That was a news article. The journal article is here:
http://ascopubs.org/doi/full/10.1200/JCO.2014.57.4111
In all FFQs, participants were asked how often on average (never to ≥ six servings per day) during the previous year they had consumed grapefruit (half), oranges (one), and grapefruit and orange juices (one small glass [6 oz]). Overall citrus consumption was calculated as the sum of these individual products. Grapefruit and grapefruit juice were asked as a single item in the 2002 and 2006 FFQs.
http://ascopubs.org/doi/full/10.1200/JCO.2014.57.4111
The article was a survey, the study never measured psoralen contents in the oranges, orange juice, grape fruits and grape fruit juices of the surveyed_people.
We know that commercialized orange juice removes oils and then adds back citrus oils with their flavour packs in their processing of oj.
https://www.toxinless.com/orange-juice
It seems that citrus oils (lemon oils, lime oils, bergamot oil, grapefruit oil...) contains the psoralens, coumarins, furocoumarins:
http://pubs.acs.org/doi/pdf/10.1021/jf901209r [locked article...]
http://pubs.acs.org/doi/pdfplus/10.1021 ... 0705.ch020 [locked article, but go find a place to unlock]
The thing is that the psoralen used to be extracted from bergamot oranges, via bergamot orange oil:
https://en.wikipedia.org/wiki/Bergamot_orange
shows us that other food items like Earl Grey Tea, Lady Grey Tea, Marmalade, and even candies like Turkish Delights, could be a source of citrus oil ingestion! Hence it is not fair of the study to ask the surveyed_people if they only ate citrus fruits and drank citrus juice... but they should ask surveyed_people if they ate other foods with citrus oils or even if they used perfumes with psoralen ingredients like from bergamot oils...!
It is also known that psoralen is present in figs, celery, parsley, fennel, carrots, parsnips ... and in some herbs and spices...!
https://en.wikipedia.org/wiki/Psoralen
Hence other dietary sources of possible psoralen ingestion were not accounted for in that survey.
What we need is to know the quantity.
i.e. grams of psoralen, in grapefruit, in oranges of species Bergamot, Seville, Navel, Valencia, ..., in citrus Tangerine, Mandarin,...etc.
Without a study that displays QUANTITY, and LOCATION i.e. in oils or in pulp, in additives, in flavour packs...
We cannot really conclude that orange juice has psoralens!
And we cannot correlate that orange juice is to melanoma!
