Does Mastectomy is the only option of treatment for Paget"s disease?

whether paget's Disease is cancerous in all cases?

Yes. Paget's disease is cancerous in all cases and Mastectomy is the treatment suggested.

Paget's disease of the breast usually requires surgery. Which type of surgery you need depends on the condition of the skin around your nipple and how advanced the underlying cancer is.

Surgical options include:

Simple mastectomy. This procedure involves removing your breast, but not the lymph nodes in your armpit (axillary lymph nodes). Simple mastectomy is recommended in cases in which an underlying breast cancer exists but it isn't invasive breast cancer and it hasn't spread to the lymph nodes.
Skin-sparing mastectomy. In this procedure, the surgeon makes an incision around your nipple and areola, removes all of the underlying breast tissue, but preserves the overlying skin. Breast reconstruction with breast implants or with your body's own tissue (autologous reconstruction) is usually performed during the same surgery. Skin-sparing mastectomy isn't an option if the skin changes associated with Paget's disease have spread beyond your areola.
Modified radical mastectomy. A modified radical mastectomy is generally recommended if you have invasive breast cancer or extensive in situ breast cancer with spread to the lymph nodes in your armpit. In this procedure, the surgeon removes the breast, the lining of the chest wall muscle and some of your underarm (axillary) lymph nodes. The lymph nodes are checked for signs that the cancer has spread (metastasized).
Lumpectomy. Lumpectomy is a breast-conserving procedure in which the surgeon removes only the diseased portion of your breast. This procedure is generally considered if you have Paget's disease of the breast but no other evidence of underlying breast cancer. Your surgeon makes an incision around your areola, removing it and your nipple along with a wedge- or cone-shaped section of your breast. The surgeon focuses on removing as little breast tissue as possible, while ensuring that the tissue removed includes an outer margin of normal cells so that only healthy cells remain. Lumpectomy to treat Paget's disease of the breast almost always requires follow-up radiation therapy as adjuvant treatment. Lumpectomy wouldn't be recommended if you can't have radiation therapy for some reason. Nipple reconstruction may be an option after your treatment.-

It is my understanding that Padget's disease IS cancer, but there is very little information on it. But the web sites I have found it on describe it as in the nipple aureole area and treatment options are not discussed. Other breast cancers are treated by just removing the tumor unless it has spread or the woman ops for a total mastectomy. Try this site;
http://www.herbtime.com/InformationPages...

Paget's disease of the nipple? That is a duct carcinoma requiring surgery.