The sci­ence of can­cer clus­ters is no sim­ple mat­ter

Dis­ease sur­veil­lance sys­tems should in­clude pub­lic con­sul­ta­tion and ed­u­ca­tion

The Hamilton Spectator - - COMMENT - Niko Yian­nakou­lias is an as­so­ciate pro­fes­sor in the School of Ge­og­ra­phy and Earth Sciences at McMaster Univer­sity. His re­search spe­cial­ties in­clude en­vi­ron­men­tal health and dis­ease clus­ter de­tec­tion. NIKO YIAN­NAKOU­LIAS

Re­cent con­cerns over a can­cer clus­ter at Hamil­ton’s Cathe­dral High School trig­ger feel­ings of em­pa­thy, con­cern and even fear, and raise ques­tions about the safety of our en­vi­ron­ment.

Of­fi­cials have con­cluded that can­cer cases at Cathe­dral can’t be at­trib­uted to air qual­ity in the school, but un­der­stand­ably, the fear of can­cer re­mains.

While it’s in­ap­pro­pri­ate to ig­nore the con­cerns of peo­ple af­fected by or wor­ried about can­cer clus­ters in their com­mu­nity, it is of­ten chal­leng­ing for pub­lic of­fi­cials to adopt a rig­or­ous and trans­par­ent process for in­ves­ti­gat­ing these ap­par­ent clus­ters that also sat­is­fies all com­mu­nity stake­hold­ers.

As a thought ex­per­i­ment, imag­ine a com­mu­nity of a 1,000 peo­ple, in which a per­son has re­cently been di­ag­nosed with brain can­cer, and where peo­ple have had fears about an in­dus­trial fa­cil­ity for decades.

As news of the di­ag­no­sis spreads, other mem­bers of the com­mu­nity dis­close their own en­coun­ters with can­cer — an­other brain tu­mour, a few cases of breast and prostate can­cer, and a rare form of leukemia.

All told, there are 12 cases of can­cer in the com­mu­nity that year. Com­pared to the na­tional can­cer rate (around 5 in 1,000 per year) this seems very high.

To­gether, the can­cer and the en­vi­ron­men­tal con­cerns cre­ate lo­cal buzz: news sto­ries, wa­ter-cooler con­ver­sa­tions and pres­sure on of­fi­cials to do some­thing about it.

Typ­i­cally we might want to de­ter­mine whether this clus­ter is a sta­tis­ti­cal anom­aly or a gen­uine con­cern by de­ter­min­ing the “sta­tis­ti­cal sig­nif­i­cance” of the 12 cases of can­cer. How­ever, this proves to be more of a chal­lenge than you might think.

Con­sider the casino game of roulette. Based on how roulette wheels are de­signed, we know with cer­tainty that in the long run play­ers will lose money, but every once in a while a per­son gets lucky and wins 10 or more favourable spins in a row.

These play­ers aren’t win­ning be­cause they are prog­nos­ti­cat­ing ge­niuses. With tens of thou­sands of peo­ple play­ing roulette every year we should ex­pect a few to have amaz­ing runs of good luck, even if the lon­grun prob­a­bil­ity is los­ing.

In fact, if we know how many roulette games are played every year, we can ac­tu­ally de­ter­mine ap­prox­i­mately how many re­ally lucky roulette play­ers to ex­pect.

Un­for­tu­nately, we have no equiv­a­lent ref­er­ence for can­cer clus­ters. Canada is a coun­try of about 35 mil­lion, within which there is a seem­ingly in­fi­nite num­ber of pos­si­ble “com­mu­ni­ties” of peo­ple, de­pend­ing on how you de­fine them. Some com­mu­ni­ties are city neigh­bour­hoods, some are groups of small vil­lages, and some are apart­ment com­plexes. Some are peo­ple work­ing within an in­dus­try or teach­ers in a school.

For any sin­gle com­mu­nity of 1,000 peo­ple, 12 cases would seem like an anom­aly that can’t be ex­plained by ran­dom­ness. But with many, many thou­sands (or even mil­lions) of pos­si­ble com­mu­ni­ties in the coun­try, it is also a sta­tis­ti­cal cer­tainty that some of them would have at least 12 cases of can­cer in a given year, even if the true risk of can­cer were the same across the coun­try.

In re­sponse, then, it might be tempt­ing to dis­miss all but the most strik­ing can­cer clus­ters as sta­tis­ti­cal anom­alies, but this does lit- tle to com­fort mem­bers of com­mu­ni­ties that di­rectly or in­di­rectly ex­pe­ri­ence the ef­fects of can­cer. More­over, it could con­trib­ute to an al­ready wor­ri­some ten­sion be­tween con­cerned cit­i­zens and seem­ingly un­sym­pa­thetic tech­nocrats.

For­tu­nately, there are things that can be done to im­prove the sit­u­a­tion. Pro­vin­cial gov­ern­ments have been work­ing for years on de­vel­op­ing dis­ease sur­veil­lance sys­tems, in­clud­ing some that are co-or­di­nated at the fed­eral level, to rou­tinely mon­i­tor in­fec­tious and non-in­fec­tious dis­eases.

An im­por­tant di­men­sion of this process should in­clude pub­lic con­sul­ta­tion to de­fine com­mu­ni­ties worth mon­i­tor­ing ahead of time; that is, be­fore the sus­pected clus­ters of dis­ease emerge.

This would pro­vide in­for­ma­tion equiv­a­lent to the num­ber of “roulette play­ers,” and serve as an im­por­tant ref­er­ence point for dis­tin­guish­ing be­tween real clus­ters and ex­pected sta­tis­ti­cal anom­alies.

Once com­mu­ni­ties are de­fined, of­fi­cials can rou­tinely mon­i­tor — and pub­licly dis­close — the sta­tus of tar­get dis­eases in these com­mu­ni­ties.

Con­sul­ta­tion and trans­parency could help build pub­lic trust, and the rou­tine mon­i­tor­ing of a known set of com­mu­ni­ties of con­cern would im­prove the sci­en­tific rigour nec­es­sary to de­ter­mine whether or not a clus­ter is a sta­tis­ti­cal ac­ci­dent or a se­ri­ous health con­cern.

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