Here is past post from AAPC thread (Armen) that is excellent:From ICD 9 official guidlines:
If the treatment is directed at the malignancy, designate the malignancy as the principal diagnosis.
The only exception to this guideline is if a patient admission/encounter is solely for the administration of chemotherapy, immunotherapy or radiation therapy, assign the appropriate V58.x code as the first-listed or principal diagnosis, and the diagnosis or problem for which the service is being performed as a secondary diagnosis.
When a primary malignancy has been previously excised or eradicated from its site and there is no further treatment directed to that site and there is no evidence of any existing primary malignancy, a code from category V10, Personal history of malignant neoplasm, should be used to indicate the former site of the malignancy. Any mention of extension, invasion, or metastasis to another site is coded as a secondary malignant neoplasm to that site. The secondary site may be the principal or first-listed with the V10 code used as a secondary code