Colorectal cancer (CRC) is a serious heterogeneous disease that stands in third place in cancer incidence and represents the second cause of death in the world (nearly1.8 million patients newly diagnosed and 1 million patients who die every year) [1]. CRC has become predominant cancer in Western countries, which could be partially explained by the aged population and adverse lifestyle habits such as smoking, increased consumption of red meat and alcohol, lack of physical activity related to obesity, and diabetes, connected usually with low diversity of intestine microflora. Risk factors also include positive family history reflecting individual genetic equipment [2]. The screening programs aim to identify patients with precancerous lesions or those with resectable CRC stages 0, I, and II, who have a generally better prognosis than symptomatic patients with the pre-existing disease [3]. Despite screening programs, many patients are diagnosed in the stages III and IV of CRC that lead to a worse overall prognosis [4]. According to data from National Cancer Institute from the United States of America (USA) [5], five years survival rates for stage IV account only for 12% at colon cancer (CC) and 13% at rectal cancer (RC), while detection at an early stage I can increase the chance to survive up to 92% at CC and 88% at RC [6].

Several screening methods such as stool testing, blood testing, and endoscopic and radiological examination are currently available [7,8]. Polyps or tumors can manifest by microscopic bleeding (so-called occult bleeding). First-line test detecting occult bleeding is a fecal occult blood test (FOBT). Guaiac fecal occult blood test (gFOBT) detects hemoglobin (Hb) by peroxidase activity. Nevertheless, gFOBT is insufficiently specific to human hemoglobin and connected with a risk of false-positive results and omission of small polyps or non-bleeding polyps [7]. Despite its disadvantages, it has been able to contribute to a 33% reduction in CRC mortality [9]. Nowadays, gFOBT has predominantly been replaced by a fecal immunochemical test (FIT) based on antibody assay, which provides qualitative and quantitate results on Hb concentration per gram feces. Positive results of gFOBT or FIT are followed by endoscopic examination [8].

Endoscopic methods include colonoscopy examination, sigmoidoscopy, or capsule endoscopy. Colonoscopy is the main investigative method of the large bowel, this technique provides visualization of the entire large intestine, precise localization, biopsy, or complete removal of a potential precancerous lesion in a single session [10]. Early polypectomy leads to a 76–90% reduction in CRC incidence [11]. The weakness of this method is its invasiveness, it is an unpleasant procedure requiring several days of diet restriction and bowel preparation. These could pose an obstacle for many people, and among other things, it is expensive with the necessary presence of a very well-trained examiner [12].

Sigmoidoscopy compared to colonoscopy reduces time-consuming examination and patient discomfort and provides a lower risk of complications without the need for sedation, but allows investigation of only the rectum and the sigmoid. The study of sigmoidoscopy screening of individuals between 55 and 64 years in the United Kingdom (UK), indicated subsequent CRC incidence reduction by 33% and mortality by 43% [13].

Colon capsule endoscopy (CCE) is a non-invasive method suitable for individuals who are unwilling to undergo colonoscopy because of discomfort or any other obstacles. Meta-analysis showed that CCE for any polyp has a specificity of 89% and sensitivity of 73%. Though CCE is not as accurate as colonoscopy, it could decrease the need for its application [14].

CRC screening by radiology using computed tomographic (CT) colonography is able to visualize the entire colorectum and with no need for sedation. Even though it still requires bowel preparation, it is a relatively non-invasive method. This technique can detect only large adenomas and tumors with size ≥10 mm, nevertheless with sensitivity of 90% [15].

Screening methods based on blood testing were enriched by a highly promising biomarker, methylated gene septin9 (^mSEPT9) in the last few years. ^mSEPT9 is released from CRC cells into circulation and is detectable in peripheral blood. A recent study showed that ^mSEPT9 assay, approved by Food and Drug Administration (FDA) in the USA, has a higher specificity (94.5%) than FOBT at advanced stages of CRC, but not at asymptomatic patients with early neoplasia [16,17].

Several types of a lesion can be histologically described from the colonoscopy biopsy. A colon polyp is a small clump of cells that forms on the lining of the colon epithelium. There are two main classes of polyps, non-neoplastic and neoplastic (Table 1) [18]. In general, the larger the neoplastic adenoma the greater the risk of cancer. Table 2 shows the recommended follow up after patient polypectomy [19,20,21]. Although the recommended surveillance guideline has been widely accepted, clinicians still detect the incidence of CRC (<10%) developed during the initial colonoscopy and the subsequent follow-up examination. This subgroup of CRCs is referred to as interval CRC (I-CRC) and represents one of the problems that screening programs face [22].

Non-Neoplastic		Neoplastic
Sporadic	Hereditary	Sporadic		Hereditary
Hyperplastic polyps	Hyperplastic polyposis	Benigns adenomas:	Tubular	Familial adenomatous polyposis (FAP)
			Villous
			Tubulovillous
Inflammatory polyps	Juvenile polyposis	Serrated adenomas:	Sessile serrated	Hereditary non-polyposis colorectal cancer (HNPCC)
Juvenile polyps			Traditional serrated
	Peutz-Jeghers syndrome	Malignant lesions:	Carcinoma in situ
				MUTYH associated polyposis
Lymphoid polyps			Intramucosal CRC
			Invasive CRC

Neoplasia Found	Recommanded Interval for Colonoscopy Examination	Comment
Small rectal hyperplastic polyps	10 years	Exception are patients with hyperplastic polyposis syndrome, who need more intensive follow up.
One or two small (<1 cm) tubular adenomas with only low-grade dysplasia	5–10 years	The precise timing within this interval should be based on other clinical factors (such as prior colonoscopy findings, family history, and the preferences of the patient and judgment of the physician).
3 to 10 adenomas, or any adenoma ≥ 1 cm, or any adenoma with villous features, or high-grade dysplasia	3 years	Adenomas must have been completely removed. If the follow up colonoscopy is normal or shows only 1 or 2 small, tubular adenomas with low-grade dysplasia, then the interval for the subsequent examination should be 5 years.
More than 10 adenomas at one examination	< 3 years	The interval should be based on the clinician judgement and consider the possibility of an underlying familial syndrome.
Sessile adenomas that are removed piecemeal	2 to 6 months	Once complete removal has been established, subsequent surveillance needs to be individualized based on the endoscopist’s judgment. Completeness of removal should be based on both endoscopic and pathologic assessments.

Transition of Adenoma to Carcinoma in Colon

The colon epithelium is constantly and rapidly renewing tissue. Old cells on the top of the villus are released into the lumen and replaced with new cells raised from colonic crypts. On the bottom of colonic crypts are stem cells that proliferate and differentiate into the cellular compartment of colon epithelium [31]. Vogelstein et al. [32] proposed the classical model of tumor evolution in the large bowel (Figure 1). Cells with high WNT signaling activity arise from aberrant crypts and evolve into a tubular or tubule-villous polyp. The subsequent proliferation of polyp may lead to the development of early adenoma with a low grade of dysplasia. Early adenoma expanses into advanced adenoma with a high grade of dysplasia and with increasing accumulation of mutations in daughter cells progressing ultimately further into carcinoma [2,32,33].

Each mutation that provides tumor cell-selective growth advantage is called driver mutation. This advantage slightly increases the growth rate of clonal expansion around 0.4% and is increasing with every new driving mutation [34]. Driver mutations enhance the accumulation of a large number of somatic mutations due to altering the cell condition and reduce the population fitness landscape. The predominant mutations, so-called passenger mutations, are mutations without selective growth advantage. With each clonal expansion of cancer cells, heterogeneous passenger mutations are generated that constitute the enormous variations of unique tumors [35].

Thanks to the next-generation sequencing (NGS) technique, thousands of mutations in the human genome were identified and some of them contribute to malignant evolution [36]. The driver mutations in the APC gene, predominantly frameshift at codon 1,554 [37], provide cell-selective growth advantage [32], and cause loss of cell ability to control the concentration level of protein β-catenin in the cytoplasm. β-catenin implements in the WNT signaling pathway and its concentration imbalances lead to uncontrolled growth and cell division [38]. Following mutations in TP53 or SMAD4 genes induce transformation into a malignant tumor, which overgrows into basal tissue and has an ability to metastasize into lymph nodes and distant organs [27].