The age of the person is not in itself a reason not to operate, but their general performance status needs to be adequate for a major operation.One particular feature that is evaluated is the encouraging presence, or discouraging absence, of a clear layer or plane of fat creating a barrier between the tumor and the vessels. Traditionally, an assessment is made of the tumors proximity to major venous or arterial vessels, in terms of "abutment" (defined as the tumor touching no more than half a blood vessels circumference without any fat to separate it), "encasement" (when the tumor encloses most of the vessels circumference), or full vessel involvement. : 22  A resection that includes encased sections of blood vessels may be possible in some cases, particularly if preliminary neoadjuvant therapy is feasible, using chemotherapy: 36  and/or radiotherapy. : 29–30 Even when the operation appears to have been successful, cancerous cells are often found around the edges ("margins") of the removed tissue,  when a pathologist examines them microscopically (this will always be done), indicating the cancer has not been entirely removed. Furthermore, cancer stem cells are usually not evident microscopically, and if they are present they may continue to develop and spread. An exploratory laparoscopy (a small, camera-guided surgical procedure) may therefore be performed to gain a clearer idea of the outcome of a full operation. For cancers involving the head of the pancreas, the Whipple procedure is the most commonly attempted curative surgical treatment.
Classifications
German entomologist Fritz Zumpt describes myiasis as "the infestation of live human and vertebrate animals with dipterous larvae, which at least for a period, feed on the hosts dead or living tissue, liquid body substances, or ingested food". For modern purposes however, this is too vague. For example, feeding on dead or necrotic tissue is not generally a problem except when larvae such as those of flies in the family Piophilidae attack stored food such as cheese or preserved meats; such activity suggests saprophagy rather than parasitism; it even may be medically beneficial in maggot debridement therapy (MDT).Currently myiasis commonly is classified according to aspects relevant to the case in question:

The classical description of myiasis is according to the part of the host that is infected. This is the classification used by ICD-10. For example:dermal
sub-dermal
cutaneous (B87.0)
creeping, where larvae burrow through or under the skin
furuncular, where a larva remains in one spot, causing a boil-like lesion
nasopharyngeal, in the nose, sinuses or pharynx (B87.3)
ophthalmic or ocular, in or about the eye (B87.2)
auricular, in or about the ear
gastric, rectal, or intestinal/enteric for the appropriate part of the digestive system (B87.8)
urogenital (B87.8)
Another aspect is the relationship between the host and the parasite and provides insight into the biology of the fly species causing the myiasis and its likely effect.