Novel weight-based dose threshold for 18F-NaF PET-CT imaging using advanced PET-CT systems: a potential tool for reducing radiation burden.

Abstract

Fluorine-18-sodium fluoride (F-NaF) PET/CT is an important tool for detecting and evaluating metastatic bone cancer. Besides traditional dose metrics, recent methods such as real-time dose mapping, dose calculation from DICOM information, and their relevance to entrance skin exposure are currently in use to reduce the radiation burden. In this study, we have analyzed the data of 1062 patients retrospectively to evaluate patterns of absorbed dose for institutional weight-based dose protocol as compared with fixed dose method guidelines of Society of Nuclear Medicine and Molecular Imaging (SNMMI). The effective dose imparted by F-NaF (internal exposure) was calculated by using coefficient 0.089 mrem/mCi (0.024 mSv/MBq) according to ICRP publication 106. To estimate the effective dose from whole-body CT scan (external exposure), volume CT dose index (mGy) and dose length product (mGy cm) were directly obtained from the display screen of CT workstation. Effective dose was calculated by multiplying DLP (mGy cm) with ICRP conversion coefficient 'k' 0.015 (mSv/mGy cm). Of the total 1062 patients, there were metastases in 44% (464), probable malignancy in 9% (96), negative findings in 40.5% (433), equivocal findings in 3% (32), and probable benignancy in 3.5% (37). All patients were injected with an institutional agreed protocol of 2.22 MBq/kg (0.06 mCi/kg). The mean injected activity for entire population came out to be 4.79±0.99 mCi. The mean effective absorbed doses were 3.37±0.70 and 5.5±1.35 mSv for F-NaF alone and CT alone, respectively. The mean cumulative effective dose of combined F-NaF PET and CT scan was calculated to be 8.8±1.8 mSv. The minimum absorbed dose for our method was as follows: 1.37 mSv for Kuwait Cancer Control Center vs. 4.44 mSv for SNMMI. Absorbed dose for maximum injected activity was as follows: 5.7 mSv for Kuwait Cancer Control Center vs. 8.88 mSv for SNMMI. Our weight-based doses were also lower when compared with minimum and maximum absorbed doses of technetium-99m-methylene diphosphonate as per SNMMI guidelines (i.e. 4.21 and 6.32 mSv, respectively). Our weight-based dose protocol showed a significant reduction in absorbed doses (i.e. 69.14% for F-NaF and 67.45% for technetium-99m-methylene diphosphonate) when compared with fixed dose method guidelines of SNMMI. It is suggested that further prospective studies may be performed in adult population, to evaluate dosimetry efficacy of F-NaF at a low dose of 0.06 mCi/kg as inferred in our current study.