Radiation Therapy Principles and Side Effects Flashcards

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?

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.

Which dosimetric parameter best predicts the risk of radiation-induced pneumonitis in a lung cancer patient receiving thoracic radiotherapy?

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.

A patient with head and neck cancer develops trismus 8 months after completing chemoradiation. Which structure's fibrosis most directly causes this complication?

Answer: Temporomandibular joint and masticatory muscles

Trismus results from radiation-induced fibrosis of the masticatory muscles and temporomandibular joint, causing limited mouth opening.

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?

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.

During radiation therapy planning, what does the term 'R50' measure in SBRT?

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.

A patient receiving radiation to the pelvis develops lymphedema of the lower extremities 1 year after treatment. What is the primary pathophysiologic mechanism?

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.

Which of the following best describes the concept of 'dose painting' in radiation therapy?

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.