The effect of three dimensional activity distribution on the dose planning of radioimmunotherapy for patients with advanced intraperitoneal pseudomyxoma

Intraperitoneal radioimmunotherapy: Auger electron emitters for solid tumors

Evaluation of: Boudousq V, Ricaud S, Garambois V et al.: Brief intraperitoneal radioimmunotherapy of small peritoneal carcinomatosis using high activities of noninternalizing 125I-labeled monoclonal antibodies. J. Nucl. Med. 51, 1748–1755 (2010). Mesothelioma, pseudomyxoma peritonei, ovarian and colon cancers, and several other malignancies produce peritoneal carcinomatosis in their advanced stages. This condition is typically managed with curative intention by means of a radical cytoreductive surgery and intraperitoneal hyperthermic chemotherapy. The method has been shown to improve the survival of some patients with peritoneal dissemination. However, the intracavitary instillation of chemotherapy results in a nonuniform drug distribution and allows penetration of the drug only into the outermost layer of the cancer nodule. Authors of the current study demonstrate that 125I-radiolabeled monoclonal antibody, 35A7, which recognizes anti-carcinoembryonic antigen, can be used effectively and safely with low toxicity in the therapy of small volume peritoneal carcinomatosis after cytoreductive surgery.

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.

Pseudomyxoma peritonei

Pseudomyxoma peritonei is a rare form of mucinous ascites associated with peritoneal and omental implants. The origin is controversial, and recent immunohistochemical and molecular genetic evidence suggests the appendix to be the likely site. The condition often presents as an incidental finding at laparotomy. Ultrasonography, computed tomography and magnetic resonance imaging aid in preoperative diagnosis. Treatment remains controversial, surgery being the main stay. The role of intraperitoneal and systemic chemotherapy is poorly defined. We review the literature on the pathology, clinical features and treatment options in pseudomyxoma peritonei.

Comparison of four scatter correction methods for patient whole-body imaging during therapeutic trials with iodine-131

BACKGROUND

Activity in regions of interest can be measured using serial whole-body scintigraphic images to estimate the dose received by a patient after therapeutic injections. As scatter and attenuation introduce biases in quantitative measurements, these phenomena need to be corrected to allow accurate determination of tracer concentration.

METHODS

The feasibility of iodine-131 whole-body imaging in list mode was studied over an extended spectrum (0-750 keV) in order to compare four scatter correction methods by the geometric mean approach (20%, Dual Energy Window, Triple Energy Window, and Spectral Factor Analysis methods). All data were corrected for attenuation using a Transmission Attenuation Correction prototype from Sopha Médical Vision international. The half-life of an iodine-131 standard source was calculated from scatter-corrected anterior views. Whole-body activities, using the Day 0, Hour 1, image as a reference (calibration from an administered dose) and an external calibration source (calibration from an imaged known-activity source), were calculated for three patients undergoing a radioimmunotherapy trial in order to assess the reliability of quantification by the geometric mean approach.

RESULTS

Patient studies confirmed the clinical feasibility of this type of acquisition. As expected, all methods allowed determination of an accurate half-life for the calibration source. A slight impact of scatter correction was observed in quantification with calibration from an administered dose. For quantification with calibration from an imaged known-activity source, whole-body activity was overestimated by +100% to +200% with the 20% window, depending on the size of the patient, whereas errors were about +50% with scatter correction. However, the influence of patient morphology was less marked when a scatter correction method was used.

CONCLUSIONS

When the geometric mean approach is used together with a sophisticated transmission acquisition device for quantification with calibration from an administered dose, the 20% energy window appears to be adequate. However, for quantification with calibration from an imaged known-activity source, accurate activity estimates cannot be obtained even when scatter correction is used to compensate for the influence of patient morphology.