Page 239 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

218 PART III Therapeutic Modalities for the Cancer Patient

Immobilization and Localization Techniques progression-free survival time (PFS) for T1 tumors (less than
2 cm) and T2 tumors (2–4 cm) is 86%; it is only about 30% for
In a retrospective study of 57 dogs with
127
As radiation delivery techniques become more conformal and treat-
T3 tumors (over 4 cm).
VetBooks.ir ment margins are reduced, the importance of precise patient posi- AAs that were treated with RT reported that the overall median
time to first event and median survival time (MST) were 1210
tioning and target localization/verification become increasingly
important. A variety of techniques are used to achieve the requisite and 1441 days, respectively. 128 Dogs younger than 8.3 years old
level of accuracy for any given patient and RT procedure. A port film (the median age in the study) had a significantly longer MST
is a radiograph taken, typically by the treatment beam, at the begin- (2322 days) than dogs older than 8.3 years (1106 days). Dogs that
ning of an RT and periodically throughout the treatment course to received doses higher than 40 Gy had significantly longer MST
ensure proper radiation positioning and alignment of the treatment than dogs that received 40 Gy or less (2994 days vs. 143 days). It
beam and patient for 3D-CRT. Commonly obtained as a double was previously suggested that RT could result in transformation
exposure from the treatment machine with an initial exposure of the of AAs into malignant epithelial tumors, but this possibility has
treatment field followed by exposure of a wide field of the surround- since been refuted. 126,128
ing anatomy, it results in a radiographic image with a dark central area Canine oral SCCs are responsive to radiation, although the
indicative of the treatment field with lighter exposure of the surround- prognosis is site dependent, with tumors located more rostrally
ing anatomy for reference. Digital radiography technology is now fre- having better probability of control. 129 Tumors at the base of
quently implemented and may be integrated into a linear accelerator. the tongue or tonsil are highly metastatic and are likely to recur
An electronic portal imaging device (EPID) is a digital imaging plate locally or regionally. In one study of oral SCC the reported PFS
mounted on a linear accelerator with a robotic arm for extending and at 1 and 3 years is 72% and 40%, respectively, for all T stages. 130
retracting the plate in precise positions. EPIDs allow for more routine Another study of fractionated RT (48–57 Gy in 3–4 Gy frac-
or daily treatment localization/verification, with rapid image acquisi- tions) in 14 dogs with oral SCC, the median disease-free interval
tion, analysis, and storage of these images. An on-board imager (OBI) (DFI), and MST were 365 and 450 days, respectively. 131 In cats,
is a high-resolution digital imaging system (x-ray source and digital oral SCC has a poor prognosis. 132 Although many cats show
capture system) separate from the primary beam from the treatment an initial response and may even show dramatic reductions in
machine itself. OBI systems are typically kilovoltage energy systems tumor size, tumor recurrence is common. Combining curative-
that provide superior contrast to images generated from a megavolt- intent RT with etanidazole or mitoxantrone therapy has resulted
age linear accelerator beam. CBCT platforms are integrated into lin- in MSTs of 116 to 170 days. 81,133 Seven cats with mandibular
ear accelerators whereby a volumetric CT image can be generated by SCC were treated with hemimandibulectomy and mandibular
rotating the x-ray source and digital reading plate around the patient node excision followed by RT; the MST was 420 days, but some
during an exposure. CBCT allows for improved localization, particu- cats do not adapt to bilateral mandibulectomies. 134 Nine cats
larly for soft tissue structures that are difficult to immobilize relative to with oral SCC received accelerated RT (14 fractions of 3.5 Gy
cutaneous or bony anatomic landmarks. Surface-guidance RT utilizes in 9 days), which resulted in an MST of 86 days. 135 In a different
3D camera technology or other means to accurately detect the patient approach, SRT (20 Gy) was delivered to 18 cats with oral SCC.
surface, often in real-time during RT. Acute radiation effects were minimal and initially cats improved
clinically, but MST was only 106 days. 136 Although RT is likely
to play a role in the treatment of this disease, new approaches
TUMORS COMMONLY TREATED WITH are indicated.
RADIATION Oral fibrosarcomas (FSAs) are unlikely to metastasize but
can be difficult to control locally. Oral FSAs are less radiosensi-
Oral Tumors tive than AAs and SCCs, although tumor control probabilities
ranging from 33% to 67% at 1 year have been reported. 137,138
Many oral tumors, such as acanthomatous ameloblastomas (AAs) In one study of oral FSAs treated with RT, the PFS at 1 year and
and rostral squamous cell carcinomas (SCCs), respond well to both 3 years were 76% and 55%, respectively. 130 Surgical cytoreduction
complete surgical excision and RT. For less radiation responsive improves the probability of tumor control by RT. 137 This should
tumors such as fibrosarcomas, complete surgical excision is pre- be taken into consideration during surgical planning with a focus
ferred; however, postoperative RT is recommended after incom- on removing gross disease and obtaining a tension-free closure. An
plete histologic excision (see Chapter 23, Section A). For large oral MST of 540 days was reported in eight dogs with oral sarcomas
tumors, aggressive surgery can lead to functional and/or cosmetic treated with surgery followed by RT. 139 In tumors too large to be
abnormalities. In these cases pretreatment planning for both surgery surgically resected to a subclinical level, RT alone is indicated; the
and RT may be indicated. 125 RT after surgery should start as soon probability of long-term tumor control is low with conventional
as possible and can start immediately after surgery. However, in the RT, but new strategies using IMRT or SRT are currently being
face of excessive tension on the suture line or indication of delayed evaluated.
healing, RT should be postponed until healing has occurred. The Malignant melanoma (MM) is the most common oral tumor
therapeutic gain from following surgery with RT will be lost if the in dogs and is associated with a high rate of regional and distant
radiation is delayed for too long. The introduction of IMRT allows metastasis. 140 Higher doses of radiation per fraction (4 Gy and
better sparing of important structures in the area and minimizes above) are believed to improve response rates for melanoma. 111
inadvertent dosing to salivary glands, tongue, lips, and skin, making In one study, 38 dogs with nonmetastatic oral MM were treated
patients more comfortable during treatment. 122 with 48 Gy delivered in 4 Gy fractions on a Monday-Wednesday-
Friday schedule. 130 The overall median PFS was 17.8 months, and
Efficacy of Treatment the median PFS was 38.0 months, 11.7 months, and 12.0 months
AAs are very radiation responsive. Tumor control with RT can for dogs T1, T2, and T3 tumors, respectively. In a retrospective
be close to 90%. 126 A relationship has been demonstrated study of 140 dogs, most of which had regional or distant metas-
between tumor (T) stage and local control. 127 The reported 3-year tasis at presentation, 141 coarsely fractionated (9–10 Gy weekly