Quantitative bremsstrahlung single photon emission computed tomographic imaging: use for volume, activity, and absorbed dose calculations

Efficacy of voxel-based dosimetry map for predicting response to trans-arterial radioembolization therapy for hepatocellular carcinoma: a pilot study

OBJECTIVE

Typical clinical dosimetry models for trans-arterial radioembolization (TARE) assume uniform dose distribution in each tissue compartment. We performed simple voxel-based dosimetry using post-treatment 90Y PET following TARE with 90Y-resin microspheres and investigated its prognostic value in a pilot cohort.

METHOD

Ten patients with 14 hepatocellular carcinoma lesions who underwent TARE with 90Y-resin microspheres were retrospectively included. The partition model-based expected target tumor dose (TDp) was calculated using a pretreatment 99mTc-macroaggregated albumin scan. From post-treatment 90Y-microsphere PET and voxel-wise S-value kernels, voxel-based dose maps were produced and the absorbed dose of each lesion (TDv) was calculated. Heterogeneity of intratumoral absorbed doses was assessed using the SD and coefficient of variation of voxel doses. The response of each lesion was determined based on contrast-enhanced MRI or CT, or both. Lesion responses were classified as local control success or failure. Prognostic values of dosimetry parameters and clinicopathological factors were evaluated in terms of progression-free survival (PFS) of each lesion.

RESULTS

TDv was significantly different between local control success and failure groups, whereas tumor size, TDp and intratumoral dose heterogeneity were not. Univariate survival analysis identified serum aspartate transaminase level ≥40 IU/L, tumor size ≥66 mm and TDv <81 Gy as significant prognostic factors for PFS. However, only TDv was an independent predictive factor in the multivariate analysis (P = 0.022). There was a significant correlation between TDv and PFS (P = 0.009; r = 0.669).

CONCLUSIONS

In TARE, voxel-based dose index TDv can be estimated on post-treatment 90Y PET using a simple method. TDv was a more effective prognostic factor for TARE than TDp and clinicopathologic factors in this pilot study. Further studies are warranted on the role of voxel-based dose and dose distribution in TARE.

Comparison of 90Y SIRT predicted and delivered absorbed doses using a PSF conversion method

PURPOSE

The aims of this study were to develop and apply a method to correct for the differences in partial volume effects of pre-therapy Technetium-99 m (^99mTc)-MAA SPECT and post-therapy Yttrium-90 (⁹⁰Y) bremsstrahlung SPECT imaging in selective internal radiation therapy, and to use this method to improve quantitative comparison of predicted and delivered ⁹⁰Y absorbed doses.

METHODS

The spatial resolution of ^99mTc SPECT data was converted to that of ⁹⁰Y SPECT data using a function calculated from ^99mTc and ⁹⁰Y point spread functions. This resolution conversion method (RCM) was first applied to ^99mTc and ⁹⁰Y SPECT phantom data to validate the method, and then to clinical data to assess the power of ^99mTc SPECT imaging to predict the therapeutic absorbed dose.

RESULTS

The maximum difference between absorbed doses to phantom spheres was 178%. This was reduced to 27% after the RCM was applied. The clinical data demonstrated differences within 38% for mean absorbed doses delivered to the normal liver, which were reduced to 20% after application of the RCM. Analysis of clinical data showed that therapeutic absorbed doses delivered to tumours greater than 100 cm³ were predicted to within 52%, although there were differences of up to 210% for smaller tumours, even after the RCM was applied.

CONCLUSIONS

The RCM was successfully verified using phantom data. Analysis of the clinical data established that the ^99mTc pre-therapy imaging was predictive of the ⁹⁰Y absorbed dose to the normal liver to within 20%, but had poor predictability for tumours smaller than 100 cm³.

Quantitative Imaging of Alpha-Emitting Therapeutic Radiopharmaceuticals

Targeted alpha therapy (TAT) is an active area of drug development as a highly specific and highly potent therapeutic modality that can be applied to many types of late-stage cancers. In order to properly evaluate its safety and efficacy, understanding biokinetics of alpha-emitting radiopharmaceuticals is essential. Quantitative imaging of alpha-emitting radiopharmaceuticals is often possible via imaging of gammas and positrons produced during complex decay chains of these radionuclides. Analysis of the complex decay chains for alpha-emitting radionuclides (Tb-149, At-211, Bi-212 (decayed from Pb-212), Bi-213, Ra-223, Ac-225, and Th-227) with relevance to imageable signals is attempted in this mini-review article. Gamma camera imaging, single-photon emission computed tomography, positron emission tomography, bremsstrahlung radiation imaging, Cerenkov luminescence imaging, and Compton cameras are briefly discussed as modalities for imaging alpha-emitting radiopharmaceuticals.

The Impact of PET and SPECT on Dosimetry for Targeted Radionuclide Therapy

Targeted radionuclide therapy (TRT) is an increasingly used treatment modality for a range of cancers. To date, few treatments have involved the use of dosimetry either to plan treatment or to retrospectively ascertain the absorbed dose delivered during treatment. Also the correlation between absorbed dose and biological effect has been difficult to establish. Tomographic methods permit the determination of the activity volume on a macroscopic scale at different time points. Proper attenuation correction in tomographic imaging requires a patient-specific attenuation map. This can be obtained from scintillation-camera transmission scanning, CT or by using segmented scatter-emission images. Attenuation corrections can be performed either on the projection images, on the reconstructed images, or as part of an iterative reconstruction method. The problem of image quantification for therapy radionuclides, particularly for I-131, is exacerbated by the fact that most cameras are optimised for diagnostic imaging with Tc-99m. In addition, problems may arise when high activities are to be measured due to count losses and mis-positioned events, because of insufficient pile-up and dead time correction methods. Sufficient image quantification, however is only possible if all effects that degrade the quantitative content of the image have been corrected for. Monte Carlo simulations are an appealing tool that can help to model interactions occurring in the patient or in the detector system. This is helpful to develop and test correction techniques, or to help to define detectors better suited to quantitative imaging. PET is probably the most accurate imaging method for the determination of activity concentrations in tissue. PET imaging can be considered for pre-therapeutic treatment planning but ideally requires the use of a radioisotope from the same element as that used for treatment (e.g. I-124 for I-131; Y-86 for Y-90). Problems, however are that--some of the positron emitting isotopes have a shorter half-life--non-standard quantification procedures have to be performed--the availability of the radiopharmaceutical is presently limited; Many 3D-tools and -techniques are now available to the physicist and clinician to enable absorbed dose calculations to both target and critical organs-at-risk. The challenge now facing nuclear medicine is to enable this methodology to be routinely available to the clinic, to ensure common standard operating procedures between centres and in particular to correlate response criteria with absorbed dose estimates.

Phantom study of fusion image of CT and SPECT with body-contour generated from external Compton scatter sources

PURPOSE

A phantom study was conducted to evaluate the feasibility of body contour definition with Compton scatter photons from external sources of technetium-99m pertechnetate (Tc-99m) to create a fusion image of CT and SPECT images.

METHODS

External sources of 1 mCi (37 MBq) Tc-99m were placed on each collimator, and body-contour SPECT images were obtained with an energy window of 100 keV +/- 25% for detecting 90 degrees and 180 degrees Compton scatter photons of Tc-99m from the body surface in water-filled cylindrical and hexagonal phantoms, and in a chest phantom with a Tc-99m-avid simulated lung nodule and multimethod surface markers. In the chest phantom, each transaxial SPECT slices was registered with the corresponding CT slice by using image-matching soft ware. A summation of the registered images yielded a three-dimensional (3-D) fusion image of this phantom.

RESULTS

This method clearly visualized the body contour on all the SPECT slices in all the phantoms except for the complex hexagonal phantom. There was no significant difference between the known and SPECT-measured diameters of the cylindrical phantom. The fit of CT and SPECT images of the chest phantom was achieved with a mean alignment error of 5% in visual inspection, which was improved to 0.2% after correction of the magnification of the SPECT images according to the resultant dimensional differences. The 3-D fusion image of this phantom effectively visualized the anatomic location of the lung nodule and surface markers.

CONCLUSION

This simple method effectively provided boundary information on the cold phantoms. Although further improvements in the registration technique with CT images are desirable, the body-contour SPECT image obtained by this method has the potential for accurately creating a 3-D fusion image with CT images, and is a feasible way of anticipating the anatomical localization of a target tissue.

Complete remission of nonresectable pancreatic cancer after infusional colloidal phosphorus-32 brachytherapy, external beam radiation therapy, and 5-fluorouracil: a preliminary report

This is a preliminary report of five patients diagnosed with locally advanced nonresectable pancreatic cancer who achieved improved quality of life, delay of local progression, and reduction of biomarker CA 19-9 after infusion of colloidal phosphorus 32 (32P) and administration of combined chemoradiotherapy. A phase II trial using intratumoral colloidal 32P delivery for nonresectable pancreatic cancer without metastases is in progress. Patients initially were given infusions of decadron followed by macroaggregated albumin and 30 mCi colloidal 32P to the interstitial space of the tumor by two infusions 1 week apart. Through this method, doses ranging from 750,000 to 1,800,000 cGy were delivered. After administration of colloidal 32P, external radiation to a dose of 6000 cGy minimum tumor dose, including regional lymph nodes, was given concomitantly with four intravenous infusions of 500 mg bolus 5-fluorouracil on alternating days within the first 2 weeks after initiation of external radiation. All five of these patients demonstrated cessation of local tumor growth or regression of disease on serial computed tomography scans for a minimum of 10 months from completion of therapy. Three of these patients have survived without local disease progression over 24 months from initiation of therapy, with one patient approaching 36 months. CA 19-9 values for all patients declined within weeks after completion of therapy. This new method of isotope delivery has resulted in reduction of tumor volume, normalization of the biomarker CA 19-9, and improved performance status in those patients who have localized nonresectable disease without dissemination.

Selective tumor irradiation by infusional brachytherapy in nonresectable pancreatic cancer: a phase I study

PURPOSE

Selective high-dose radiation of solid tumors has been a goal of radiation oncology. The physiological barriers of solid tumors (high interstitial tumor pressure, reduced tumor vascularity, and poor perfusion) have been major barriers in achieving significant tumor dose of systemically infused radioconjugates. Direct tumor infusional brachytherapy overcomes these barriers and leads to selective high tumor doses.

METHODS AND MATERIALS

The development of interstitial tumor infusion of macroaggregated albumin (MAA) followed by colloidal chromic phosphate 32P has overcome solid tumor obstacles in 47 patients with nonresectable pancreatic cancer in a Phase I dose escalation study. The colloidal 32P infusion was followed by external radiation and five fluorouracil.

RESULTS

Of the 28 patients with cancer limited to the pancreas, 15 of 16 patients retained 86-100% (mean 96%) of the infused colloidal 32P isotope. While the other 12 patients had partial shunting to the liver, shunting to the liver was due to high interstitial resistance with tumor dose deposition of 17-88% (mean 52 %). Of the 19 patients with metastatic pancreas cancer, colloidal 32P tumor deposition ranged from 22 to 100% of the infused dose (mean 79%). The less than optimal tumor deposition led to our increasing the MAA from 600,000 to 1.5-2.5 million particles. Interstitial dexamethasone 2 mg and later 4 mg was infused first and prevented liver shunting by somehow reducing tumor resistance. The median survival in 28 Phase I patients with nonresectable pancreas cancer without metastasis, was 12 months. No significant toxicity occurred when treatment was limited to two infusions with as much as 30 mCi each. The maximum tumor dose was 17,000 Gy (1.700,000 cGy). In 19 nonresectable pancreatic cancer patients with metastasis, a 6.9 months median survival was observed.

CONCLUSIONS

Infusional brachytherapy is an outpatient procedure that delivers high-dose radiation selectively to pancreatic cancer. Results of the Phase I study in nonresectable pancreas cancer has led to a national multiinstitutional Phase II trial.

Partition model for estimating radiation doses from yttrium-90 microspheres in treating hepatic tumours

A uniform distribution of yttrium-90 (90Y) microspheres throughout the entire liver has always been assumed for dose calculation in treating hepatic tumours. A simple mathematical model was formulated which allows estimation of the activities of a therapeutic dose of 90Y microspheres partitioned between the lungs, the tumour and the normal liver, and hence the radiation doses to them. The doses to the tumour and normal liver were verified by intra-operative direct beta-probing. The percentage of activity shunted to the lung and the tumour-to-normal tissue ratio (T/N) were obtained from gamma scintigraphy using technetium-99m-labelled macroaggregated albumin (MAA) which simulates the 90Y microspheres used in subsequent treatment. The intrahepatic activity was partitioned between the tumour and the normal liver based on the T/N and their masses determined from computerized tomography slices. The corresponding radiation doses were computed using the MIRD formula. The estimated radiation doses were correlated with the doses directly measured using a calibrated beta-probe at laparotomy by linear regression. The radiation doses to the tumour and the normal liver, estimated using the partition model, were close to that measured directly with coefficients of correlation for linear regression: 0.862 for the tumours and 0.804 for the normal liver compartment (P<0.001). The partition model permits a distinction between the radiation doses received by the tumour and the normal liver to be made and the doses thus estimated are close to the actual doses received. The optimal doses to the tumour and normal liver and hence the required quantity of 90Y microspheres to be administered can be easily predetermined.