Physics Against Cancer Practice Flashcards

A complete set of vocabulary flashcards covering the physics, biology, and clinical terminology found in the Physics Against Cancer lecture series.

Absorbed dose

Energy deposited per unit mass; $1\,Gy$ equals $1\,J/kg$.

Equivalent dose

Absorbed dose multiplied by a radiation weighting factor ($w_R$); unit is the Sievert ($Sv$).

Linear Energy Transfer (LET)

Energy deposited locally per unit track length, usually measured in $keV/\mu m$; used as a proxy for track density.

Relative Biological Effectiveness (RBE)

Ratio comparing the dose of a reference radiation to the dose of test radiation for the same biological effect.

Oxygen Enhancement Ratio (OER)

The ratio of dose required in hypoxia divided by the dose required in air (oxic conditions) to achieve the same biological effect.

Tumor Control Probability (TCP)

The probability that no clonogenic tumor cells survive treatment.

Normal Tissue Complication Probability (NTCP)

The probability that normal tissue develops a treatment-related complication.

Gross Tumor Volume (GTV)

The clinically visible or demonstrated volume of the tumor.

Clinical Target Volume (CTV)

The Gross Tumor Volume ($GTV$) plus a margin for microscopic malignant disease extension.

Planning Target Volume (PTV)

The Clinical Target Volume ($CTV$) plus margins for setup uncertainties and organ motion.

Intensity-Modulated Radiotherapy (IMRT)

A photon treatment method that modulates beam intensity to create concave dose distributions and spare organs at risk.

Linear-Quadratic (LQ) Model

A cell-survival model defined by the equation $S(D) = \exp(-\alpha D - \beta D^2)$, where $\alpha$ and $\beta$ describe the linear and quadratic cell-kill terms.

Alpha/Beta value (\alpha/\beta)

The dose at which the linear ($\alpha D$) and quadratic ($\beta D^2$) cell-kill components are equal; it measures the survival curve bend and tissue repair capacity.

Bragg Peak

The sharp dose peak that occurs near the end of a charged particle's range in tissue.

Spread-Out Bragg Peak (SOBP)

The superposition of multiple Bragg peaks over depth to cover the full thickness of an extended tumor target.

Linear Accelerator (LINAC)

A machine that accelerates electrons to produce high-energy treatment photons (X-rays) or electron beams.

Multi-Leaf Collimator (MLC)

A system of opposing motorized metal leaves used to shape and modulate radiation fields.

Direct Action

Radiation biological effect where the radiation ionizes DNA or breaks molecular bonds directly; more dominant with high-$LET$ radiation.

Indirect Action

Biological effect where radiation ionizes water to produce free radicals that then damage DNA; strongly influenced by oxygen levels.

Single-Strand Break (SSB)

A break in only one strand of the DNA double helix; usually easier for the cell to repair than a $DSB$.

Double-Strant Break (DSB)

A break in both DNA strands close together; recognized as the key lethal lesion in radiobiology.

Clustered Damage

Multiple DNA lesions, such as $SSBs$ and base damages, occurring within a few nanometers of each other, making repair harder and misrepair more likely.

Nonhomologous End Joining (NHEJ)

A fast but error-prone DNA repair pathway for double-strand breaks that is dominant in the $G1$ phase of the cell cycle.

Homologous Recombination Repair (HRR)

A high-fidelity DNA repair pathway for double-strand breaks using the sister chromatid as a template; most active in late $S$ and $G_2$ phases.

Base Excision Repair (BER)

The cellular pathway responsible for repairing small base lesions such as oxidation or deamination.

Nucleotide Excision Repair (NER)

The pathway responsible for repairing bulky DNA lesions, such as those caused by chemical adducts or UV radiation.

Gompertz Model

A mathematical tumor growth model where growth slows as the tumor increases in size, eventually approaching a limiting mass.

Herman-Savage-West (HSW) Model

A mechanistic model of tumor/spheroid growth that accounts for metabolism, oxygen diffusion, and inhibition by necrosis.

Basic Interaction Volume (BIV)

A nanometer-scale volume (approx. $2-10\,nm$) used in nanodosimetry to count ionizations near the DNA segment.

Lethal Interaction Volume (LIV)

A target volume of approximately $2-50\,nm$ where clustered damage events can lead to a lethal biological event.

Pencil Beam Scanning (PBS)

A particle therapy delivery method using magnetically scanned narrow beams and energy changes to cover a target volume.

Relative Risk (RR)

The ratio of observed cases ($O$) to expected cases ($E$): $RR = O/E$.

Excess Relative Risk (ERR)

The relative increase in risk above the expected level: $ERR = (O - E) / E$.

Excess Absolute Risk (EAR)

The number of extra cases per person-year: $EAR = (O - E) / PY$.

Risk Equivalent Dose (RED)

A dose quantity used in models to transform physical dose into a value proportional to the risk of second-cancer induction.

Organ Equivalent Dose (OED)

The average risk equivalent dose ($RED$) over an organ, used for relative comparisons of cancer risk between treatment plans.

Compton Scattering

The dominant photon interaction in the therapeutic energy range, where a photon scatters and ejects an electron from an atom.

Bremsstrahlung

Radiation emitted when high-speed electrons are decelerated near the nuclei of a target material; the source of therapeutic X-rays.

Skin-Sparing Effect

The phenomenon in megavoltage photon beams where the dose at the skin surface is lower due to the build-up of secondary electrons.

Penumbra

The blurred edge of a radiation beam where the dose falls off laterally, caused by the size of the source and lateral electron transport.

Beam Hardening

The process where lower-energy photons are removed from a beam as it passes through material, increasing the average energy/quality of the beam.

Multiple Coulomb Scattering (MCS)

The accumulated scattering of charged particles by atomic nuclei, which causes lateral broadening of beams and smears the Bragg peak.

Latency Period

The time interval between the initial radiation exposure and the subsequent clinical appearance of a second cancer.

Clonogenic Cell

A cell that retains the ability to proliferate indefinitely and form a colony; tumor control requires the elimination of all such cells.

One-Track Event (OTE)

In the track-event model, a lethal biological event produced by a single radiation track.

Two-Track Event (TTE)

In the track-event model, a lethal biological event produced by the interaction of two sublethal events from separate tracks.

Fractionation

The practice of dividing the total radiation dose into smaller daily sessions to allow for normal tissue repair and repopulation.

Hypoxia

A biological state of low oxygen concentration that increases a cell's radioresistance to low-$LET$ radiation.

Isocenter

The geometric point in space where the treatment machine's rotation axes and the radiation beams intersect.

Monte Carlo Simulation

A computational method that relies on random sampling to simulate particle tracks and energy deposition patterns at various scales.

Gy

Gray

IGRT

Intensity-modulated radiotherapy: Photon treatment using non-uniform beam intensity to shape concave dose distributions.

Ionizing Radiation

Radiation energetic enough to ionize atoms and molecules. Molecule Loses electrons → becomes a positive ion (cation) because it has more protons than electrons.

Local control

Probability or rate that the tumor remains controlled at the treated site.

OAR

Organ at risk: Normal organ or tissue whose dose must be limited.

X-ray

Dominant radiation in modern photon radiotherapy.

Biological Dose

Dose weighted by biological effectiveness, for example Gy(RBE) or Gye.

Photon

Neutral quantum of electromagnetic radiation; interacts weakly in tissue.

Electron

Charged particle that ionizes atoms readily and deposits much of the dose.

Coulomb Scattering

Charged-particle deflection by electric fields of nuclei or electrons.

Attenuation

Reduction of photon number with depth; often exponential: n = n0 exp(-mu d).

Photon tail

Low-level dose beyond the electron range from bremsstrahlung photons.

Bending Magnet

Magnet that bends accelerated electrons toward the target or treatment head.

Flattening Filter

Filter that homogenizes the transverse photon beam profile.

Collimator

Device that shapes the radiation field.

Cross-firing beams

Multiple beams from different directions to concentrate dose in the target.

Distal fall-off

Rapid dose decrease after the Bragg peak. Important for sparing tissues beyond the target.

Range uncertainty

Uncertainty in where charged particles stop.

Lateral Scattering

Sideway spreading of particles. Heavier particles spread less than light particles.

Initiation

Radiation-induced mutation creating potentially premalignant cells - First part of second-cancer models.

Nanodosimetry

Measurement of ionization patterns in nanometer-scale volumes. Connects physical track structure to DNA damage

probability.

Second primary cancer

A new cancer with different histology after a latency period. Different from recurrence of the original tumor.

Ionization cluster

Group of ionizations produced in a small nanometric volume. Cluster size helps predict complex damage.

Basic interaction volume

Small volume representing a DNA-size target segment. Often modeled as about 2 nm and 10 base pairs.

Lethal interaction volume

Larger nanoscale target where clustered damage can become lethal. Composed of several BIVs.

Track structure

Spatial pattern of energy deposition around a particle path. Dose is not smooth at the nanometer scale.

Free radical

Highly reactive molecule or atom with an unpaired electron.

DNA lesion

Any radiation-induced chemical damage to DNA. Includes base damage, SSBs and DSBs.

Base lesion

Damage to a DNA base. Often repaired by BER.

Complex DSB

DSB accompanied by nearby base or sugar damage.

Cytoplasm

Cell material outside the nucleus.

Colony-forming ability

Ability of a cell to grow into a colony in vitro. Loss of this ability defines radiation cell kill.

Radioresistance

Cell or tissue ability to survive radiation. Depends on repair, oxygen, cell cycle and radiation

type.

Repair Capability

Ability to repair sublethal or potentially lethal DNA damage. High repair capability changes survival curve

shoulder.

Repopulation

Cell proliferation during or after treatment. Can reduce treatment effectiveness if too strong.

Alpha component

Linear cell-kill term in LQ model, usually single-track damage. Dominates at low dose/high LET.

Beta component

Quadratic cell-kill term in LQ model, often two-event damage interaction. Important for fractionation and repair.