why are they bald?

Just finished watching this outstanding movie, very original and touching without being sappy. Did I miss something though, why are the boys bald? They aren't chemo-patient bald, they are like skin-head bald, is it to show camaraderie with their mates that are receiving chemo, or something of a sense of belonging? It just, never to me, seemed to get any explanation, unless it is so was so obvious as to be implied. In the comments someone remarked that the English subtitles were lacking somewhat, I could tell from my limited Spanish that this was at times the case. So I wonder if this was lost in the translation?