AOCNP Radiation Therapy Principles and Side Effects 4 — Questions and Answers
Question 1: A patient undergoing pelvic radiation for cervical cancer reports diarrhea, cramping, and rectal bleeding 6 weeks after completing treatment. What is the most likely diagnosis?
- Acute radiation proctitis
- Chronic radiation enteritis (Correct answer)
- Clostridium difficile colitis
- Radiation-induced colorectal cancer
Correct answer: Chronic radiation enteritis
Symptoms appearing weeks to months after completing pelvic radiation are consistent with chronic radiation enteritis, which can develop from 6 weeks to years post-treatment.
Question 2: Which dosimetric parameter best predicts the risk of radiation-induced pneumonitis in a lung cancer patient receiving thoracic radiotherapy?
- Maximum dose to the spinal cord
- Mean lung dose (MLD) (Correct answer)
- V20 of the contralateral lung only
- D95 of the planning target volume
Correct answer: Mean lung dose (MLD)
Mean lung dose (MLD) is one of the strongest predictors of radiation pneumonitis risk, with higher MLD correlating with increased incidence.
Question 3: A patient with head and neck cancer develops trismus 8 months after completing chemoradiation. Which structure's fibrosis most directly causes this complication?
- Parotid glands
- Temporomandibular joint and masticatory muscles (Correct answer)
- Cervical lymph nodes
- Submandibular glands
Correct answer: Temporomandibular joint and masticatory muscles
Trismus results from radiation-induced fibrosis of the masticatory muscles and temporomandibular joint, causing limited mouth opening.
Question 4: Stereotactic body radiation therapy (SBRT) for a peripheral lung tumor uses a biologically effective dose (BED) of 100 Gy or more. This high BED is thought to achieve tumor control partly through which mechanism beyond direct DNA damage?
- Increased tumor oxygenation from vascular normalization
- Ablation of tumor vasculature causing indirect tumor cell death (Correct answer)
- Enhanced repair of sublethal damage in tumor cells
- Upregulation of tumor suppressor genes
Correct answer: Ablation of tumor vasculature causing indirect tumor cell death
High-dose-per-fraction SBRT is believed to destroy tumor vasculature, causing indirect tumor cell death through ischemia in addition to direct cytotoxicity.
Question 5: During radiation therapy planning, what does the term 'R50' measure in SBRT?
- The ratio of prescription dose to the 50% isodose volume
- The distance from the isocenter to the 50% isodose line
- The ratio of the 50% isodose volume to the planning target volume (Correct answer)
- The volume receiving 50% of the maximum dose
Correct answer: The ratio of the 50% isodose volume to the planning target volume
R50 (conformity index) is the ratio of the 50% isodose volume to the PTV, used to evaluate the conformality and fall-off of SBRT plans.
Question 6: A patient receiving radiation to the pelvis develops lymphedema of the lower extremities 1 year after treatment. What is the primary pathophysiologic mechanism?
- Arterial insufficiency from radiation arteritis
- Obstruction of lymphatic channels due to radiation-induced fibrosis (Correct answer)
- Deep vein thrombosis from prolonged immobility
- Hypoalbuminemia from radiation enteritis
Correct answer: Obstruction of lymphatic channels due to radiation-induced fibrosis
Pelvic radiation can cause fibrosis and obstruction of lymphatic channels, impairing lymphatic drainage and leading to lower extremity lymphedema.
Question 7: Which of the following best describes the concept of 'dose painting' in radiation therapy?
- Applying topical agents to irradiated skin to reduce dermatitis
- Delivering higher radiation doses to metabolically active or hypoxic tumor subvolumes (Correct answer)
- Painting the treatment field on the patient's skin for daily setup verification
- Using multiple beam angles to achieve a uniform dose across the target
Correct answer: Delivering higher radiation doses to metabolically active or hypoxic tumor subvolumes
Dose painting uses functional imaging (e.g., PET, MRI) to identify tumor subvolumes requiring higher doses, such as hypoxic or FDG-avid regions, and boosts dose selectively to those areas.
AOCNP Radiation Therapy Principles and Side Effects 4
A patient undergoing pelvic radiation for cervical cancer reports diarrhea, cramping, and rectal bleeding 6 weeks after completing treatment.
What is the most likely diagnosis?