Correct Answer: A. Intensity is inversely proportional to squared distance
This question tests the fundamental physics principle governing radiation dose delivery in radiotherapy. The correct answer reflects the inverse square law, which states that radiation intensity is inversely proportional to the square of the distance from the source: I ∝ 1/d². This principle is foundational to all brachytherapy and external beam radiotherapy planning. In the context of endometrial carcinoma treatment (commonly managed with intracavitary brachytherapy in Indian centers), understanding dose distribution is critical. As distance from the radiation source doubles, the intensity falls to one-quarter; tripling the distance reduces intensity to one-ninth. This law explains why dose rates drop rapidly with distance, necessitating precise applicator placement and dose calculation in gynecological brachytherapy. The inverse square law is the physical basis for dose optimization in intracavitary applications, where the uterine cavity is the target and adjacent tissues (bladder, rectum) must be spared. This principle is universally applicable and forms the backbone of radiotherapy dosimetry in Indian oncology practice.
Why the other options are wrong
B. Rapidly proliferating cells are most radioresistant — This is the opposite of radiobiology fact. Rapidly proliferating cells (like bone marrow, intestinal crypts, reproductive tissues) are most radiosensitive, not radioresistant. Slowly dividing or non-dividing cells (nerve, bone, muscle) are radioresistant. This is Bergonié and Tribondeau's law. The NBE trap exploits confusion between radiosensitivity and radioresistance in cell kinetics. C. Small bowel is most radioresistant — Small bowel is actually highly radiosensitive due to rapid epithelial cell turnover in crypts of Lieberkühn. It is one of the most radiosensitive organs in the abdomen, making it a dose-limiting structure in pelvic radiotherapy for endometrial cancer. Radioresistant tissues include bone, cartilage, and nerve. This option confuses organ radiosensitivity with physics principles. D. Small blood vessels are most radioresistant — Small blood vessels are radiosensitive and are among the first structures to show radiation injury (endothelial damage, thrombosis). Large blood vessels show greater radioresistance than small ones. This option misrepresents vascular radiosensitivity and is a distractor unrelated to the inverse square law governing dose distribution.
High-Yield Facts
- Inverse square law: Radiation intensity ∝ 1/d² — doubling distance reduces intensity to 1/4
- Bergonié–Tribondeau law: Rapidly dividing cells (bone marrow, intestinal crypts, reproductive tissue) are radiosensitive; slowly dividing cells (nerve, bone, muscle) are radioresistant
- Small bowel is the most radiosensitive abdominal organ and dose-limiting structure in pelvic radiotherapy for endometrial cancer
- Intracavitary brachytherapy for endometrial cancer relies on inverse square law to deliver high dose to tumor while sparing bladder and rectum
- Dose rate in brachytherapy drops rapidly with distance; 1 cm distance change significantly alters dose to adjacent organs
Mnemonics
INVERSE SQUARE LAW (Physics) Intensity ∝ 1/d² — Double distance → Dose drops to 1/4. Use when calculating dose at different distances in brachytherapy applicator placement. RADIOSENSITIVE vs RADIORESISTANT (Bergonié–Tribondeau) FAST dividers = SENSITIVE (bone marrow, gut crypts, gonads, lens). SLOW dividers = RESISTANT (nerve, bone, cartilage, muscle). Use to predict organ toxicity in pelvic radiotherapy.
NBE Trap
NBE pairs radiotherapy physics (inverse square law) with radiobiology distractors (cell radiosensitivity, organ radiosensitivity) to test whether students confuse dose distribution physics with cellular/tissue radiosensitivity. The trap is that options B, C, D all reference radiosensitivity/radioresistance, which are radiobiology concepts, not physics principles.
Clinical Pearl
In Indian gynecological oncology centers, endometrial cancer brachytherapy applicators are placed in the uterine cavity to exploit the inverse square law: high dose to the tumor-bearing endometrium, but dose falls steeply with distance, protecting the bladder (anterior) and rectum (posterior). A 1 cm shift in applicator position can halve the dose to adjacent organs — this is why precise applicator placement and dose calculation are critical in Indian cancer centers following ICRU guidelines.
_Reference: Robbins Ch. 7 (Neoplasia); Harrison Ch. 87 (Oncology); KD Tripathi Ch. 50 (Anticancer Drugs & Radiotherapy)_