CMD CMD Clinical Oncology & Disease Sites

Question 1

For prostate cancer treated with definitive external beam radiotherapy, which structure is a primary organ at risk (OAR) that the dosimetrist must carefully contour and spare?

Accepted Answer

Rectum

Answer

The rectum is the most critical OAR for prostate EBRT; dose-volume constraints (e.g., V70Gy < 20%) are used to minimize the risk of late rectal toxicity.

Question 2

In breast cancer radiotherapy following lumpectomy, what is the target volume that typically receives the boost dose?

Accepted Answer

Tumor bed (lumpectomy cavity)

Answer

The tumor bed (surgical cavity) receives a boost dose after whole-breast irradiation to reduce local recurrence, particularly in younger patients with high-risk features.

Question 3

Which OAR dose constraint is most critical when treating head and neck cancers to prevent permanent xerostomia?

Accepted Answer

Mean parotid dose < 26 Gy

Answer

Keeping the mean parotid gland dose below approximately 26 Gy reduces the risk of long-term xerostomia, which is the most common debilitating late effect of head and neck RT.

Question 4

For lung cancer radiotherapy, what is a key pulmonary dose-volume constraint used to predict radiation pneumonitis risk?

Accepted Answer

Mean lung dose (MLD) ≤ 20 Gy

Answer

Mean lung dose ≤ 20 Gy and V20 ≤ 30–35% are standard constraints; exceeding these thresholds significantly increases the risk of symptomatic radiation pneumonitis.

Question 5

In whole-brain radiation therapy (WBRT), which structure should be considered for sparing to reduce neurocognitive decline?

Accepted Answer

Hippocampus

Answer

Hippocampal avoidance WBRT (HA-WBRT) spares the hippocampus (memory center) to reduce radiation-induced neurocognitive toxicity, as validated in the NRG CC001 trial.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD CMD Clinical Oncology & Disease Sites