Page 1298 - Small Animal Internal Medicine, 6th Edition

1270 PART XII Oncology

BASIC PRINCIPLES OF CHEMOTHERAPY toxicities. It is customary to name the protocol after the
first letters of each drug in the combination (e.g., VAC for
VetBooks.ir Chemotherapeutic agents predominantly kill cells in rapidly vincristine, doxorubicin [or Adriamycin], and cyclophos-
phamide). As a general rule, combination chemotherapy
dividing tissues. To exploit the tumoricidal effect of different
chemotherapeutic drugs, it is common practice to combine
three or more drugs to treat a given malignancy. These drugs results in more sustained remissions and prolonged survival
times than single-agent chemotherapy; this is thought to
are selected on the basis of the following principles: Each result from the fact that multiagent chemotherapy delays
should be active against the given tumor type, act by a dif- (or even prevents) the development of drug-resistant clones.
ferent mechanism of action, and not have superimposed In some cases, single-agent chemotherapy is as effective as

Log number of cancer cells
Plateau growth phase;
M DIFFERENTIATION DEATH SURGERY-XRT
low GF and MI; high DT

G 2 G 0

G 1 Log growth phase;
high GF and MI; short DT
Time

FIG 76.3
S The effect of surgical or radiotherapeutic intervention on
tumor kinetics. After cytoreduction, cells are recruited from
FIG 76.1 the G 0 phase and the tumor returns to the exponential
Mammalian cell cycle. Cells in mitosis (M) can differentiate phase. DT, Doubling time; GF, growth factor; MI, mitotic
and subsequently die (the rule in normal tissues); they can index; XRT, radiation therapy. (From Couto CG: Principles
also progress to G 0 (true resting phase), from which they of chemotherapy. In Proceedings of the tenth annual Kal
can be recruited by a variety of stimuli (see text). G 1, Gap Kan symposium for the treatment of small animal diseases:
1; S, DNA synthesis; G 2, gap 2. oncology, Kalkan Foods, Inc, 1986, Vernon, Calif, p 37.)

Low GF and MI; prolonged DT
10 14

DEATH
10 12
Tumor mass 1 kg
Log number of cancer cells 10 6 8 Radiographic or ultrasonographic diagnosis possible Tumor mass 1 g
10
10
TUMOR FIRST PALPABLE

10
(tumor mass 150 mg)
10

10 2 4 High GF and MI; short DT

Time
FIG 76.2
Tumor (cell) kinetics. Additional information on tumor kinetics can be found in the text. GF,
Growth fraction; MI, mitotic index; DT, doubling time. (From Couto CG: Principles of
chemotherapy. In Proceedings of the tenth annual Kal Kan symposium for the treatment of
small animal diseases: oncology, Kalkan Foods, Inc, 1986, Vernon, Calif, p 37.)