Treatment Planning Flashcards

For heterogeneity corrections, the treatment planning system converts CT numbers to:

Answer: Relative electron density

A CT-number-to-electron-density calibration curve lets the dose algorithm model tissue inhomogeneities.

For megavoltage photons, the dose to tissue just beyond a large section of lung is generally:

Answer: Higher than in a homogeneous water calculation

Low-density lung attenuates less, so the dose distal to the lung is higher than an uncorrected calculation predicts.

Which 4DCT reconstruction is commonly used to help contour the ITV of a lung tumor?

Answer: Maximum intensity projection (MIP)

The MIP shows the highest-density voxel across all phases and outlines the full range of tumor motion.

A digitally reconstructed radiograph (DRR) is used primarily to:

Answer: Act as a reference image for comparison with treatment portal or kV images

The DRR is made from the planning CT and acts as the reference for checking patient position on treatment images.

According to QUANTEC, to limit severe xerostomia when both parotids are spared, the combined mean parotid dose should be kept below about:

Answer: 25 Gy

QUANTEC recommends a mean dose under 25 Gy to both parotids together, or under 20 Gy to at least one parotid.

The beam's eye view (BEV) display in treatment planning shows:

Answer: The patient anatomy from the viewpoint of the radiation source

BEV projects the anatomy and contours as seen from the source, which helps with shaping fields and blocks.

The main purpose of a field-in-field technique in breast planning is to:

Answer: Reduce hot spots and improve dose homogeneity

Small MLC-shaped subfields block high-dose regions for part of the treatment, which reduces hot spots.