HPTC RT Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

ALARA principle

Dose should be kept as low as reasonably achievable, with economic and social factors considered. It applies to occupational, public, and patient exposure and is implemented through time, distance, and shielding.

Inverse square law

Exposure rate from a point source is inversely proportional to the square of the distance. Doubling distance reduces intensity to one quarter, so increasing distance is an effective protection tool.

Occupational dose limit

Annual effective dose limit for radiation workers is 50 mSv (5 rem). Lens limit is 150 mSv and skin or extremity limit is 500 mSv per year under ICRP recommendations.

Half-value layer

Thickness of material that reduces beam intensity to half. About 3.32 HVL reduces intensity to roughly 10%, and transmission follows exponential attenuation I = I₀e^(−μx).

Declared pregnant worker limit

After a voluntary written declaration of pregnancy, the dose limit to the embryo/fetus is 5 mSv (0.5 rem) for the entire gestation period, with dose monitored and recorded.

PTV margin concept

The planning target volume adds margins to the clinical target volume to account for setup uncertainty, organ motion, and patient movement, ensuring the prescribed dose reaches the target.

Homogeneity index

A plan metric describing dose uniformity across the target, often calculated as (D2 − D98)/D50 or (Dmax − Dmin)/Dmean. Lower values indicate a more uniform dose distribution.

Conformity index

Ratio of the prescription isodose volume to the target volume. A value near 1 indicates the high-dose region conforms well to the PTV without excessive normal tissue irradiation.

Electron energy selection

Electron energy is chosen so the 90% isodose covers the target depth, roughly energy divided by 3 to 3.5 in centimeters. Bolus brings dose to the surface for superficial lesions.

CT simulation setup

Acquire planning CT with the same immobilization, patient position, and isocenter marks used at treatment. Thin slices (≤3 mm) improve contouring and dose calculation accuracy.

SAD versus SSD setup

In SAD technique the source-to-axis distance is fixed (usually 100 cm) and the gantry rotates around isocenter. In SSD technique the source-to-skin distance is fixed and field size is set at the surface.

Wedge factor

Ratio of dose with a wedge to dose without it at the same point. It reduces monitor units delivered and must be applied in MU calculations; verify wedge orientation matches the plan.

Monitor unit calculation

MU equals prescribed dose divided by the product of dose rate, output factor, depth factor, scatter factors, wedge factor, and tray factor, with inverse square corrections applied.

Patient-specific IMRT QA

Measurement of the planned fluence in a phantom before first treatment. Gamma analysis at 3%/3 mm with a 10% threshold typically requires at least 90–95% of points to pass.

Systematic versus random error

Systematic setup errors are consistent offsets corrected by adjusting the plan or isocenter. Random errors vary day to day and are reduced by better immobilization and daily imaging.

Radiation dermatitis grading

Grade 1 is faint erythema, grade 2 moderate to brisk erythema, grade 3 moist desquamation, and grade 4 life-threatening skin necrosis. Skin care includes mild soap, patting dry, and avoiding friction.

Neutropenic precautions

When absolute neutrophil count falls below 500/µL, implement hand hygiene, avoid crowds, screen visitors for illness, and report fever of 38°C or higher immediately.

Acute radiation effects timeline

Fatigue and erythema typically appear in weeks 2–4, mucositis after week 2, and cytopenias later in treatment. Effects are graded using CTCAE criteria and managed symptomatically.

Radiation pneumonitis

An inflammatory lung reaction occurring 1–6 months after thoracic irradiation. Symptoms include cough, dyspnea, and low-grade fever; risk rises with dose, volume, and concurrent chemotherapy.

Linear-quadratic model

Surviving fraction equals e^(−αD−βD²). The α/β ratio is about 10 Gy for tumors and early effects and about 3 Gy for late normal-tissue effects.

BED formula

Biologically effective dose equals nd[1 + d/(α/β)], where n is fractions and d is dose per fraction. It allows comparison of different fractionation schedules on a common scale.

Four R's of radiobiology

Repair of sublethal damage, repopulation of cells, redistribution through the cell cycle, and reoxygenation of hypoxic tumor cells all influence fractionated radiotherapy response.

Oxygen enhancement ratio

Ratio of dose under hypoxic versus well-oxygenated conditions producing the same effect. For photons it is about 2.5–3, explaining why hypoxic tumor cells are more radioresistant.

Brachytherapy dose rate categories

Low dose rate is below 2 Gy/h, medium dose rate is 2–12 Gy/h, and high dose rate exceeds 12 Gy/h. Common HDR sources deliver around 10–12 Gy/h with iridium-192.

Daily linac QA

Checks performed before the first patient include output constancy, laser alignment, door interlock, audiovisual monitoring, and beam-on indicator, with results recorded per TG-142.

TG-51 calibration

Absorbed dose to water protocol using an ion chamber with calibration factor N_D,w and beam quality correction k_Q, measured at 10 cm depth in water at reference conditions.

Iridium-192 half-life

Approximately 73.8 days. Source strength decays over time, so HDR treatment times must be recalculated and source exchange occurs at scheduled intervals.

MLC leaf position tolerance

Leaf position accuracy is typically verified to within about 1 mm. Tests include leaf positioning, speed, and interleaf leakage, performed monthly and annually per TG-142.

Gamma analysis criteria

Comparison of measured and calculated dose using dose difference and distance-to-agreement. Common criteria are 3%/3 mm with a 10% low-dose threshold, requiring 90–95% passing points.

Temperature-pressure correction

Ion chamber readings are corrected to reference conditions using P_tp = (273.15 + T)/295.15 × 101.33/P, where T is in Celsius and P in kilopascals.

In-vivo dosimetry

Measurement of dose on or in the patient during treatment using diodes or TLDs. Entrance dose is typically verified within about 5% of the calculated value.

Error disclosure duty

Report errors and near-misses through the institutional incident system, disclose to the patient per policy, and document the event and corrective actions accurately.