ABR Radiation Oncology cheat sheet

Photon attenuation: I = I0·e^(−μx); μ depends on photon energy and material density; μ/ρ is the mass attenuation coefficient (units cm²/g).

Half-value layer (HVL) = ln2/μ ≈ 0.693/μ; tenth-value layer (TVL) = ln10/μ ≈ 2.303/μ.

Percent depth dose (%DD) increases with higher energy, larger field size, and longer SSD; it decreases with greater depth and larger source-to-surface distance for a fixed field size.

Tissue-air ratio (TAR) and tissue-phantom ratio (TPR) are independent of SSD; TMR (tissue-maximum ratio) replaces TAR for isocentric high-energy techniques.

Mayneord factor (F) converts %DD between SSDs: F = ((SSD2 + dmax)/(SSD2 + d))² × ((SSD1 + d)/(SSD1 + dmax))².

Brachytherapy dose rate: dose rate in Gy/h at distance d scales as 1/d² for a point source; TG-43 formalism uses dose-rate constant Λ, geometry factor G(r,θ), radial dose function g(r), and anisotropy function F(r,θ).

Electron beam depth: therapeutic range R90 ≈ E(MeV)/4 cm; practical range Rp ≈ E/2 cm in water; dose falls steeply after R90.

Photon beam quality: %DD at 10 cm depth in water is the typical beam-quality specifier for megavoltage photons; dmax depth increases with energy (e.g., ~1.5 cm at 6 MV, ~2.5-3 cm at 10-18 MV).

IMRT/VMAT QA: gamma analysis compares measured vs. calculated dose; 3%/3 mm criteria are common; passing rate is dose-difference and distance-to-agreement combined.

Linear-quadratic model: SF = e^(−αD − βD²); α/β is the dose at which linear and quadratic contributions are equal; low α/β (~3 Gy) for late-responding tissues, high (~10 Gy) for early-responding tumors.

BED = n·d·(1 + d/(α/β)); EQD2 = BED/(1 + 2/(α/β)); these convert fractionation schedules to a common scale.

Four R's of radiobiology: repair (sublethal damage), repopulation, redistribution (cell-cycle reassortment), and reoxygenation.

Oxygen enhancement ratio (OER) ≈ 2.5-3 for sparsely ionizing radiation (X-rays); OER ≈ 1 for high-LET radiation such as alpha particles and neutrons.

Relative biological effectiveness (RBE) = dose of reference radiation / dose of test radiation producing the same effect; high-LET radiation has RBE > 1.

Cell survival curve: D0 (mean lethal dose) is the dose reducing survival to 37% on the exponential portion; extrapolation number n describes the shoulder.

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

DNA double-strand breaks are the principal lethal lesion; repair via nonhomologous end joining (NHEJ) and homologous recombination (HR).

Tumor control probability (TCP) and normal tissue complication probability (NTCP) both rise with dose; therapeutic ratio is maximized when TCP exceeds NTCP.

Hodgkin lymphoma: mantle-field era replaced by involved-site/involved-node RT; combined modality with ABVD chemotherapy is standard for early-stage disease.

Prostate cancer: risk stratification uses PSA, Gleason score, and T stage; dose escalation to ≥78 Gy improves biochemical control in intermediate/high-risk disease.

Breast-conserving therapy: whole-breast RT after lumpectomy reduces local recurrence; hypofractionation (e.g., 40-42.5 Gy in 15-16 fractions) is standard for many patients.

Lung cancer: SBRT (e.g., 3-5 fractions of 10-20 Gy) is standard for medically inoperable early-stage NSCLC; concurrent chemoradiation for locally advanced disease.

Head and neck cancer: concurrent cisplatin with RT is standard for locally advanced squamous cell carcinoma; HPV-positive oropharyngeal cancer has better prognosis.

Rectal cancer: neoadjuvant chemoradiation (e.g., 50.4 Gy in 28 fractions with 5-FU) or short-course RT (25 Gy in 5 fractions) followed by surgery.

Cervical cancer: concurrent cisplatin-based chemoradiation with brachytherapy boost is standard; brachytherapy is essential for local control.

Brain metastases: whole-brain RT versus stereotactic radiosurgery; SRS preferred for limited number of small metastases.

Palliative RT: common schedules include 8 Gy × 1, 20 Gy × 5, and 30 Gy × 10; single-fraction 8 Gy is effective for painful bone metastases.

Inverse square law: dose rate ∝ 1/d²; doubling distance reduces dose rate to one-quarter.

Field size effects: larger fields increase scatter and %DD; collimator scatter factor (Sc) and phantom scatter factor (Sp) multiply to give total scatter factor (Scp).

Wedge factor and tray factor modify delivered dose; dynamic wedges use moving collimator jaw motion.

CT simulation: Hounsfield units relate to electron density; heterogeneity corrections use either equivalent path length or convolution/superposition algorithms.

Gamma Knife uses cobalt-60 sources; CyberKnife uses a linear accelerator on a robotic arm with real-time tracking.

Proton therapy: Bragg peak deposits dose at a defined depth; spread-out Bragg peak (SOBP) covers a target volume.

Dosimetry: 1 Gy = 1 J/kg; absorbed dose is energy deposited per unit mass.

Fractionation spares late-responding tissues more than early-responding tissues because of their lower α/β.

Accelerated repopulation can occur during a prolonged course of RT; this is a rationale for not extending overall treatment time.

Hyperfractionation uses smaller doses more frequently (e.g., 1.2 Gy BID) to increase total dose while limiting late effects.

Hypofractionation uses larger doses per fraction; it is favored when α/β of the tumor is low (e.g., prostate).

Radiosensitizers include oxygen and hypoxic cell sensitizers; radioprotectors (e.g., amifostine) reduce normal tissue injury.

Bystander effect: irradiated cells can induce damage in non-irradiated neighboring cells; abscopal effect is a systemic response outside the irradiated field.

Cell cycle checkpoints: G1/S and G2/M; loss of p53 impairs G1 arrest and increases radiosensitivity in some contexts.

Esophageal cancer: neoadjuvant chemoradiation (e.g., 50.4 Gy with carboplatin/paclitaxel) followed by surgery for locally advanced disease.

Anal cancer: definitive chemoradiation (mitomycin C + 5-FU with RT) is standard; surgery reserved for salvage.

Bladder cancer: bladder-preserving trimodality therapy (TURBT + chemoradiation) is an alternative to cystectomy in selected patients.

Pediatric tumors: craniospinal irradiation for medulloblastoma; proton therapy reduces integral dose and late effects.

Stereotactic body RT (SBRT) uses high dose per fraction with steep dose gradients; requires accurate immobilization and image guidance.

Total body irradiation (TBI) is used in conditioning regimens before hematopoietic stem cell transplant; lung and kidney dose limits apply.

Palliative care: 8 Gy single fraction is effective for bone metastases; 10 Gy × 1 or 20 Gy × 5 for brain metastases depending on prognosis.

Monitor unit (MU) calculation for photons: MU = Dose / (output × Scp × %DD × ISF × wedge × tray).

Isocentric technique: SAD setup; dose at isocenter is independent of SSD for a given field size at isocenter.

Quality assurance: daily output constancy, monthly mechanical checks, annual calibration per TG-51 (photons) and TG-21/TG-51 for electrons.

TG-51 calibration: absorbed dose to water calibration factor N_D,w and beam quality conversion k_Q; D_w = M × N_D,w × k_Q.

Brachytherapy: TG-43 parameters include dose-rate constant Λ, geometry function G, radial dose function g, and anisotropy function F.

Electron beams: bolus is used to bring high dose to the surface; electron energy determines depth of penetration.

Surviving fraction after fractionated RT: SF_total = (SF_single)^n if fractions are independent and fully repaired between fractions.

Dose-rate effect: lower dose rates allow more repair and reduce biological effect for a given total dose.

LET (linear energy transfer) is energy deposited per unit path length; high LET produces dense ionization and complex DNA damage.

Tumor hypoxia reduces radiosensitivity; reoxygenation during fractionated RT improves killing of hypoxic cells.

Apoptosis, necrosis, mitotic catastrophe, and senescence are modes of radiation-induced cell death.

Normal tissue tolerance: TD5/5 and TD50/5 describe dose thresholds for 5% and 50% complication probability at 5 years.

Lymphoma: involved-site RT uses smaller fields than involved-node RT; dose typically 20-30 Gy for early-stage favorable disease.

Prostate brachytherapy: low-dose-rate seeds (I-125, Pd-103) or high-dose-rate iridium; dose escalation improves biochemical control.

Breast: regional nodal irradiation (supraclavicular, internal mammary) is considered for node-positive disease.

CNS: glioblastoma standard is concurrent temozolomide with RT (60 Gy in 30 fractions) followed by adjuvant temozolomide.

Head and neck: altered fractionation (hyperfractionation or accelerated) improves local control in some settings.

Sarcoma: preoperative RT (50 Gy) or postoperative RT (60-66 Gy) with surgery; limb-sparing approaches are standard.

Photon interaction probabilities: photoelectric effect ∝ Z³/E³; Compton scattering depends on electron density and is roughly independent of Z; pair production requires >1.022 MeV.

Beam hardening: lower-energy photons are preferentially absorbed, shifting the spectrum to higher energy.

Inverse square law applies to point sources; for extended sources and brachytherapy, use TG-43 geometry factors.

Dose gradient index and conformity index are used to evaluate treatment plan quality.

Image guidance: kV imaging, cone-beam CT, and ultrasound; setup margins account for systematic and random errors.

Patient safety: TG-100 covers failure mode and effects analysis (FMEA); TG-142 covers linac QA.

Alpha/beta ratio: low α/β tissues (late effects: spinal cord, kidney, bladder) are spared more by fractionation than high α/β tumors.

BED for a schedule with n fractions of dose d: BED = nd(1 + d/(α/β)); EQD2 = BED/(1 + 2/(α/β)).

Oxygen enhancement ratio (OER) is the ratio of dose in hypoxia to dose in air for the same effect; OER ≈ 2.5-3 for X-rays.

High-LET radiation has reduced OER and reduced cell-cycle dependence of radiosensitivity.

Repair of sublethal damage (SLD) and potentially lethal damage (PLD) affects survival; split-dose experiments demonstrate SLD repair.

Tumor heterogeneity and cancer stem cells contribute to radioresistance.

Lung SBRT: peripheral tumors may receive 3 fractions of 18-20 Gy or 4-5 fractions; central tumors require fractionation to limit toxicity.

Esophageal cancer: RT dose 50.4 Gy with concurrent chemotherapy is common in the neoadjuvant setting.

Cervical cancer: brachytherapy dose is prescribed to point A (historically) or to target volume with MRI-based planning.

Palliative bone metastases: single 8 Gy is equivalent to multifraction regimens for pain relief; retreatment is common.

Prostate cancer: androgen deprivation therapy combined with RT improves survival in intermediate- and high-risk disease.

Percent depth dose depends on energy, field size, and SSD; TMR is independent of SSD and used for isocentric calculations.

Scatter factors: Sc (collimator) depends on field size at isocenter; Sp (phantom) depends on field size at depth; Scp = Sc × Sp.

Brachytherapy dose falls off approximately as 1/r² near a point source; anisotropy and radial dose functions correct for encapsulation and absorption.

Electron beam: surface dose is high; bolus increases surface dose; electron range in cm ≈ energy (MeV)/2.

IMRT QA: patient-specific QA verifies dose delivery; gamma index with 3%/3 mm is a common acceptance criterion.

Fraction size matters: larger dose per fraction increases late effects more than early effects because late tissues have lower α/β.

Accelerated repopulation during RT can reduce tumor control; overall treatment time should be minimized when repopulation is significant.

Radiosensitivity varies through the cell cycle: M > G2 > G1 > S (late S most resistant).

DNA repair pathways: NHEJ repairs double-strand breaks in all phases; HR is restricted to S/G2.

Tumor control probability increases with dose and with smaller tumor volume; NTCP increases with dose and volume of normal tissue irradiated.

Radioprotectors such as amifostine can reduce xerostomia and mucositis in selected settings.

Glioblastoma: standard RT dose is 60 Gy in 30 fractions with concurrent and adjuvant temozolomide.

Hodgkin lymphoma: early-stage favorable disease may be treated with chemotherapy alone or combined modality; RT fields are involved-site.

Rectal cancer: short-course RT (25 Gy in 5 fractions) followed by surgery is an alternative to long-course chemoradiation.

Breast cancer: whole-breast RT after lumpectomy reduces recurrence; boost to tumor bed may be considered for higher-risk patients.

Palliative RT for brain metastases: whole-brain RT (e.g., 30 Gy in 10 fractions or 20 Gy in 5 fractions) for multiple metastases.

Photon beam energy selection: higher energy gives deeper penetration and lower skin dose but may increase exit dose and neutron production at >10 MV.

Dosimetric quantities: exposure (C/kg), air kerma (Gy), absorbed dose (Gy); 1 Gy = 1 J/kg.

TG-51 reference conditions: 10 × 10 cm field, 100 cm SSD, depth of dmax or 10 cm depending on energy.

Brachytherapy source strength: air kerma strength (U = 1 μGy·m²/h) or apparent activity; TG-43 formalism uses these.

Treatment planning: forward planning for 3D conformal; inverse planning for IMRT/VMAT with dose-volume constraints.

Patient-specific QA for IMRT/VMAT includes gamma analysis and point-dose measurements.

Linear-quadratic model: SF = e^(−αD − βD²); at low doses the linear term dominates; at high doses the quadratic term dominates.

α/β ratio: low for late-responding tissues (~2-3 Gy), high for early-responding tumors (~10 Gy).

BED and EQD2 allow comparison of different fractionation schedules.

OER ≈ 2.5-3 for X-rays; high-LET radiation has OER near 1.

Repair, repopulation, redistribution, and reoxygenation are the 4 R's of radiobiology.

Prostate cancer: dose escalation to ≥78 Gy improves biochemical control in intermediate/high-risk disease.

Lung cancer: SBRT is standard for medically inoperable early-stage NSCLC; concurrent chemoradiation for locally advanced disease.

Head and neck cancer: concurrent cisplatin with RT is standard for locally advanced squamous cell carcinoma.

Cervical cancer: concurrent cisplatin-based chemoradiation with brachytherapy boost is standard.

Palliative bone metastases: 8 Gy single fraction is effective and commonly used.

Inverse square law: dose rate ∝ 1/d².

HVL = 0.693/μ; TVL = 2.303/μ.

%DD increases with energy, field size, and SSD.

TMR is independent of SSD; used for isocentric calculations.

Brachytherapy TG-43 uses Λ, G(r,θ), g(r), and F(r,θ).

Electron range: R90 ≈ E/4 cm; Rp ≈ E/2 cm.

MU calculation includes Scp, %DD, ISF, wedge, and tray factors.

SF = e^(−αD − βD²); α/β is the dose where linear and quadratic contributions are equal.

BED = nd(1 + d/(α/β)); EQD2 = BED/(1 + 2/(α/β)).

OER ≈ 2.5-3 for X-rays; ≈1 for high-LET.

Radiosensitivity: M > G2 > G1 > S.

Four R's: repair, repopulation, redistribution, reoxygenation.

DSBs are the principal lethal lesion; repaired by NHEJ and HR.

TCP and NTCP both rise with dose; therapeutic ratio is maximized when TCP > NTCP.

Hodgkin lymphoma: combined modality with ABVD; involved-site RT.

Prostate cancer: dose escalation ≥78 Gy improves biochemical control.

Breast: hypofractionation (40-42.5 Gy in 15-16 fractions) is standard for many patients.

Lung: SBRT for early-stage inoperable NSCLC; concurrent chemoradiation for locally advanced.

Head and neck: concurrent cisplatin with RT for locally advanced SCC.

Rectal: neoadjuvant chemoradiation or short-course RT followed by surgery.

Cervical: concurrent cisplatin-based chemoradiation with brachytherapy boost.

Brain metastases: SRS for limited small metastases; whole-brain RT for multiple.

Palliative: 8 Gy × 1 is effective for bone metastases.

Photon attenuation: I = I0·e^(−μx).

HVL = 0.693/μ; TVL = 2.303/μ.

%DD depends on energy, field size, and SSD.

TAR/TPR are independent of SSD; TMR replaces TAR for isocentric high-energy techniques.

Mayneord factor converts %DD between SSDs.

Brachytherapy dose rate scales as 1/d² for a point source.

Electron therapeutic range R90 ≈ E/4 cm; practical range Rp ≈ E/2 cm.

Photon beam quality: %DD at 10 cm in water.

IMRT/VMAT QA: gamma analysis with 3%/3 mm criteria.

LQ model: SF = e^(−αD − βD²); α/β is the dose at which linear and quadratic contributions are equal.

Low α/β (~3 Gy) for late-responding tissues; high (~10 Gy) for early-responding tumors.

BED = n·d·(1 + d/(α/β)); EQD2 = BED/(1 + 2/(α/β)).

Four R's: repair, repopulation, redistribution, reoxygenation.

OER ≈ 2.5-3 for X-rays; ≈1 for high-LET.

RBE = dose of reference radiation / dose of test radiation producing the same effect.

D0 is the dose reducing survival to 37% on the exponential portion; n is the extrapolation number.

Radiosensitivity: M and G2 most sensitive; late S least sensitive.

DSBs are the principal lethal lesion; repaired by NHEJ and HR.

TCP and NTCP both rise with dose; therapeutic ratio maximized when TCP > NTCP.

Hodgkin lymphoma: mantle-field era replaced by involved-site/involved-node RT; combined modality with ABVD is standard for early-stage disease.

Prostate cancer: risk stratification uses PSA, Gleason score, and T stage; dose escalation to ≥78 Gy improves biochemical control in intermediate/high-risk disease.

Breast-conserving therapy: whole-breast RT after lumpectomy reduces local recurrence; hypofractionation (e.g., 40-42.5 Gy in 15-16 fractions) is standard for many patients.

Lung cancer: SBRT (e.g., 3-5 fractions of 10-20 Gy) is standard for medically inoperable early-stage NSCLC; concurrent chemoradiation for locally advanced disease.

Head and neck cancer: concurrent cisplatin with RT is standard for locally advanced squamous cell carcinoma; HPV-positive oropharyngeal cancer has better prognosis.

Rectal cancer: neoadjuvant chemoradiation (e.g., 50.4 Gy in 28 fractions with 5-FU) or short-course RT (25 Gy in 5 fractions) followed by surgery.

Cervical cancer: concurrent cisplatin-based chemoradiation with brachytherapy boost is standard; brachytherapy is essential for local control.

Brain metastases: whole-brain RT versus stereotactic radiosurgery; SRS preferred for limited number of small metastases.

Palliative RT: common schedules include 8 Gy × 1, 20 Gy × 5, and 30 Gy × 10; single-fraction 8 Gy is effective for painful bone metastases.