Chapter 3 of 6
Beam Setup
Beam Placement
There are several methods to place the isocenter and define field borders, depending on the technique employed. This section will cover the most commonly used approaches.
For the half-beam block (HBB) technique, the isocenter is positioned within the lung, and the field is blocked medially to minimize beam divergence. The field originates from the setup marks. In this example, the setup mark is at 100 cm Source-to-Surface Distance (SSD) at the setup BB, corresponding to slice Z=0. If the setup marks are placed elsewhere, the field begins at the Z-slice of that specific mark. This approach restricts shifts to two directions: laterally along the X-plane and anterior/posterior along the Y-plane.
First, determine the separation for the tangential field.
Next, locate the midpoint of this separation and reposition the isocenter to this “mid-separation” point.
With the isocenter placement established, proceed to define the field borders.
HBB Tangential Breast Field Placement
For standard tangential field placement without a half-beam block:
Standard Tangential Breast Field Placement
Three-dimensional (3D) breast treatments typically employ a tangential field arrangement.
Review the fields, ensuring proper isocenter placement and appropriate field margins on the axial view (Z-plane).
When configuring tangential fields, consider the following: ensure at least 2 cm of flash and approximately 2 cm of lung bite.
If the field flash exceeds 2 cm, this is acceptable and does not compromise the plan. However, be mindful of the multileaf collimator (MLC) travel limitation of 14.5 cm. For this reason, aim to maintain flash at approximately 2 cm.
Field Borders and Target Coverage for Both Techniques
For both techniques, rotate the collimator to align with the chest and lung slant, ensuring a consistent lung bite.
Tangential Field Arrangement
Field Arrangement: Medial and Lateral Tangents Using Half-Beam Block
Left Medial Tangent = 300 degrees
Left Lateral Tangent = 120 degrees
Tangential Field Arrangement
Field Arrangement: Medial and Lateral Tangents Without Half-Beam Block
Left Medial Tangent = 300 degrees
Left Lateral Tangent = 124 degrees
Check Skin Render
Before proceeding to planning, verify the skin render to identify any errors and ensure the field borders adequately encompass the breast tissue.
Summary
With the setup complete, we can now develop a plan similar to those demonstrated.
Next: Treatment Planning