By 1960, Carson had more than enough research material, and the writing was progressing rapidly. In addition to the thorough literature search, she had investigated hundreds of individual incidents of pesticide exposure and the human sickness and ecological damage that resulted. However, in January, a duodenal ulcer followed by several infections kept her bedridden for weeks, greatly delaying the completion of "Silent Spring". As she was nearing full recovery in March (just as she was completing drafts of the two cancer chapters of her book), she discovered cysts in her left breast, one of which necessitated a mastectomy. Though her doctor described the procedure as precautionary and recommended no further treatment, by December Carson discovered that the tumor was malignant and the cancer had metastasized. Her research was also delayed by revision work for a new edition of "The Sea Around Us" and by a collaborative photo essay with Erich Hartmann. Most of the research and writing was done by the fall of 1960, except for the discussion of recent research on biological pest controls and investigations of a handful of new pesticides. However, further health troubles slowed the final revisions in 1961 and early 1962. While writing the book Carson chose to hide her illness so that the pesticide companies couldn't use it against her (she worried that if the companies knew, they would use it as ammunition to make her book look untrustworthy and biased).