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had cirrhosis with Child-Pugh score <8. The prescribed dose was 40 Gy in four fractions. Dose reduction was permitted for normal organ dose constrains. The bile duct (BD) was delineated from the common bile duct to the first bifurcation of left and right intrahepatic duct. In addition, the central hepatobiliary tract (cHBT) was defined by a 10 or 15 mm expansion of the portal vein from the splenic confluence to the first bifurcation of left and right portal veins. We analyzed the clinical and dosimetric parameters, including multiple dose-volume histogram endpoints: D max (the maximum point dose), D mean (the mean dose), V 40Gy (volume of cHBT that received 40 Gy), V 37Gy , V 34Gy. Receiver operator curves (ROC) defined optimal dosimetric thresholds for analysis. HB toxicities were graded according to Common Terminology Criteria for Adverse Events version 4.0 and we defined grade 3+ HB toxicity as a severe HB toxicity. Results: Median follow-up duration was 9.9 months after SABR. Eight out of 28 patients (28.6%) experienced severe HB toxicity. Among clinical and dosimetric parameters, V 40Gy of cHBT with 10 mm expansion were highly associated with severe HB toxicity: V 40Gy >40 cm 3 (relative risk [RR] Z 3.2, P < 0.011). However, clinical or other dosimetric factors, D max or D mean of BD and cHBT, did not have predictive value. The risk of severe HB toxicity for V 40Gy 20 cm 3 , > 20 cm 3 , > 30 cm 3 , and > 40 cm 3 are 8.3% (n Z 1/12), 41.2% (n Z 7/17), 46.2% (n Z 6/13), and 54.5% (n Z 6/11), respectively. Conclusion: SABR to the central liver lesions should be used with caution due to the risk of HB toxicity. Radiation doses to cHBT are associated with development of severe HB toxicity. We suggest that V 40Gy <20 cm 3 as a potential dose constraint for cHBT when delivered in four fractions.

Flyby images of (486958) Arrokoth (Ultima Thule, 2014 MU69) show a comparatively bright “necklace” in the neck, or cleft, between its two lobes, in contrast to its generally low albedo. We suggest that the necklace may be the result of thermally controlled ice deposition. The necklace is found in the most (orbitally averaged) shaded part of the surface. It may consist of clean, high albedo, ice condensed from vapor sublimed by dirty, low albedo, ice elsewhere; ice accumulates where the maximum temperatures are the lowest. Ammonia and propane have the necessary mesovolatile vapor pressure. Surrounding gas in the proto-Solar System would facilitate redeposition of molecules sublimed by warmer parts of the surface into the cleft, as well as smoothing the surface and explaining, by hydrodynamic drag, Arrokoth’s slow (compared to its breakup rate) rotation. Alternatively, a layer of hoarfrost thick enough (≳ 0.1 μ) to have a high albedo could have formed more recently.