34th Annual Scientific Meeting proceedings

Introduction
Adrenalectomy remains the standard of care for adrenal tumours of any type, though active monitoring may be appropriate or reasonable for some smaller lesions, incidentalomas etc. However, patients may be elderly, have co-morbidities (including due to secondary endocrinopathies, hypertension or arrhythmia), be poor anaesthetic candidates or have challenging presentations. The latter may include caval invasion necessitating more complex surgery, extensive caval extension, adjacent organ invasion or be truly inoperable. Even if some tumours are technically operable, owners may decline surgery if there is metastatic disease (even oligometastatic disease), suspicious regional lymphadenopathy or nodular liver/spleen on CT, the “solitary 2mm pulmonary nodule”, a higher risk surgery is required etc. Indeed, given the reported incidence of peri-operative mortality of up to 26%, nonsurgical options are necessary.

**See thyroid carcinoma notes for explanations on the different types of RT**

Why is RT for adrenal tumours not routine and well documented?
Historically, patients were aligned/positioned for radiotherapy based on palpable bony landmarks and measuring distances on radiographs/CT scans and thereafter aligning to ink marks placed on the skin. Obviously, this is very crude and resulted in the need for large error margins (treating with radiotherapy fields that were much bigger than technically needed) to account for errors in patient positioning on a day to day basis. Such huge fields would still result in uncertainty in terms or irradiating the target and would result in lots of normal tissue included in the field – AKA more side effects (especially gastrointestinal).      

Even when radiotherapy imaging became more prevalent and better, positioning was still usually performed with orthogonal kV radiographs – so usually using bony anatomy to ensure accurate patient positioning. Some soft tissue structures can be seen this way, but smaller intraabdominal lesions cannot. Therefore, as most organs move to some degree within the abdomen between treatments (inter-fraction motion), whilst error margins could be reduced, they were still large.

Now it is feasible to use cone-beam CT scans (incorporated into the linac) to not only align patient anatomy overall, but also to be able to see the soft tissue target and match it perfectly with respect to the radiation field. Essentially this gives great confidence that the target is being irradiated, but importantly it means the margins for error can be reduced dramatically – meaning less normal tissue in the field – meaning less toxicity and potential for both dose escalation and shorter treatment protocols.

Respiratory motion
Probably the biggest frustration in treating intra-abdominal targets is their movement during treatment (intra-fraction motion). This can be dramatic (4cm in some directions for liver) and is more obvious in lesions near the diaphragm. However, it is variable and affected by breed, conformation and respiratory rate and tidal volume. It can also vary within individuals within and between treatments. Essentially it can be very unpredictable. Whilst treatments are quick these days, most treatments (“beam on time”) will be at least 1-5 minutes. Therefore respiratory motion management needs to be considered for tumours such as adrenal tumours.

Fractionated radiation therapy
Conventional, highly fractionated (i.e. 16-20 daily treatments), daily, definitive-intent radiation therapy is unlikely to be a reasonable option for adrenal tumours. Having to deal with the previously setup issues every day over a long period would be time consuming and given the requirement for advanced imaging anyway, shorter protocols would be preferred.

Current Published Studies (all canine)

1.  Stereotactic Radiation (n=9 adrenocortical tumours)

• 30-45 Gy in 3-5 days
• MST = 1030 days
• No significant radiotoxicity
• ~30% volume reduction
• 2/3 dogs with HAC had endocrine resolution

2.  Stereotactic Radiation (n=8 phaeochromocytoma)

• 11 Gy x 3 (33 Gy) over 3 consecutive days (n=6)
• 7 Gy x 5 (35 Gy) on an alternate-day basis (n=2)
• CR (n=2), PR (>50-85% reduction; n=2), SD (n=2), not checked (=2)
• N=3 presented with acute haemoabdomen – all resolved post-SBRT without need for further therapy
• Most clinical signs resolved
• MST not able to be calculated

3.  Hypofractionated Radiation (n=3)

• Standard planning and delivery
• 6.5 Gy x 4, once per week (26 Gy total) (n=2)
• 7.9 Gy x 3, once per week (23.7 Gy total) (n=1)
• Two died of metastasis  with controlled primary tumours and one died of metastasis but with a progressive primary tumour after 15 months

Conclusion
Radiotherapy represents a valuable therapeutic option in the management of adrenal tumours. By effectively targeting cancer cells while minimizing harm to surrounding healthy tissues, radiotherapy can significantly impact tumour control and enhance the quality of life for affected dogs. It is essential for surgeons and radiation oncologists to collaborate closely to ensure precise treatment planning and delivery.

It is clear that adrenal tumours can respond to many different RT fractionation schemes, and the ultimate choice will be based on tumour size, location, costs, institutional availability of equipment, patient co-morbidities etc.

Although complications and side effects may occur, they are generally manageable, offering hope for improved outcomes and extended survival times. Continued research and advancements in veterinary radiotherapy techniques hold promise for further enhancing the efficacy of this treatment approach.