Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization.

BACKGROUND: PET/CT has recently been shown to be a viable alternative to traditional post-infusion imaging methods providing good quality images of 90Y-laden microspheres after selective internal radiation therapy (SIRT). In the present paper, first we assessed the quantitative accuracy of 90Y-PET using an anthropomorphic phantom provided with lungs, liver, spine, and a cylindrical homemade lesion located into the hepatic compartment. Then, we explored the accuracy of different computational approaches on dose calculation, including (I) direct Monte Carlo radiation transport using Raydose, (II) Kernel convolution using Philips Stratos, (III) local deposition algorithm, (IV) Monte Carlo technique (MCNP) considering a uniform activity distribution, and (V) MIRD (Medical Internal Radiation Dose) analytical approach. Finally, calculated absorbed doses were compared with those obtained performing measurements with LiF:Mg,Cu,P TLD chips in a liquid environment. RESULTS: Our results indicate that despite 90Y-PET being likely to provide high-resolution images, the 90Y low branch ratio, along with other image-degrading factors, may produce non-uniform activity maps, even in the presence of uniform activity. A systematic underestimation of the recovered activity, both for the tumor insert and for the liver background, was found. This is particularly true if no partial volume correction is applied through recovery coefficients. All dose algorithms performed well, the worst case scenario providing an agreement between absorbed dose evaluations within 20%. Average absorbed doses determined with the local deposition method are in excellent agreement with those obtained using the MIRD and the kernel-convolution dose calculation approach. Finally, absorbed dose assessed with MC codes are in good agreement with those obtained using TLD in liquid solution, thus confirming the soundness of both calculation approaches. This is especially true for Raydose, which provided an absorbed dose value within 3% of the measured dose, well within the stated uncertainties. CONCLUSIONS: Patient-specific dosimetry is possible even in a scenario with low true coincidences and high random fraction, as in 90Y-PET imaging, granted that accurate absolute PET calibration is performed and acquisition times are sufficiently long. Despite Monte Carlo calculations seeming to outperform all dose estimation algorithms, our data provide a strong argument for encouraging the use of the local deposition algorithm for routine 90Y dosimetry based on PET/CT imaging, due to its simplicity of implementation.

Absorbed dose to lesion and clinical outcome after liver radioembolization with 90Y microspheres: a case report of PET-based dosimetry.

A 54-year-old woman with metastatic colorectal carcinoma underwent liver radioembolization with (90)Y resin microspheres. Microsphere biodistribution was assessed 2 h after the treatment through a 20-min long (90)Y PET scan. Isodose map and lesion dose-volume histogram (DVH) were then evaluated using a MATLAB-based code. Response to therapy was assessed performing a (18)F-FDG PET 6 months after the treatment. At (90)Y PET the patient showed a well-defined horseshoe-shaped hepatic lesion with hot margins and a cold core. The lesion presented a heterogeneous DVH with a hot margin receiving an average radiation dose as high as 287 Gy and a cold area receiving an average radiation dose of 70 Gy approximately. Six months after the treatment the patient reported a complete remission of tumour areas which received a high radiation dose, while progression of metastases was observed in the area that presented scarce microsphere localization at (90)Y PET. According to our experience, the use of (90)Y PET voxel dosimetry may provide a useful tool to assess possible correlations between microsphere biodistribution and clinical outcome of the treatment. In agreement with current literature findings, an average radiation dose greater than approximately 100 Gy may be required to sterilize liver metastases.

90Y PET-based dosimetry after selective internal radiotherapy treatments.

OBJECTIVES: The decay of 90Y has a minor branch to the O+ first excited state of 89Zr, the de-excitation of which to the fundamental state is followed by a ß+­ß- emission that has been used recently for biodistribution assessment after selective internal radiotherapy (SIRT) treatments. The purpose of the present study is to demonstrate the feasibility of 90Y PET imaging for dose assessment after radioembolization with 90Y microspheres. METHODS: Activity quantification was validated through preliminary phantom studies using a cylindrical body phantom composed of six inserts of different volumes filled with a calibrated amount of 90Y microspheres. A GE Discovery ST PET/CT scanner provided with bismuth germinate (BGO) crystals was used for image acquisition. Images were reconstructed with an ordered subset expectation­maximization method. The effect of object size and the effect of the number of iterations on dose evaluation and volume recovery were investigated. Microsphere dose distribution was then evaluated on one patient (one lesion) who underwent liver SIRT treatment. Dose calculations were made with a MATLAB-based code developed in our department. Dedicated Monte Carlo calculations were executed to evaluate dose S-values for the 90Y source. The activity distribution derived from 90Y PET acquisitions was convolved with the voxel S-values to obtain a three-dimensional absorbed dose distribution and dose­volume histograms. RESULTS: Dosimetry studies carried out on the body phantom with ordered subset expectation­maximization algorithm, three iterations, provided an accuracy of 7.62% in determining the absorbed dose in the largest insert. The dose difference increases as the insert size reduces. Preliminary results on a patient provided a high-resolution absorbed dose distribution map. An average dose of 139.3 Gy was evaluated for the tumor area, with a maximum dose as high as 237.9 Gy. The absorbed dose to the healthy liver was below the tolerance dose of 35 Gy (33.8 Gy). A clear correlation between absorbed dose and tumor response was observed at 18F-fluorodeoxyglucose PET acquired 6 months after treatment. CONCLUSION: According to our experience, 90Y PET is a promising and reliable technique for microsphere dose assessment and might pave the way for a patient-specific PET-based dosimetry after liver SIRT treatments.

90Y-PET for the assessment of microsphere biodistribution after selective internal radiotherapy.

OBJECTIVES: To demonstrate the feasibility of 90Y-PET imaging for biodistribution assessment after selective internal radiotherapy treatments with 90Y-microspheres, comparing the results with 99mTc-macroaggregated albumin (MAA) images obtained with single-photon emission computed tomography. METHODS: Preliminary studies were performed with the aim of evaluating the imaging system spatial resolution and scanner sensitivity for detecting annihilation photons. Subsequently, microsphere distribution was evaluated in 10 patients who underwent liver selective internal radiotherapy treatment. 99mTc-MAA and 90Y-microsphere were simultaneously injected for immediate monitoring after treatment. For each patient, the metastases detected with 90Y-PET and 99mTc-MAA were assessed and compared with 18F-fluorodeoxyglucose-PET (18F-FDG-PET) obtained before treatment and used as an imaging benchmark procedure. The correlation between these techniques was thus investigated in terms of matching lesions. Lesions were considered true positive in the case of matching with 18F-FDG-PET. The sensitivity of both techniques was evaluated as the true-positive fraction of detected spots in the treated liver sectors. RESULTS: With our experimental setup, a maximum scanner sensitivity of 0.577 and 0.077 cps/MBq was obtained for three-dimensional and two-dimensional acquisitions, respectively. A good correlation was obtained between images obtained before and after treatment, with 90Y-PET being by far the most accurate technique in detecting microsphere distribution and tumor nonhomogeneity areas. A sensitivity as high as 0.91 was obtained with 90Y-PET, whereas 99mTc-MAA imaging showed a SE of 0.75. CONCLUSION: 90Y-PET is a promising and reliable technique for microsphere biodistribution evaluation after liver selective internal radiotherapy treatment. Because of the better resolution and the possibility to perform computed tomography fusion, 90Y-PET images are more accurate than 99mTc-MAA single-photon emission computed tomography, which is now considered the gold standard for biodistribution assessment.