The presenting problem should ideally be the diagnosis the patient has come to see the doctor with.
While it is tempting to just put "Cancer" we all know that there are multitudes of different cancers and even benign conditions that are treated with radiotherapy. In an effort to standardise diagnoses the WHO has created the International Classification of Disease - the currently edition is volume 10 (ICD-10). This is covered in more detail in the Coding part of the website.
In some respects the presenting problem may be a manifestation of the cancer rather than the cancer itself, especially in palliative settings. Symptoms are not usually included in ICD-10 unless the diagnosis is elusive (there are, in fact, a bunch of Rxx codes that refer to symptoms rather than diagnoses). So, a person with pain may have a diagnosis of secondary malignant neoplasm of a particular bone (all classified as C79.5). A patient with hypercalcaemia has a diagnosis of whatever their primary cancer is (and should be referred on to the medical oncologist perhaps).