I.	Introduction Marsha Kinney, James Cook and Steven Swerdlow

More ways to err than one would like to imagine: The discussion today represents more than half a century of experience the faculty has had in trying avoid errors in diagnostic hematopathology – an arduous and not always completely successful task. Our course will emphasize not only ways in which one can easily fall into a deep black hole, but also how to stay on the high ground. There are many sources of potential errors in pathology in general and hematopathology in specific. Inadequate histologic sections are a guarantee for disaster at some point, and attention to detail is critical as always. Attempting to do too much in too little time, a curse many of us are faced with, is not optimal. Concern for sleep deprivation must extend to practicing pathologists, in addition to our trainees. Many of our cases illustrate specific ways in which potential errors arise. The first step in avoiding errors is to know one's options in terms of possible diagnoses – it is impossible to diagnose an entity that you didn't even know existed. We also need to take into account as much information as possible in diagnosing cases including patients' histories – something that is frequently not handed to us upon a golden platter. The location of mass lesions can also provide important clues to the diagnosis; however, not all disorders at a given site will be those that are the most common. It is inappropriate to diagnose a MALT lymphoma just because you have a lymphoma in a gastric biopsy. While our growing armamentarium of ancillary studies is a way to stay out of trouble, they can also get you into trouble sometimes by providing potentially misleading data. Getting back a flow cytometry report that says "polyclonal B-cells and heterogeneous T-cells without an aberrant phenotype" doesn't rule out a non-Hodgkin lymphoma or even a B-cell lymphoma. Focussing on one pathologic finding and ignoring others that are present can also lead you down the wrong yellow brick road. Finding reactive follicles in a lymphoid proliferation might make you feel somewhat reassured until you think through the list of lymphomas where reactive follicles are actually an expectation! Related to all of the above, being one step behind the rest of the field is another excellent way to end up trying to climb out of a major cess pool. Nevertheless, it is acknowledged that this can be a near impossible task in a world where it seems hard some days just to get your basic work done. And then there are the "needle in a hay stack" cases – I nearly lost some good friends that way at a USCAP evening session because the needles were not apparent in all of the sections that were distributed. Our agenda: Classification of malignant lymphomas and the avoidance of errors Specific benign entities that can be easily misdiagnosed as a lymphoma Cases 1 and 2 – Dr. Kinney Malignant lymphomas mimicking reactive processes Cases 3-5 – Dr. Cook Cases 6-7 – Dr. Swerdlow Recuperation time Malignant lymphomas that can go totally unrecognized Case 8 – Dr. Kinney Malignant lymphomas that look like lymphomas of a different type and which have very different clinical implications Case 9 – Dr. Kinney Cases 10 & 11 – Dr. Cook Case 12 – Dr. Swerdlow Quiz! Case 13 – Dr. Swerdlow