Tc99m-MAA

Cholecystitis is a rare but dolorous complication after Y90-radioembolization of liver malignancies.

To decide the occlusion of the cystic artery (CA) to prevent cholecystitis after Y90 radioembolization using an algorithm.

In 130 patients, the gallbladder was at risk of embolization as the right liver lobe was targeted. Precautionary measures (e.g. coil occlusion of the cystic artery) were decided by enhancement of the gallbladder in pre-treatment Tc99m-MAA SPECT/CT and performed directly before Y90 radioembolization. In non-enhancing cases, the CA was left open. The outcome was determined by clinical symptoms of acute or chronic cholecystitis as well as imaging and laboratory parameters. Findings were additionally classified according to the Tokyo Guidelines of acute cholecystitis.

Only 16 patients demonstrated enhancement of the gallbladder in Tc99m-MAA SPECT/CT. Including additional indications from angiographic findings, prophylactic measures were scheduled in 22 patients (standard of care). Thus, 121 patients were at risk of non-target embolization to the gallbladder during Y90 microsphere administration (investigative arm). Four cases (3.0%) of cholecystitis occurred by clinical presentation: two patients with onset of acute symptoms within 48 h after Y90 radioembolization (“embolic cholecystitis”) and two patients with late onset of symptoms (“radiogenic cholecystitis”). The incidence of cholecystitis was not significantly more frequent without indication of precautionary measures (investigative cohort 2.9% vs. standard of care 4.7%; P = 0.53).

The overall incidence of cholecystitis after Y90 radioembolization is low. Determination of cystic artery intervention using Tc99m-MAA SPECT/CT successfully balances the incidence of symptomatic cholecystitis with unnecessary vessel occlusion.

We have read the comments to our recently published article about Y90-radioembolization of lung metastases via the bronchial artery with great interest, and we welcome the additional perspective on dosimetry for Y90-radioembolization outside the liver [1, 2]. However, some thoughts must be added. First, it should be noted that the authors of the letter deduce their approach from a systematic description of the compartmental distribution as utilized in radioembolization of liver malignancies [2]. This model is then extended to cater different application scenarios (lung, kidney). However, other than implied in the letter dosimetry for Y90-radioembolization of the liver generally remains highly controversial with no uniform conclusions between numerous research groups [3–6]. We wish to highlight the fact that while dosimetric considerations naturally are a hallmark of radioembolization, in case of lung treatment gaining more basic knowledge must be prioritized to ensure patient safety. Radiobiological effects and kinetic properties of the compartments and substances for dosimetric modeling are widely unknown—not in regard to the target volume, but also to normal lung tissue. As we have learned from radioembolization of liver malignancies, dosimetry predicting activity patterns and uptake in different morphologic structures (tumor, liver tissue, extrahepatic tissue) is challenging. In contrast to the letter’s authors, we challenge the predictive value of Tc-99m-MAA as a surrogate for pretherapeutic dosimetry. The essential correlation between the distribution of the surrogate (Tc-99m-MAA) and the distribution of the therapeutic agent (Y-90-SIRSpheres) is generally weak. Several groups have discussed discordant accumulation patterns for both substances [7–9]. Just recently, we have published a patient series where Tc-99mMAA accumulation in colorectal liver lesions did not correlate with clinical response [10]. In light of these assumptions supplemented by the different morphology of lung tissue compared with liver the eligibility of the Tc99m-MAA as surrogate for radioembolization of lung malignancies must be reevaluated. In addition, we do not believe that in lung tissue glass spheres with a higher Y90 load and lower embolic potential compared with resin spheres have theoretical advantages in safety and efficacy. As a matter of fact, the bronchial artery blood pool comprises a very low volume compared with the hepatic artery. We hypothesize that the temporary embolic effect of resin particles has been an integral part of the observed strong activity accumulation in the lung tumors. During fluoroscopy, it became apparent that the resin spheres almost immediately embolized (temporarily) the capillary bronchial artery system, resulting in a lowdose exposure of bronchus or lung tissue given the relatively low Y90 load per particle compared with glass spheres. This might explain why relatively low activity applied in our patients led to an objective response; it seems that due to the embolic effect described most of the dose accumulated in the tumor blood pool rather than the bronchial artery system. For that reason, vascular stasis must not be excluded a priori by a dosimetric model as proposed by the authors of the letter, but it is part of the treatment success. However, this aspect seems of utmost importance and must be clarified before tumor dosimetry can be considered. It is our conviction that dosimetric models for radioembolization should commit themselves more to the effects of the 3D-dose distribution. Establishing a mean dose does O. S. Groser H. Amthauer J. Ricke (&) Department of Radiology and Nuclear Medicine, University of Magdeburg, Magdeburg, Germany e-mail: jens.ricke@med.ovgu.de