is verification of a planned dose on the border of hip prosthesis–acetabulum surface. At the examined energy — 6 MV — a dose results in decrease at the border of the medium, to achieve up to 10%. To verify this hypothesis, a water-filled phantom (soft tissue equivalent) was used with bone fragments (imitating hip joint) and metallic elements (hip joint endoprostheses) placed in a working stand. On acetabulum surface, thermoluminescent microdosimeters (TLD) based on lithium fluoride (LiF) was placed. The irradiation by medical linear accelerator was performed. The planned dose is higher compared with measured dose by approx. 9.8% (1.112 vs. 1.003 Gy for 2 Gy of fraction dose). It was confirmed that the treatment planning system overestimates the dose on the surface of acetabulum.