Treatment Planning Flashcards
How are MB pts simulated?
MB simulation: supine or prone, neck extended (so PA spine field does not exit through the mouth), head mask, shoulders positioned inferiorly (to allow for lat cranial fields). Depending on institutional experience, can simulate supine, which allows better airway access during anesthesia, most places are now doing supine technique.
What modalities of RT have been used for CSI?
Photons, electrons (at MDACC in yrs past), and protons are the more commonly used RT modalities for CSI therapy.
In CSI, which fields are placed 1st?
Spinal fields are placed 1st (to allow calculation of collimator angle for the cranial field based on spinal field beam divergence).
Cranial fields are placed 2nd (down to C5–6 or as low as possible but need to ensure lats do not go through shoulder).
By what angle are the cranial field collimators rotated?
Arctan (one-half length of sup spine field/SSD), which matches the cranial field to the spine field divergence
What are the borders of the spine field(s)?
Superior: matched to cranial field
Inferior: end of thecal sac (near S2–3, check on sagittal spine MRI)
Lateral: 1 cm past pedicles (some centers plan with wider margins in sacrum to cover neural foramina)
By what angle is the couch kicked and in which direction?
Couch kick for CSI: arctan (one-half length of cranial field/SAD); couch kicked toward side treated to match cranial field divergence (for breast, kick is away)
What is a potential problem with a couch kick?
Couch could be rotated in the opposite direction than intended. At MDACC, Tx are usually planned without a couch kick to eliminate moving table in wrong direction. A small amount of overlap occurs lat to cord, but doses used are relatively low and amount of overlap is decreased by feathering the junction.
If multiple spinal fields are used, what is the skin gap? At what depth is the match?
With multiple spine fields, the skin gap = ([0.5 × Length1 × d]/SSD1) + ([0.5 × Length2 × d]/SSD2)where d is the depth of the match, which is typically at the ant cord edge.
How is “feathering” done? Why is it used?
here are several techniques, and feathering is dependent upon institutional experience. Feathering helps reduce hot and cold spots in a plan. At MDACC, several techniques are used, including inter- and intrafractional junctioning for photons, electron junction technique, and proton junction technique.
Interfractional junctioning may be modulated with field-in-field technique and consists of moving junction superiorly 0.5 cm on 7th and 13th fx. This creates 12 fields with junctions.
Intrafractional junctioning may be modulated with step-and-shoot technology. 3 junction control points at 1-cm gaps (i.e., 0, 1, and 2 cm with the use of MLCs) are created, and each control point delivers one-third of the fractional dose. MLC leaves remain outside the field to ensure min interleaf leakage.
Where should the isocenter be placed in the cranial field for CSI? What cranial structure should be assessed for adequate coverage?
For the half-beam block technique, the isocenter should be placed behind the lenses to minimize divergence of beams into the opposite lens; the cribriform plate is not optimally visualized on conventional simulation films. A generous margin must be given in this area, or CT contours of the cribriform plate can be outlined to ensure coverage.
What CSI techniques can be employed if the entire spine cannot be included in 1 field?
The practitioner can increase the SSD (i.e., 100 cm → 120 cm) or rotate the collimator using a single field, but if the length is >36–38 cm, then 2 spinal fields are needed, with the inf field’s isocenter placed at the junction (using half-beam block to minimize the cold spot). Match at L1–2, as this is the area where the depth of cord changes the most.
What are the borders for the spinal and cranial fields in traditional, prone technique CSI?
