Correlations of Total Pelvic Spongiosa Volume With Both Anthropometric Parameters and Computed Tomography–Based Skeletal Size Measurements

A dosimetry procedure for organs-at-risk in 177Lu peptide receptor radionuclide therapy of patients with neuroendocrine tumours

PURPOSE

Peptide receptor radionuclide therapy with ¹⁷⁷Lu-DOTATATE has become a standard treatment modality in neuroendocrine tumours (NETs). No consensus has yet been reached however regarding the absorbed dose threshold for lesion response, the absorbed dose limit to organs-at-risk, and the optimal fractionation and activity to be administered. This is partly due to a lack of uniform and comparable dosimetry protocols. The present article details the development of an organ-at-risk dosimetry procedure, which could be implemented and used routinely in a clinical context.

METHODS

Forty-seven patients with NETs underwent ¹⁷⁷Lu-DOTATATE therapy. Three SPECT/CT images were acquired at 4, 24 and 144-192 h post-injection. Three blood samples were obtained together with the SPECT/CT acquisitions and 2 additional samples were obtained around 30 min and 1 h post-injection. A bi-exponential fit was used to compute the source organ time-integrated activity coefficients. Coefficients were introduced into OLINDA/EXM software to compute organ-at-risk absorbed doses. Median values for all patients were computed for absorbed dose coefficient D/A₀ and for late effective half-life T_1/2eff for kidneys, spleen and red marrow.

RESULTS

Dosimetry resulted in a median[interquartile range] of 0.78[0.35], 1.07[0.58] and 0.028[0.010] Gy/GBq for D/A₀ and of 55[9], 71[9] and 52[18] h for T_1/2eff for kidneys, spleen and red marrow respectively.

CONCLUSIONS

A dosimetry procedure for organs-at-risk in ¹⁷⁷Lu-DOTATATE therapy based on serial SPECT/CT images and blood samples can be implemented routinely in a clinical context with limited patient burden. The results obtained were in accordance with those of other centres.

Pelvic and Lower Gastrointestinal Tract Anatomical Characterization of the Average Male

OBJECTIVE

Colorectal surgeons report difficulty in positioning surgical devices in males, particularly those with a narrower pelvis. The objectives of this study were to (1) characterize the anatomy of the pelvis and surrounding soft tissue from magnetic resonance and computed tomography scans from 10 average males (175 cm, 78 kg) and (2) develop a model representing the mean configuration to assess variability.

METHODS

The anatomy was characterized from existing scans using segmentation and registration techniques. Size and shape variation in the pelvis and soft tissue morphology was characterized using the Generalized Procrustes Analysis to compute the mean configuration.

RESULTS

There was considerable variability in volume of the psoas, connective tissue, and pelvis and in surface area of the mesorectum, pelvis, and connective tissue. Subject height was positively correlated with mesorectum surface area (P = .028, R² = 0.47) and pelvis volume ( P = .041, R² = 0.43). The anterior-posterior distance between the inferior pelvic floor muscle and pubic symphysis was positively correlated with subject height ( P = .043, r = 0.65). The angle between the superior mesorectum and sacral promontory was negatively correlated with subject height ( P = .042, r = -0.65). The pelvic inlet was positively correlated with subject weight ( P = .001, r = 0.89).

CONCLUSIONS

There was considerable variability in organ volume and surface area among average males with some correlations to subject height and weight. A physical trainer model created from these data helped surgeons trial and assess device prototypes in a controllable environment.

A method to correct for radioactivity in large vessels that overlap the spine in imaging-based marrow dosimetry of lumbar vertebrae

Accurate marrow dosimetry for radionuclide therapy based on imaging methods has been challenging because of a variety of factors. One of the uncertainties in image quantification of lumbar vertebrae is correction for radioactivity in large blood vessels anterior to the vertebrae. We developed a method to correct for background radioactivity contributed from blood in large vessels and tested it in a pilot study.

METHODS

CT images of 26 patients receiving (111)In- or (131)I-labeled conjugates were used to measure the inside diameters of the aorta and inferior vena cava (IVC) at the top of L2 and the bottom of L4 and to measure the length of this vessel segment. The volume was calculated for this vessel segment, and then the radioactivity in that volume at each imaging time was determined using a time-variant blood radioactivity concentration as established by serial blood samples. This vessel segment typically overlapped with lumbar vertebrae in anterior and posterior whole-body images. The contribution of this background radioactivity to the cumulated activity of the lumbar spine region of interest (ROI) from serial gamma-camera images was determined, taking into account differences in attenuation between vessel segments and lumbar vertebrae.

RESULTS

The total blood volumes varied from 25 to 94 mL, with a mean of 51 mL. This mean is 76% of the mean marrow volume of 3 lumbar vertebrae measured in some of these patients. Thirteen of the 14 patients evaluated for aortic position had the aortic segment completely within the L2-L4 ROI. For the IVC, a mean of 72% was in the L2-L4 ROI. Adjusting for radioactivity in major blood vessels that were in the ROI led to lower marrow dose estimates.

CONCLUSION

To improve the accuracy of lumbar spine imaging-based marrow dosimetry, one can adjust radioactivity in the large vessels by methods that measure the volume, position, and depth of vessels in the ROI.

CT volumetry of the skeletal tissues

Computed tomography (CT) is an important and widely used modality in the diagnosis and treatment of various cancers. In the field of molecular radiotherapy, the use of spongiosa volume (combined tissues of the bone marrow and bone trabeculae) has been suggested as a means to improve the patient-specificity of bone marrow dose estimates. The noninvasive estimation of an organ volume comes with some degree of error or variation from the true organ volume. The present study explores the ability to obtain estimates of spongiosa volume or its surrogate via manual image segmentation. The variation among different segmentation raters was explored and found not to be statistically significant (p value >0.05). Accuracy was assessed by having several raters manually segment a polyvinyl chloride (PVC) pipe with known volumes. Segmentation of the outer region of the PVC pipe resulted in mean percent errors as great as 15% while segmentation of the pipe's inner region resulted in mean percent errors within approximately 5%. Differences between volumes estimated with the high-resolution CT data set (typical of ex vivo skeletal scans) and the low-resolution CT data set (typical of in vivo skeletal scans) were also explored using both patient CT images and a PVC pipe phantom. While a statistically significant difference (p value <0.002) between the high-resolution and low-resolution data sets was observed with excised femoral heads obtained following total hip arthroplasty, the mean difference between high-resolution and low-resolution data sets was found to be only 1.24 and 2.18 cm3 for spongiosa and cortical bone, respectively. With respect to differences observed with the PVC pipe, the variation between the high-resolution and low-resolution mean percent errors was a high as approximately 20% for the outer region volume estimates and only as high as approximately 6% for the inner region volume estimates. The findings from this study suggest that manual segmentation is a reasonably accurate and reliable means for the in vivo estimation of spongiosa volume. This work also provides a foundation for future studies where spongiosa volumes are estimated by various raters in more comprehensive CT data