Can placing the mobile phone on the breast (covered by the shirt) for a few seconds cause a tumor? how much radiation do you expose yourself to by placing your mobile phone on your breast (covered by the shirt) for a few seconds? thank you?

What you described would not cause any harmful effects. You can reduce your risk of cancer by adopting healthy habits: Eat a mostly vegetarian diet, avoid alcohol, tobacco and street drugs, avoid excessive exposure to sun, maintain healthy weight, get HPV vaccine, exercise 30 minutes each day, drink enough water so your urine is mostly colorless. Wish you good health!