REFERENCES 1. Gates VL, Marshall KG, Salzig K, Williams M, Lewandowski RJ, Salem R. Outpatient single-session yttrium-90 glass microsphere radioemboli- zation. J Vasc Interv Radiol 2014; 25:266–270. 2. van den Hoven AF, Prince JF, van den Bosch MAAJ, Lam MGEH. Hepatic radioembolization as a true single-session treatment. J Vasc Interv Radiol 2014; 25:1143–1144. 3. Salem R, Parikh P, Atassi B, et al. Incidence of radiation pneumonitis after hepatic intra-arterial radiotherapy with yttrium-90 microspheres assuming uniform lung distribution. Am J Clin Oncol 2008; 31:431–438. 4. Leung TW, Lau WY, Ho SK, et al. Radiation pneumonitis after selective internal radiation treatment with intraarterial 90yttrium-microspheres for inoperable hepatic tumors. Int J Radiat Oncol Biol Phys 1995; 33: 919–924. 5. Lin M. Radiation pneumonitis caused by yttrium-90 microspheres: radio- logic findings. AJR Am J Roentgenol 1994; 162:1300–1302. 6. Wright CL, Werner JD, Tran JM, et al. Radiation pneumonitis following yttrium-90 radioembolization: case report and literature review. J Vasc Interv Radiol 2012; 23:669–674. 7. O’Doherty J, Scuffham J, Hinton P. The importance of scatter correction for the assessment of lung shunting prior to yttrium-90 radioembolization therapy. Nucl Med Commun 2011; 32:628–634. 8. Yu N, Srinivas SM, Difilippo FP, et al. Lung dose calculation with SPECT/ CT for (90)Yttrium radioembolization of liver cancer. Int J Radiat Oncol Biol Phys 2013; 85:834–839. 9. Jha AK, Zade AA, Rangarajan V, et al. Comparative analysis of hepato- pulmonary shunt obtained from pretherapy 99mTc MAA scintigraphy and post-therapy 90Y Bremsstrahlung imaging in 90Y microsphere therapy. Nucl Med Commun 2012; 33:486–490. The Limitations of Theoretical Dose Modeling for Yttrium-90 Radioembolization From: David Liu, MD, FSIR, FRCPC Darren Klass, MD, PhD, MRCS, FRCR Mark Westcott, MD Andrew S. Kennedy, MD Department of Radiation Oncology Research (A.S.K.) Sarah Cannon Research Institute Nashville, Tennessee; Department of Interventional Oncology (M.W.) Lenox Hill Hospital New York, New York; and Angio/Interventional Section Department of Radiology (D.K., D.M.L.) University of British Columbia JP G873 Vancouver, BC V5Z1M9, Canada Editor: We appreciate the opportunity comment on the article by Lam et al (1) published in the Journal of Vascular Interventional Radiology regarding radioembolization with resin microspheres as it relates to whole-liver absorbed radiation doses using the simplified medical internal radiation dosimetry (MIRD) formula (com- monly used with glass microspheres), as opposed to the currently recommended body surface area (BSA) for- mula, in the metastatic colorectal carcinoma (mCRC) population. The authors conclude that the administered activity of yttrium-90 ( 90 Y) did not correlate with the theoretical whole-liver absorbed dose (D WL ), and that therefore the BSA formula may be inappropriate. We respectfully disagree with this conclusion, because a number of assumptions were made that ultimately may have led to an incorrect interpretation of the data and ultimately to an erroneous conclusion. The first incorrect assumption is that stasis during resin microsphere delivery results in too little radiation implanted in the tumor. We note that this report (1) was the result of retrospective review of patients treated before the replacement of sterile water with nonionic contrast media and 5% dextrose in water, which is known to reduce vessel spasm and has shown a signi- ficant reduction in stasis. Nevertheless, the percentage of reported cases of stasis (28.9%), as well as the incidence of gastrointestinal ulceration (11.1%), caused by the nontarget distribution of 90 Y are greater than the 1.8% incidence published in larger single-center and multi- center trials—even in the current “bevacizumab era” (2). We would invite further comments regarding their experiences with the revised technique. The second incorrect assumption is that single- compartment MIRD partition accurately estimates tumor radiation. The BSA-based prescribed activity was used retrospectively to calculate DWL for each patient. DWL was calculated using the MIRD formula of 50 Gy per 1 GBq activity/kg tissue (similar to the dose calculation used in Glass microsphere activity determination). From this statement, it becomes apparent that a fundamental question must be asked: what does a theoretical uniform exposure mean in terms of tumor uptake and parenchyma exposure in D WL ? The entire basis of arterial embolo- therapy is based on the axiom that higher concentrations of therapy accumulates in the tumor as a result of preferential hepatic arterial neovascularization, a fact that is not considered with D WL . In contrast, the BSA method inherently incorporates the degree of tumor infiltration. It was concluded that patients with larger livers were relatively “underdosed,” and patients with smaller livers were relatively “overdosed,” without formal definition (1). Because D WL uses absolute parenchyma and tumor mass in its calculation, it is inherent to this method that patients with larger livers will require greater amounts of radiation. Therefore, the conclusions regarding over- and underdosing represent a logical fallacy—circulus in probando (circular logic)—with respect to comparison versus the BSA method. Ironically, many experts believe http://dx.doi.org/10.1016/j.jvir.2014.03.025 D.M.L. receives a salary from Sirtex Medical (North Sydney, Australia). A.S.K. received funding from an unrestricted research grant from Sirtex Medical. Neither of the other authors has identified a conflict of interest. Liu et al ’ JVIR 1146 ’ Letter to the Editor