Hyperpigmentation following radiotherapy

Last July I completed 15 sessions of radiotherapy which caused blistering around the scar area of my breast. Not long after I noticed patches of hyperpigmentation - like sun damage freckles, on my chest and on the side of the affected breast (I went for a tissue expander, and I’m going for a new implant in the autumn, and it’s on that breast). I’m fair skinned and was advised post-surgery to slap on Factor 50 sunblock under my clothes, but have noticed the patches of hyperpigmentation have got darker. Does anyone know if the patches fade over time? I’m not a sunbather, so those areas of skin won’t blend in at any point.
Thanks in advance for any insight or responses.