CMD CMD Clinical Oncology & Disease Sites 1 — Questions and Answers
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?
- Rectum (Correct answer)
- Spinal cord
- Optic chiasm
- Parotid gland
Correct answer: Rectum
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?
- Tumor bed (lumpectomy cavity) (Correct answer)
- Entire ipsilateral breast
- Regional lymph nodes
- Contralateral breast
Correct answer: Tumor bed (lumpectomy cavity)
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?
- Mean parotid dose < 26 Gy (Correct answer)
- Max spinal cord dose < 45 Gy
- Mean mandible dose < 50 Gy
- Max brainstem dose < 54 Gy
Correct answer: Mean parotid dose < 26 Gy
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?
- Mean lung dose (MLD) ≤ 20 Gy (Correct answer)
- Max spinal cord dose ≤ 45 Gy
- V20 of heart ≤ 30%
- Mean esophagus dose ≤ 34 Gy
Correct answer: Mean lung dose (MLD) ≤ 20 Gy
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?
- Hippocampus (Correct answer)
- Cerebellum
- Brainstem
- Pituitary gland
Correct answer: Hippocampus
Hippocampal avoidance WBRT (HA-WBRT) spares the hippocampus (memory center) to reduce radiation-induced neurocognitive toxicity, as validated in the NRG CC001 trial.
Question 6: What is the standard clinical target volume (CTV) margin concept added around the gross tumor volume (GTV) in radiation therapy?
- Subclinical microscopic disease spread beyond the visible tumor (Correct answer)
- Setup uncertainty and organ motion
- Penumbra of the radiation beam
- Dose gradient around the 50% isodose line
Correct answer: Subclinical microscopic disease spread beyond the visible tumor
The CTV margin around the GTV accounts for potential microscopic tumor extension not visible on imaging, based on known patterns of disease spread for each tumor site.
CMD CMD Clinical Oncology & Disease Sites 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?