TLD Measurements on Patients being treated with a Taylor Spatial Frame : Using Radiation from Na¹⁸F PET/CT Studies and from Naturally Occurring Radioisotopes

Abstract:

Background: In an ongoing study conducted at Karolinska Institutet & Karolinska University Hospital, Positron Emission Tomography (PET)/Computed Tomography (CT) scans are performed on patients with tibia fractures and deformations treated with Taylor Spatial Frames (TSFs) in order to monitor their bone remodeling progress. Each patients receive an administration of approximately 2 MBq/kg bodyweight of Na¹⁸F associated with PET scans on two sessions, six and twelve weeks after the attachment of the TSF. These PET/CT scans provide information about the progress of the healing bone and can be used to estimate the optimal time point for de-attachment of the TSF. The Standardized Uptake Value (SUV) is used as a measure of the rate of bone remodeling for these patients, however, there is a need for verification of this practice by a method independent of the PET scanner. Furthermore, information regarding the biodistribution of the Na¹⁸F throughout the body of these patients and the effects of the TSF on the CT scan X‑rays is required. Additionally, an investigation of alternative methods that have the potential to provide similar information with a lower absorbed dose to the patients is desirable.

Materials and methods: Thermoluminescent Dosimeters (TLDs) were attached on the skin at the position of the heart, urinary bladder, femurs, fracture, and the contralateral tibia of twelve patients during the first one hour and five minutes after the administration of the Na¹⁸F. Additional TLD measurements were performed during the CT scan of two of these patients. From the PET scan images, SUVs at the fracture site of these patients were collected. An investigation of the possibility of exploiting the “naturally” occurring bone seeking radionuclide Strontium-90 (⁹⁰Sr) in the human body to gain information about the fracture site was undertaken. Using a ⁹⁰Sr source, three different detection techniques were evaluated and a practical methodology for in vivo measurements on the tibia fracture patients was developed. As it was concluded that TLD based measurements were the most suitable technique for this purpose, and it was tested on five patients with tibia fractures.

Results: From the collected TLD data, it was concluded that for these patients the urinary bladder is the organ receiving the greatest amount of absorbed dose and the organ most affected as the administered activity exceeds 2 MBq/kg. On average, a three times higher surface dose was measured on the tibia fracture compared to the un-fractured tibia. A linear relationship between the surface dose and SUV_max was shown. A strong positive correlation between the activity concentration at the fracture site and the amount of injected activity was found, and it was demonstrated that this also affects the SUVs. For patients who were administered different amounts of Na¹⁸F for the two PET scans, maximum activity concentrationwas less affected than mean activity concentration. It was concluded that TSF’s effect on the scatter of the X-rays to organs higher up in the body is negligible. Regarding “naturally” occurring ⁹⁰Sr in the human body, no higher activity at the fractured tibia compared to the non‑fractured tibia could be found.

Conclusions: This project assessed the accumulation of Na¹⁸F in the fracture site of patients treated with TSF by a method independent of the PET scanner. The methodology of using SUVs as an indicator for bone remodeling was verified. It was shown that the uptake of Na¹⁸F by the fracture site is strongly correlated to the amount of injected activity. The importance of considering the amount of injected activity when evaluating and comparing SUVs was highlighted. In vivo measurements using LiF:Mn TLDs did not indicate any quantifiable higher concentration of ⁹⁰Sr at the fracture in the tibia bone.