Evaluation of planar versus hybrid SPECT image methodology for bone marrow dosimetry during 177Lu-DOTATATE treatments reveals the obstacles with bone marrow metastases and cross-irradiation for the SPECT activity concentration quantification

• Introduction. The aim of this study is to compare the recently developed planar image-based method for bone marrow dosimetry with a hybrid method using SPECT/CT imaging at 24 h.p.i. of 177Lu-DOTATATE. Predictive ability of the methods is compared by investigating correlations of determined absorbed bone marrow dose, with haematological toxicity during the course of four treatment cycles. The aim is also to investigate the activity distribution in the vertebral column, and how the hybrid methodology can be optimized. Methods and Materials. 45 patients with advanced neuroendocrine tumours treated with 177Lu-DOTATATE at Sahlgrenska University Hospital in 2011-2016 (ILUMINET-study, EUDRACT nr 2011-etc) were included in this study. Absorbed bone marrow doses were calculated as the sum of the cross-doses from high-uptake organs and the remainder of the body, and the self-dose. Cross-doses were determined by time-activity curves created using planar images and a two-compartment method in the image platform PhONSAi. The self-dose was calculated using the time-activity concentration curve for the remainder of the body adjusted with the activity concentration determined in spheres placed in the vertebral bodies in SPECT-images. To improve recovery and reduce cross-irradiation of false counts in the SPECT-image, we utilized the Monte Carlo based reconstruction code SARec. Three activity concentrations were calculated to represent the activity concentration in the bone marrow; one mean (mean SPECT) and one median (median SPECT) activity concentration based on all visible vertebras and one where vertebras enclosing metastases were manually excluded (w/o Mets SPECT). Results. The planar method, the hybrid methods mean SPECT, median SPECT, and w/o Mets SPECT, yielded absorbed bone marrow doses after treatment cycle one at 0.19 (0.12-0.32), 0.35 (0.12-1.25), 0.29 (0.11-0.92) and 0.29 (0.15-0.81) Gy/7.4 GBq, respectively. A significant dose-response relationship was established after treatment cycle one between decreased platelet counts and absorbed bone marrow dose using the planar method (p=0.025, r=-0.16). With hybrid methods, a significant correlation was firstly found after treatment cycle two between absorbed dose and decreased platelet counts using median SPECT (p=0.018, r=-0.35). Conclusion. Early significant dose-response relationships were established. Despite using SARec-reconstructed SPECT-imaging for specific measurement of activity concentration in bone marrow cavities, the hybrid methods were not able to perform better than the planar method. The hybrid methods yielded higher absorbed bone marrow doses compared to the planar method as both bone metastases and cross-irradiation will influence the activity quantification. Further studies on minimizing influence of bone metastases and cross-irradiation are on-going.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Radiologi och bildbehandling (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Radiology, Nuclear Medicine and Medical Imaging (hsv//eng)

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Cancer och onkologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Cancer and Oncology (hsv//eng)

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