"Radiotherapy Principles & Radiation Oncology Basics" — Practice

Question 1

The principal reason radiotherapy is given in multiple small fractions is

Accepted Answer

B

Question 2

Radiation-induced carcinogenesis is best described as

Accepted Answer

A

Question 3

A patient on radical head and neck radiotherapy develops severe mucositis at week four and the team considers a two-week break. What is the concern?

Accepted Answer

Accelerated repopulation. Tumour clonogens proliferate during a course of radiotherapy, and in head and neck squamous carcinoma this acceleration typically begins around three to four weeks into treatment, so surviving cells are dividing considerably faster than at the start.

A gap therefore allows the tumour to regain ground that delivered fractions had already taken, and prolonging overall treatment time is associated with reduced local control and worse survival.

The alternative is to manage the mucositis aggressively rather than to interrupt: analgesia including opioids, mouth care, nutritional support with a feeding tube where necessary, and treatment of superimposed candidal infection.

If a gap becomes unavoidable, compensation is attempted by accelerating the remainder or adding fractions, and the reasoning is documented.

Question 4

Why does anaemia reduce the effectiveness of radiotherapy?

Accepted Answer

Because roughly two-thirds of the DNA damage caused by X-rays is indirect, mediated by free radicals generated from water, and oxygen is required to make that damage permanent.

The mechanism is oxygen fixation. When a hydroxyl radical produces a DNA lesion, that lesion can be chemically restored by hydrogen donation from cellular thiols. If oxygen is present it reacts with the damaged site first, forming an organic peroxide and rendering the damage permanent and therefore lethal.

Well-oxygenated cells are consequently two to three times more sensitive than hypoxic ones, which is the oxygen enhancement ratio.

Anaemia reduces oxygen delivery, increasing the hypoxic fraction and effectively making the tumour more radioresistant. The practical response is to correct treatable causes, though trials of aggressive correction with erythropoiesis-stimulating agents have not shown benefit and have raised safety concerns.

Question 5

A patient with widespread metastatic disease has a painful bone metastasis. The team proposes six weeks of daily radiotherapy. Comment.

Accepted Answer

The fractionation is wrong for the intention.

The purpose here is palliative rather than curative, and the reasoning changes accordingly. Late effects, which are driven by dose per fraction and appear over years, matter far less in a patient whose survival is measured in months, so the constraint that forces small daily fractions in radical treatment does not apply.

The burden of treatment matters a great deal. Six weeks of daily attendance for a patient with limited mobility, pain and a short life expectancy consumes a substantial proportion of the time they have.

The evidence supports short courses. A single fraction is as effective as multi-fraction regimens for pain relief from uncomplicated bone metastases, with a somewhat higher retreatment rate that is easily managed by retreating.

Longer courses remain appropriate for impending pathological fracture or spinal cord compression, where a different technique and dose apply.

Question 6

A patient treated with radiotherapy 15 years ago develops a sarcoma within the treated field. What kind of effect is this, and what does it imply about dose?

Accepted Answer

A stochastic effect, specifically radiation-induced carcinogenesis.

Stochastic effects differ fundamentally from deterministic ones. They have no threshold dose, so no exposure can be declared entirely safe, and dose influences the probability that the effect occurs rather than its severity. A sarcoma arising after radiotherapy is no worse for having followed a higher dose; the higher dose simply made it more likely.

Deterministic effects behave in the opposite way. Cataract, skin erythema, sterility and acute radiation syndrome all require a threshold dose to occur at all, and above that threshold severity rises with dose.

Two clinical implications follow. Latency for second malignancy is long, typically a decade or more, which is why the risk weighs most heavily in children and young adults with long expected survival and matters least in elderly patients. And it is one of the principal reasons every technical advance has aimed at reducing the volume of normal tissue irradiated.

Question 7

Why is brachytherapy essential rather than optional in locally advanced cervical cancer?

Accepted Answer

Because the geometry of the disease matches the physics of the technique better than any other common cancer.

Dose from a brachytherapy source falls off according to the inverse square of distance, so intensity drops extremely steeply over a few centimetres. Placing the source within the uterine cavity and the vaginal fornices therefore delivers a very high dose to the central tumour while the rectum and bladder, lying a short distance away, receive substantially less.

External beam radiotherapy alone cannot achieve this. To deliver an equivalent central dose it would have to pass that dose through bladder and rectum, whose tolerance would be exceeded well before the tumour dose was reached.

The clinical evidence is unambiguous. Omitting brachytherapy in locally advanced cervical cancer is associated with markedly worse local control and survival, and it cannot be adequately substituted by additional external beam dose.

This matters particularly in India, where cervical cancer is a leading cause of cancer death in women and access to brachytherapy facilities is a genuine determinant of outcome.

Question 8

A hypofractionated regimen delivers a lower total dose than conventional treatment, yet carries greater risk of late fibrosis. Explain.

Accepted Answer

Because late-responding tissues are far more sensitive to the size of each fraction than to the total dose delivered.

The underlying reason is differential capacity to repair sublethal damage. Late-responding tissues, meaning slowly dividing connective tissue, vasculature and nerve, have a large capacity for repair between fractions, which is exploited by giving small doses each day. When fraction size increases, that repair capacity is progressively overwhelmed, and the biological effect on those tissues rises disproportionately.

Acutely responding tissues, meaning rapidly dividing mucosa and marrow, have less repair capacity to lose and are influenced more by total dose and overall treatment time than by fraction size.

The consequence is that a schedule delivering fewer, larger fractions can produce equivalent tumour control with a lower nominal total dose while carrying a greater late-effect burden. This is acceptable in palliative treatment, where survival is limited, and requires careful justification in radical treatment, where it is used in tumours whose repair characteristics make it favourable, such as prostate and breast cancer.

Question 9

A patient with known lung cancer develops facial swelling, distended neck veins and breathlessness worse on lying flat. Outline the reasoning and immediate management.

Accepted Answer

This is superior vena cava obstruction, one of the three situations in which radiotherapy is genuinely an emergency treatment alongside spinal cord compression and uncontrolled tumour bleeding.

The mechanism explains the signs. Extrinsic compression or thrombosis of the superior vena cava obstructs venous return from the head, neck and upper limbs, producing facial and arm swelling, distended non-pulsatile neck veins, dilated chest wall collaterals and orthopnoea. Airway compromise and raised intracranial pressure are the features that make it urgent rather than merely uncomfortable.

Immediate management combines sitting the patient upright, oxygen and dexamethasone to reduce peritumoural oedema, with contrast CT to define the level of obstruction, the causative mass and any thrombus.

Treatment then depends on histology and urgency. Endovascular stenting provides the fastest relief and is often preferred in severe cases. Radiotherapy is effective and is standard where the tumour is radiosensitive. In small cell lung cancer and lymphoma, chemotherapy alone can produce rapid response, which is why obtaining tissue before treatment matters whenever the patient is stable enough to allow it.

Practice — "Radiotherapy Principles & Radiation Oncology Basics"

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