Prepared by staff in Prevention and Cancer Control. Citation: Cancer Care Ontario. Cancer Fact: In situ cancers being reported in Ontario. February 2016. Available at http://www.cancercare.on.ca/cancerfacts. Prepared by staff in Prevention and Cancer Control. In situ cancers being reported in Ontario (Feb. 2016) In 2010–2012, 33 per cent of all in situ cancers diagnosed in Ontario were cervix. Registration of in situ cancers in the Ontario Cancer Registry began in 2010. If left untreated, an in situ cancer can progress to a malignant cancer. For the first time, the Ontario Cancer Registry (OCR) is reporting in situ cancers. An in situ cancer is an early-stage cancer in the epithelial cells (i.e., cells that line internal organs) that is still confined to the site where it began. Historically, the OCR only contained information about malignant cancers (i.e., cancers that are starting to invade nearby tissue), but in 2010 it began registering in situ cancers to provide information on the full spectrum of cancer progression. In 2010–2012, in situ cervical cancers* (N=10,777) accounted for 33 per cent of all in situ cancers* (N=32,903) diagnosed in Ontario and 85 per cent of all cervical cancers (N=12,731) — only 1,954 cervical cancers were diagnosed as malignant. The large number of in situ cancers of the cervix is not surprising given the implementation of the Ontario Cervical Screening Program, a province-wide program launched in 2000 to prevent cervical cancer by identifying the disease at an early stage when cancers are curable and easier to treat.1 The next most common in situ cancers diagnosed in 2010–2012 were melanoma (7,199 cancers), bladder (5,407 cancers), and female breast (4,559 cancers). There were 1,557 in situ cancers of all other female genital organs combined and 1,416 in situ cancers of the colon and rectum. In situ cancers made up about 44 per cent of all melanoma, 28 per cent of bladder cancers, over 10 per cent of female breast cancers, and only five per cent of colon and rectum cancers. Over 69 per cent of all in situ cancers were diagnosed in females, driven mainly by the large numbers of cervical, breast and other female genital organ cancers. For melanoma, bladder and colon and rectum, however, more in situ cancers were diagnosed in males. In situ bladder cancer is coded as in situ in the OCR, but is treated and reported as malignant. This is in line with how other provincial and territorial cancer registries report bladder cancer due to the significant morbidity and mortality associated with it, and the fact that in pathology reports it is hard to detect how much it has spread.2 It is important to seek treatment for an in situ cancer because if left untreated, it can progress to a malignant cancer. Treatment options vary by cancer site and include surgical removal and radiation therapy.3–6 In most cases, malignant cancer can be prevented by removing the in situ cancer. *All microscopically confirmed in situ cancers are included, except cervix in situ cancers microscopically confirmed by cytology. References 1. Cancer Care Ontario. Ontario Cervical Screening Program 2012 Report. Toronto, Canada: 2014. 2. Public Health Agency of Canada. Cancer in Canada – An Epidemiologic Overview. Ottawa, Canada: 2014. 3. World Health Organization. WHO Guidelines for Screening and Treatment of Precancerous Lesions for Cervical Cancer Prevention. Geneva: 2013. 4. Higgins HW II, Lee KC, Galan A, Leffell DJ. Melanoma in situ: Part II. Histopathology, treatment, and clinical management. J Am Acad Dermatol. 2015;73(2):193-203. 5. Kerlikowske K. Epidemiology of ductal carcinoma in situ. J Natl Cancer Inst Monogr. 2010;2010(41):139-41. 6. Tang DH, Chang SS. Management of carcinoma in situ of the bladder: best practice and recent developments. Ther Adv Urol. 2015;7(6):351-64.