Treatment Planning Flashcards

For an electron beam of energy E (in MeV), the approximate depth of the 80% isodose in cm is:

Answer: E/3

The therapeutic range (about 80%) is roughly E/3 cm, and the practical range is about E/2 cm.

Bolus is placed on the skin in megavoltage photon treatment mainly to:

Answer: Increase the dose at the surface

Bolus shifts the buildup region toward the skin, which raises the surface dose and removes skin sparing.

In IMRT inverse planning, the optimizer adjusts beam fluence to meet:

Answer: Dose-volume objectives and constraints set by the planner

Inverse planning starts from the desired dose objectives and calculates fluence patterns to meet them.

In tangential breast planning, a half-beam block or collimator rotation is mainly used to:

Answer: Remove beam divergence into the lung and match the deep field edges

Blocking half the beam or rotating the collimator makes the deep edges of the opposed tangents line up and limits divergence into the lung.

A cumulative dose-volume histogram (DVH) shows:

Answer: The percentage of a structure receiving at least a given dose

A cumulative DVH plots the volume receiving a dose greater than or equal to each dose value.

A common conventional-fractionation maximum dose limit for the spinal cord is approximately:

Answer: 45-50 Gy

A spinal cord maximum of about 45-50 Gy at 1.8-2 Gy per fraction keeps the myelopathy risk low (QUANTEC).

A main advantage of the isocentric (SAD) technique over the SSD technique is that:

Answer: Multiple fields can be treated without moving the patient between fields

With the target at isocenter, the gantry rotates around the patient, so no patient shifts are needed between fields.