Panty hose offer very little pressure or support to prevent blood from pooling. They are not recommended.
Yes. Be sure to follow product directions for washing, however mild detergent is usually ok. Always have an extra pair so the person can wear one pair while the other is being washed.
A pair of stockings will lose their elasticity over time. Replace at least every 6 months. (If the person is having dramatic weight loss, or weight gain, the stockings should be reevaluated to ensure that the correct size is being worn).
Cancer itself can cause changes in the way blood clots. Also, some medicines used to treat cancer, increase the risk of clot formation.