In the OECD data averaged here, I reckon this chemo therapy is counted as sick leave. On a first search, the metric seemed to be defined as "(compensated) absenteeism from work due to illness, days per employee per year"

In .nl and even .pl, once someone gets into "chemo" territory, they're no longer a full-time employee for at least part of the year. I have no idea how this is reflected in OECD data, but I would not be entirely unsurprised if this is a reprise of the "women in .nl don't work" trope, whereas in-actual-fact it's sort-of the opposite, except not full-time.