Can Stereotactic Radiation Therapy (SRT) be used to treat a dog based on the provided Computed Tomography (CT) scan results?

Stereotactic Radiation Therapy for Canine Tumors Based on CT Results

Stereotactic Radiation Therapy (SRT) is an appropriate and effective treatment option for dogs with tumors identified on CT scans, particularly for those with oligometastatic disease or when surgical options are limited. 1

Patient Selection Criteria

SRT is particularly suitable for dogs in the following scenarios:

• Oligometastatic disease (typically ≤5 lesions in ≤3 organs) 1
• Patients who are not surgical candidates or refuse surgery 1
• Tumors that require high-precision targeting to avoid damage to surrounding tissues 2
• Primary or metastatic tumors where local control is needed 3

Technical Considerations for SRT in Dogs

Imaging Requirements

• CT scanning with proper settings is essential for accurate tumor delineation:
  • For lung parenchyma: window width 1,600 and level 600 2
  • For mediastinum: window width 400 and level 20 2
• 4D-CT scanning is strongly preferred for treatment planning, especially for tumors affected by respiratory motion 2
• Intravenous contrast can improve delineation of centrally located tumors 2

Treatment Planning

• Target volume definition requires:
  • Gross Tumor Volume (GTV) based on CT findings 2
  • Clinical Target Volume (CTV) typically with a 5mm margin around the GTV 2
  • Planning Target Volume (PTV) that accounts for setup errors and breathing motion 2
• Advanced dose calculation algorithms (type B) are recommended for accurate computation of dose distributions 2

Dose Considerations

• For lung tumors, constraints should include:
  • V20 (volume receiving 20 Gy) kept at 35%-37%
  • Mean lung dose maintained at 20-23 Gy 2
• Dose fractionation for canine oral melanoma typically follows 8 Gy × four weekly fractions 3

Expected Outcomes

Based on studies in canine oral melanoma treated with multimodality approaches including radiation:

• Median time to progression: 304 days
• Median progression-free survival: 260 days
• Median tumor-specific overall survival: 510 days 3

Negative prognostic indicators include:

• Presence of gross disease
• Primary tumor ≥2 cm
• Higher clinical stage
• Presence of lymph node metastasis at diagnosis
• Caudal location in oral cavity tumors 3

Treatment Response Evaluation

CT appearance after SRT may show various patterns:

• Continuous contrast accumulation without early arterial enhancement
• Residual early arterial enhancement within 3 months after SRT
• Residual early arterial enhancement more than 3 months after SRT
• Shrinking low-density area without early arterial enhancement 4

Important note: Early assessment within 3 months may result in misleading response evaluation, as residual enhancement often disappears within 6 months 4

Practical Implementation

• Patient positioning must be stable and reproducible during all imaging procedures and treatment 2
• Interventions to reduce tumor motion may be useful in selected patients 2
• Treatment typically takes less than 1 hour on the initial day and about 20 minutes on subsequent days 5
• Treatment can be performed on an outpatient basis 5

Potential Pitfalls and Caveats

• Avoid treating centrally located tumors with very high doses per fraction (≥18 Gy) due to risk of complications 2
• For centrally located tumors, consider lower doses per fraction adapted to critical organs 2
• CT without IV contrast has limited value for tumor delineation; contrast-enhanced studies are preferred 2
• Respiratory motion must be accounted for in treatment planning, especially for thoracic tumors 2

SRT represents a non-invasive alternative to surgery with comparable outcomes for many canine tumors, offering excellent local control with minimal toxicity when properly implemented.