ABR Radiation Oncology flashcards

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

HVL

Half-value layer: thickness of material that reduces photon intensity by half; HVL = ln2/μ ≈ 0.693/μ, where μ is the linear attenuation coefficient.

Tenth-value layer

TVL is the thickness of material that reduces photon intensity to one-tenth; TVL = ln10/μ ≈ 2.303/μ.

Percent depth dose

Ratio of dose at depth to dose at dmax along the central axis, expressed as a percentage; increases with energy, field size, and SSD.

Tissue-maximum ratio

TMR is the ratio of dose at depth to dose at dmax in a phantom, measured at isocenter; it is independent of SSD and used for isocentric calculations.

Mayneord factor

F = ((SSD2 + dmax)/(SSD2 + d))² × ((SSD1 + d)/(SSD1 + dmax))²; converts percent depth dose from one SSD to another.

Scatter factor Scp

Total scatter factor is the product of collimator scatter factor (Sc) and phantom scatter factor (Sp); it accounts for field-size-dependent scatter.

TG-43 formalism

Brachytherapy dosimetry formalism using dose-rate constant Λ, geometry factor G(r,θ), radial dose function g(r), and anisotropy function F(r,θ).

Electron therapeutic range

R90, the depth at which dose falls to 90% of maximum, approximates E(MeV)/4 cm in water; practical range Rp approximates E/2 cm.

Gamma index

QA metric combining dose-difference and distance-to-agreement criteria; 3%/3 mm is a common acceptance threshold for IMRT/VMAT verification.

Linear-quadratic model

Surviving fraction SF = e^(−αD − βD²); α/β is the dose at which the linear and quadratic contributions to cell killing are equal.

Alpha/beta ratio

Dose at which linear and quadratic components of cell killing are equal; ~3 Gy for late-responding tissues and ~10 Gy for early-responding tumors.

Biologically effective dose

BED = n·d·(1 + d/(α/β)); converts a fractionation schedule to a common biological scale for comparing regimens.

EQD2

Equivalent dose in 2-Gy fractions: EQD2 = BED/(1 + 2/(α/β)); allows comparison of different fractionation schedules on a 2-Gy-per-fraction scale.

Oxygen enhancement ratio

OER is the ratio of dose under hypoxia to dose under normoxia producing the same effect; approximately 2.5-3 for X-rays and near 1 for high-LET radiation.

Relative biological effectiveness

RBE is the ratio of dose of reference radiation to dose of test radiation producing the same biological effect; high-LET radiation has RBE greater than 1.

Four R's of radiobiology

Repair of sublethal damage, repopulation, redistribution through the cell cycle, and reoxygenation; these processes shape fractionated radiation response.

D0

Mean lethal dose: the dose that reduces cell survival to 37% on the exponential portion of the survival curve; describes intrinsic radiosensitivity.

Extrapolation number

Symbol n; the extrapolation number describes the shoulder width of the cell survival curve and reflects repair capacity.

Cell-cycle radiosensitivity

Radiosensitivity is greatest in M and G2 phases and least in late S phase; proliferating cells are generally more sensitive than quiescent cells.

Tumor control probability

TCP is the probability of eradicating all clonogenic tumor cells; it increases with dose and decreases with tumor volume and clonogen number.

NTCP

Normal tissue complication probability; it increases with dose and irradiated volume and is used to evaluate treatment plan trade-offs.

Hodgkin lymphoma RT

Modern treatment uses involved-site or involved-node radiation with combined-modality chemotherapy (ABVD); mantle fields are largely historical.

Prostate dose escalation

Escalating prostate RT dose to at least 78 Gy improves biochemical control in intermediate- and high-risk disease compared with lower doses.

Breast hypofractionation

Whole-breast RT in 15-16 fractions (e.g., 40-42.5 Gy) is standard for many early breast cancer patients after lumpectomy.

Lung SBRT

Stereotactic body RT delivering high dose per fraction (e.g., 3-5 fractions of 10-20 Gy) for medically inoperable early-stage NSCLC.

Concurrent chemoradiation HNSCC

Concurrent cisplatin with radiation is standard for locally advanced head and neck squamous cell carcinoma; HPV-positive oropharyngeal cancer has better prognosis.

Rectal neoadjuvant therapy

Locally advanced rectal cancer is treated with neoadjuvant chemoradiation (e.g., 50.4 Gy with 5-FU) or short-course RT (25 Gy in 5 fractions) before surgery.

Cervical chemoradiation

Concurrent cisplatin-based chemoradiation with brachytherapy boost is standard for locally advanced cervical cancer; brachytherapy is essential for local control.

Brain metastases SRS

Stereotactic radiosurgery is preferred for a limited number of small brain metastases; whole-brain RT is used for multiple metastases.

Palliative bone RT

Single-fraction 8 Gy is effective for painful bone metastases and is comparable to multifraction regimens for pain relief.

Glioblastoma standard

Standard therapy is 60 Gy in 30 fractions with concurrent temozolomide followed by adjuvant temozolomide.

Anal cancer chemoradiation

Definitive chemoradiation with mitomycin C and 5-FU plus RT is standard for anal squamous cell carcinoma; surgery is reserved for salvage.