Accurate quantitation of activity provides the basis for internal dosimetry of targeted radionuclide therapies. This study investigated quantitative imaging capabilities at sites with a variety of experience and equipment and assessed levels of errors in activity quantitation in Single-Photon Emission Computed Tomography (SPECT) and planar imaging. Participants from 9 countries took part in a comparison in which planar, SPECT and SPECT with X ray computed tomography (SPECT-CT) imaging were used to quantify activities of four epoxy-filled cylinders containing 133Ba, which was chosen as a surrogate for 131I. The sources, with nominal volumes of 2, 4, 6 and 23 mL, were calibrated for 133Ba activity by the National Institute of Standards and Technology, but the activity was initially unknown to the participants. Imaging was performed in a cylindrical phantom filled with water. Two trials were carried out in which the participants first estimated the activities using their local standard protocols, and then repeated the measurements using a standardized acquisition and analysis protocol. Finally, processing of the imaging data from the second trial was repeated by a single centre using a fixed protocol. In the first trial, the activities were underestimated by about 15% with planar imaging. SPECT with Chang’s first order attenuation correction (Chang-AC) and SPECT-CT overestimated the activity by about 10%. The second trial showed moderate improvements in accuracy and variability. Planar imaging was subject to methodological errors, e.g., in the use of a transmission scan for attenuation correction. The use of Chang-AC was subject to variability from the definition of phantom contours. The project demonstrated the need for training and standardized protocols to achieve good levels of quantitative accuracy and precision in a multi-centre setting. Absolute quantification of simple objects with no background was possible with the strictest protocol to about 6% with planar imaging and SPECT (with Chang-AC) and within 2% for SPECT-CT.
BackgroundLow iodine diet (LID) is recommended in patients with differentiated thyroid cancer before radioiodine administration. Patients with increased thyroglobulin (Tg) level, but negative 131I whole body scan present diagnostic and therapeutic dilemma. This study was designed to evaluate the benefit of a two-week LID in patients with elevated serum Tg levels and negative 131I whole body scans.Patients and methods.For the impact assessment of two-week LID on radioiodine tissue avidity, radioiodine scans before and after LID were compared. Sixteen patients with serum Tg > 2 μg/L, negative Tg-antibodies, and negative radioiodine scans underwent two-week LID before the 131I administration. Fourteen patients underwent diagnostic scanning and two patients received radioiodine therapy. Iodine concentration in the morning urine specimens were measured in each patient, a day before and 15th day after starting LID.ResultsFollowing self-managed LID, patients were able to significantly reduce their iodine body content by 50% (range 28–65%, p<0,001). 13 patients (82%) accomplished mild iodine deficiency (50-99 μg/L) and one patient (6%) achieved targeted moderate iodine deficient state (<50 μg/L). All diagnostic post-LID scans were negative. Both post-therapy 131I scans showed radioiodine accumulation outside of normal 131I distribution (neck region and diffuse hepatic uptake). This study demonstrated that two-week LID is effective way to decrease total body iodine content, although without a visible effect on post-LID diagnostic 131I scans.ConclusionsA more stringent dietary protocol and longer iodine restriction period are probably needed to achieve targeted moderate iodine deficiency in patients preparing for 131I administration. This might result in higher radioiodine avidity of thyroid remnant/metastases.
SummaryAim: Absorbed dose to thyroid remnant tissue after 131I ablation becomes mass/size-dependent. This is a direct consequence of the small remnant size and radiation escape starts to be relevant. The self-absorbed fraction becomes mass/size-dependent. We have used Monte Carlo simulations to investigate the influence of the thyroid remnant shape upon the absorbed fraction calculation. Methods: Thyroid residue was modeled using spherical, cylindrical and elliptical shapes. Uniform beta activity distribution and unit density medium (water) within a remnant was assumed. For each of the geometrical models beta self-absorbed fraction (ϕγ) was calculated using Monte Carlo codes, while the mean absorbed dose per unit cumulated activity (Sγ) was calculated using MIRD formalism. Results: For spherical objects Ömono for mean beta energy (E = 0.182 MeV) of 131I is always greater than ϕγ calculated for the complete beta spectrum. For spheres having diameters 2–6 mm and assumption ϕγ=1, Sγ is overestimated by 11–37%. For cylinder and prolate spheroid of the same length and thickness, Sγ for cylinder is 30% smaller because of the greater mass. Similarly, elliptical cylinder and general ellipsoid of the same length and the same perpendicular dimensions (width and breadth), have similar ϕγ, while Sγ for elliptical cylinder is correspondingly smaller. Conclusion: For accurate dosimetry of thyroid remnants having masses <1 g and chordal lengths <1 cm it is necessary to calculate ϕγ for the full beta spectrum, or Sγ will be overestimated. The shape of the remnant may also be important since elongated non-spherical objects may also have ϕγ < 1.
Positron emission tomography (PET) is currently performed using either a dedicated PET scanner or scintillation gamma camera equipped with electronic circuitry for coincidence detection of 511 keV annihilation quanta (gamma camera PET system). Although the resolution limits of these two instruments are comparable, the sensitivity and count rate performance of the gamma camera PET system are several times lower than that of the PET scanner. Most gamma camera PET systems are manufactured as dual-detector systems capable of performing dual-head coincidence imaging. One possible step towards the improvement of the sensitivity of the gamma camera PET system is to add another detector head. This work investigates the characteristics of one such triple-head gamma camera PET system capable of performing triple-head coincidence imaging. The following performance characteristics of the system were assessed: spatial resolution, sensitivity, count rate performance. The spatial resolution, expressed as the full width at half-maximum (FWHM), at 1 cm radius is 5.9 mm; at 10 cm radius, the transverse radial resolution is 5.3 mm, whilst the transverse tangential and axial resolutions are 8.9 mm and 13.3 mm, respectively. The sensitivity for a standard cylindrical phantom is 255 counts.s(-1).MBq*(-1)), using a 30% width photopeak energy window. An increase of 35% in the PET sensitivity is achievable by opening an additional 30% width energy window in the Compton region. The count rate in coincidence mode, at the upper limit of the systems optimal performance, is 45 kc.s(-1) (kc=kilocounts) using the photopeak energy window only, and increases to 60 kc.s(-1) using the photopeak + Compton windows. Sensitivity results are compared with published data for a similar dual-head detector system.
We evaluated the effects on the absorbed dose to thyroid follicular cells of self-absorption of 131 I radiation (specifically, b-rays) in the follicular colloid. Methods: Thyroid follicles were modeled as colloid-filled spheres, containing a uniform concentration of 131 I and surrounded by a concentric monolayer of cells. Assuming close packing of identical follicles, we used Monte Carlo simulation to assess the absorbed dose to follicular cells. Results: Because of b-ray self-absorption in colloidal spheres with radii larger than 50 mm, the absorbed dose to follicular cells is less than the average thyroid absorbed dose. Conclusion: For the same thyroid mass, radioiodine thyroid uptake, and effective half-life, patients with follicles with colloidal sphere radii of 100, 200, 300, and 400 mm should be administered 9%, 15%, 21%, and 30% more 131 I, respectively, than patients with colloidal sphere radii of less than 50 mm, to yield the same absorbed dose to follicular cells. Radi oiodine ( 131 I-iodide) therapy is generally the treatment of choice for uncomplicated Graves disease in adults. The ideal goal is to destroy or otherwise affect enough thyroid tissue to produce euthyroidism. Despite efforts to assess clinically the target absorbed dose by accounting for the gland size and radioiodine kinetics, the success of radioiodine therapy remains largely unpredictable (1,2).The current methods define the whole thyroid as the target organ, although the biologic effects are primarily due to irradiation of thyroid follicular cells. The current article presents evidence that a 131 I absorbed dose to thyroid follicular cells differs from the mean thyroid absorbed dose and that such differences increase with increasing sizes of thyroid follicles. MATERIALS AND METHODSSystemically administered iodide is captured by the thyroid and organified and appears in the follicular colloid within minutes of administration, as demonstrated by autoradiographic studies in rats (3). In contrast, the biologic half-life of thyroidal iodine is quite long, varying from 15 to 60 d and yielding an effective halflife of 5-7 d for 131 I. This means that most 131 I decays occur within the colloid, that is, extracellularly.In the current analysis, we considered only the b-radiation of 131 I. The contribution of g-radiation to total average gland absorbed dose (average thyroid absorbed dose from both g-radiation and b-particles of 131 I) increases with the gland size and depends on the gland shape; for a 20-g gland, it is 6% (Appendix). This component is not negligible and should be taken into account. However, because of the relatively long mean free path of g-rays, compared with particle ranges, the absorbed energy distribution of g-rays is expected to be uniform across the thyroid volume (i.e., insensitive to thyroid microarchitecture).The b-radiation-absorbed dose to thyroid follicular cells, D cell , is due to b-particles emanating from their own colloid (the selfdose [D self ]) and from b-particles coming from the colloid of neighboring fol...
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