Health disparities are an important public health concern with several studies documenting racial and socioeconomic disparities in screening, incidence, stage at diagnosis, and mortality^6,8,10,14-23. As physician recommendation is a key determinant of patients’ acceptance of screening, adequate access may be an important contributor to disparity. Frequency of physician contact as well as time spent during individual visits may influence screening^24-27. Higher physician-per-population ratio may improve access, decrease waiting times and increase the opportunity for contact between the patient and physician. Compared to individuals with no physician contact, there is a nearly twofold increase in the odds of CRC screening for individuals with 1-2 physician contacts, rising to a 2.75-fold increase with more than four contacts in a year²⁷.

Higher physician density has been associated with earlier stage of diagnosis of malignant melanoma²⁸, lower incidence of cervical cancer²⁹ and earlier stage at diagnosis of breast cancer^30,31. There has been limited prior research examining the relation between physician density and CRC. In one study, supply of primary care physicians (PCP) negatively correlated with CRC incidence and mortality in Florida³². A 10-percentile increase in PCP density was associated with a 3% decrease in the odds of late-stage diagnosis³³. However, it is likely that not just the supply of PCPs, but also gastroenterologists may be an important factor. The prior studies examining the correlation between physician supply and CRC outcomes were either conducted prior to Medicare coverage of colonoscopy for routine CRC screening in 2001 or did not specifically examine the availability of gastroenterologists^32-34.

Several authors have examined the relationship between physician supply and health outcomes. Higher physician supply was associated with lower mortality rates in some, though not all studies³⁷. Three prior studies have examined the relationship between physician supply and CRC^32-34. Shipp et al. found that an increase in the number of physicians/1000 population was associated with a modestly higher rate of CRC (relative risk 1.14)³⁴. In contrast Roetzheim et al. showed that higher PCP density negatively correlated with both CRC incidence and mortality³². The same authors also found that for each 10-percentile increase in PCP supply, the odds of late-stage CRC diagnosis decreased by 5%³³. However, the converse was true for specialists, with an increase in late-stage CRC diagnosis with each 10-percentile increase in specialty physician-per-population ratio. Density of gastroenterologists did not correlate with stage at diagnosis in their study. Our findings of the association between PCP density and late-stage CRC incidence is consistent with the findings of these two latter studies^32,33. The novel finding of the association between gastroenterologist supply and late-stage CRC incidence in our study compared to prior studies³³ may be due to Medicare coverage of colonoscopy for average-risk CRC screening beginning in July 2001. Since this coverage, there has been increasing use of endoscopic modalities for CRC screening^8,10. In a prior study, Ananthakrishnan et al. found that 3.8% of all eligible Medicare patients received a screening colonoscopy in one calendar year in 2002-03⁸ compared to 1.4% reported by Ko et al. for a period prior to the expansion of Medicare coverage⁹. Correspondingly, colonoscopy comprised 42% of all screening tests in the latter period⁸ compared to 35% in 1998⁹. While a reduction in CRC mortality has been demonstrated with FOBT³⁸, this reduction may be attributable to patients with positive tests undergoing colonoscopy with removal of adenomatous polyps³⁹. A reduction in CRC mortality with use of screening colonoscopy has also been demonstrated⁴⁰. It is possible that with this present trend, gastroenterologist availability also becomes important in determining CRC incidence and outcomes.

The reduction in late-stage CRC incidence in counties with higher physician supply could have a few explanations. The specificity of our finding a negative association between late-stage CRC incidence and PCPs/gastroenterologists density, but not overall physician density suggests that availability of these two physician groups most likely to be involved in screening or early detection of CRC is an important factor. This is further supported by the fact that after the initiation of Medicare coverage of colonoscopy for average-risk CRC, gastroenterologist density became more strongly associated with reduced late-stage CRC incidence than prior to such coverage. Physician recommendation is an important determinant of CRC screening^24-26 with individuals with more frequent physician contact being more likely to undergo screening²⁷. Inadequate physician time with the patient is another barrier to screening²⁵. As county-level rates of CRC screening are not available for each county in Pennsylvania, we were unable to examine the impact of differential screening rates. Given the generally lower rates of CRC screening compared to other preventive health services, differences in screening are unlikely to be the sole factor. In addition, given the known timeline for development of CRC, changes in screening practices after 2001 are unlikely to be the sole determinants of changes in late-stage CRC diagnosis in 2004–2006. However, increase in screening has been associated with a higher proportion of early-stage CRC diagnosis even short-term⁷. Higher physician supply may also result in earlier care care-seeking behavior for patients with symptoms suggestive of CRC, resulting in early diagnosis.

It is interesting that the relationship between gastroenterologists/PCP supply and late-stage CRC differed by metropolitan status and population density. Studies examining the availability of PCPs in urban areas identified weak correlations with health outcomes⁴¹ while the link appeared to be stronger in some, but not all studies, examining rural health care. Pathman et al. found that higher number of persons per physician in each county was associated with longer travel times but no other significant barriers to care⁴¹ with no difference in the utilization of preventive health services. However, among patients who were covered under Medicaid or were uninsured, lower physician-per-population ratio was associated with lower satisfaction with care and difficulty in contacting medical personnel. Intuitively, it stands to reason that in rural areas with a scattered population, there may be longer travel times to physician offices in counties with a low physician-per-population ratio resulting in longer waiting times. Reduced access to PCP or gastroenterologists in these counties may also delay CRC screening, surveillance, or diagnostic evaluations in those with symptoms and consequently a higher incidence of late-stage cancer. In urban counties, physician density may be above the threshold for such relationships to hold true and no longer acts as a rate-limiting step. It is also interesting that the relationship with physician supply did not hold true for overall CRC incidence or mortality. There a few potential explanations for this finding. In prior studies, having had a screening endoscopy was associated with a lower risk of only late-stage diagnosis (odds ratio (OR) 0.46, 95% CI 0.22–0.98)⁶. Gross et al. demonstrated an association between increasing colonoscopy use with earlier stage at diagnosis for proximal but not distal colon lesions⁷.

Our study has a few limitations. Ecologic analysis assigns the same characteristics to each individual residing within the county. County-level socioeconomic characteristics may not represent the SES of the individual. However, we believe that measures such as availability of or access to physicians, our primary variable of interest, are more meaningful when measured over a wider geographic area. Performing such analysis at the level of zip code or census tract may be fallacious as individuals are unlikely to restrict their care to physicians within their zip codes of residence. We also did not have information on location of CRC or county-level CRC screening rates. While the Behavioral Risk Factor Surveillance System and the National Health Interview Survey track overall screening rates in the US and in select population areas, there is currently limited mechanisms for obtaining county-level screening rates for each county for any state in the US. We believe that it is important to develop such county-level or other small-geographic area level databases to track various health behaviors in order to identify high-risk populations. Another limitation of our analysis is the inability to adjust for some known individual risk factors for CRC including obesity and smoking status though adjusting for county level proportion of smokers did not influence our estimates. It is possible though unlikely that the above variables vary systematically enough with physician density to influence our results.

There are several implications to our study. Recent concerns have been raised about the potential shortage of physicians^42,43. The Lewin group projected that by the year 2020, there might be a shortage of between 1000–1500 gastroenterologists nationwide in the US⁴⁴. By demonstrating a relationship between county-level physician supply and late-stage CRC incidence, our study supports these concerns. It is important to recognize that the increase in physician supply may need to be targeted to non-metropolitan counties (comprising 12.6% of the state population) or those with low population densities (comprising approximately 12.7% of the state population). In urban counties, physician supply was not an important determinant of CRC incidence; increasing the physician-per-population supply in such counties may consequently have a limited impact. In rural counties with a population that is sparse and more spread out, availability of PCP and/or gastroenterologists may be important in reducing CRC incidence. However, it is almost important to remember that physician density represents only availability of healthcare but does not take into account affordability or acceptability of CRC screening practices⁴⁵. Physician density may have a limited impact on CRC incidence and outcomes if the other barriers to healthcare are more dominant factors.