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Aim In patients with multiple skeletal metastases, accurate estimation of absorbed doses to radiosensitive bone marrow in bone-directed systemic radionuclide therapies (RNT) is critically important from clinical dose determination standpoint. The primary aim of the present study was to estimate the radiation absorbed doses of therapeutic [177Lu]Lu-EDTMP to bone marrow by two methods viz. Medical Internal Radiation Dose (MIRD) schema and using OLINDA software and correlate with hematological toxicity. Methods A total of 15 patients diagnosed to have multiple painful skeletal metastases being treated with [177Lu]Lu-EDTMP for palliation of pain, were enrolled for this prospective study. For all patients, urine was collected immediately after infusion of [177Lu]Lu-EDTMP up to 24 h post-administration and cumulative activity excreted from body via urine was calculated. For dosimetry, patients underwent post-administration whole-body scintigraphy at five-time points: 0.5 (pre-void), 2, 24, 48 and 120 h (post-void). From the time-activity curves generated by drawing regions of interest (ROIs) on the images, number of disintegrations was determined. Absorbed doses for organs and bone lesions were calculated using OLINDA 2.2.0 software. For bone marrow dose estimates, in addition to OLINDA 2.2.0 software, MIRD schema was also adopted. Hematological profile was monitored in all patients during the treatment and post-treatment follow-up (estimating complete blood counts, every 15 d for 3 months after therapy). Results The mean ± standard deviation activity of [177Lu]Lu-EDTMP administered per patient per cycle was 2.08 ± 0.45 GBq. The results demonstrated higher uptake of [177Lu]Lu-EDTMP in bone metastases compared to normal bones. Within 2 and 24 h of administration of [177Lu]Lu-EDTMP, [177Lu]Lu activity excreted from the body was 24 ± 9% and 39 ± 14%, respectively. The mean absorbed organ doses (mean ± SD) in Gy/GBq were as follows: osteogenic cells 3.15 ± 1.85, bone marrow 0.57 ± 0.31, kidneys 0.08 ± 0.05, urinary bladder 0.32 ± 0.04, and bone lesions 2.91 ± 1.88. Strong correlation was found between (a) MIRD schema and OLINDA 2.2.0 software method for estimation of bone marrow doses (r = 0.96; P = <0.0001) and (b) Bone marrow absorbed dose and hematological toxicity (r = 0.81, P = 0.0027). Conclusion Radiation absorbed doses to the bone marrow and skeletal metastatic lesions, following therapeutic [177Lu]Lu-EDTMP were estimated using a convenient and non-invasive quantitative imaging method. The estimated bone marrow absorbed dose, either by MIRD schema or the OLINDA 2.2.0 software method, demonstrated strong correlation. Strong correlation was also observed between bone marrow absorbed dose and hematological toxicity.

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