HPTC RT Exam cheat sheet

ALARA: keep occupational dose as low as reasonably achievable, social and economic factors considered; all three radiation protection principles (time, distance, shielding) apply to every exposure.

Inverse square law: intensity is proportional to 1/d²; doubling distance from a point source cuts exposure rate to one quarter.

Barrier transmission follows I = I₀·e^(−μx); half-value layer (HVL) is the thickness reducing intensity by 50%, and 3.32 HVL reduces it to about 10% (one-tenth value layer).

Annual occupational effective dose limit is 50 mSv (5 rem); lens dose 150 mSv (15 rem); skin/extremity 500 mSv (50 rem); declared pregnant worker limit 5 mSv (0.5 rem) for the gestation period.

Public/general area limit is 1 mSv (100 mrem) per year; controlled area 5 mSv/yr; dose equivalent in unrestricted areas must stay below 0.02 mSv (2 mrem) in any one hour.

Dose = exposure × time; use pulsed fluoroscopy, last-image hold, and collimation to cut scatter; scatter at 1 m from a patient is roughly 0.1% of the primary beam intensity.

TLD/OSL badges are worn at collar level outside lead apron (or as two-badge system: one under apron at waist, one at collar) and read monthly; extremity dosimeters for high-hand-dose procedures.

Radioactive decay: A = A₀·e^(−λt), λ = ln2/T½; handle sealed sources with tongs, store in shielded containers, and survey with a calibrated meter before and after use.

Emergency rule: time, distance, shielding, and contamination control - remove patient, decontaminate before transport, and notify the radiation safety officer per institutional protocol.

CT simulation: acquire with the immobilization device in place, isocenter marked on the patient, and slice thickness ≤3 mm for most definitive plans; use the same setup as treatment.

Gross tumor volume (GTV) → clinical target volume (CTV) → planning target volume (PTV): PTV adds setup and organ-motion margin to CTV; organs at risk are contoured separately.

Dose-volume constraints: PTV coverage typically V95 ≥ 95% and D95 near prescription; OAR constraints follow published tables (e.g., spinal cord Dmax < 45–50 Gy in conventional fractionation).

Plan evaluation metrics: homogeneity index = (D2−D98)/D50 or (Dmax−Dmin)/Dmean; conformity index = prescription isodose volume / PTV volume.

Beam arrangements: coplanar vs non-coplanar; gantry, collimator, and couch angles determine entry/exit paths; collimator rotation shapes the field relative to patient axis.

Electron planning: choose energy by depth of 90% isodose (≈ E/3 to E/3.5 cm); use bolus to bring dose to surface for skin lesions.

Brachytherapy planning: dose specification per ICRU/ABS recommendations; source strength in air-kerma strength (U = µGy·m²/h), and dose rate depends on inverse square plus attenuation.

Simulation films/DRRs must match treatment geometry; verify isocenter with fiducials and record couch height, angle, and patient orientation (HFS, HFP, supine/prone).

Dose calculation algorithms differ (pencil beam vs collapsed cone vs Monte Carlo); check heterogeneity corrections in lung and bone, where simple algorithms over- or under-estimate dose.

Daily setup: verify patient identity, prescription, and treatment site; reproduce immobilization, laser alignment, and isocenter marks before beam-on.

SSD setup: source-to-skin distance fixed (typically 80–100 cm); SAD setup: source-to-axis distance fixed (100 cm) and gantry rotates around isocenter.

Field verification: check gantry, collimator, couch angles, field size, and accessory tray (wedge, block, bolus) against the plan before each field.

Wedge factors: dynamic/physical wedge reduces output; apply wedge factor to MU calculation; verify orientation and direction against plan.

IMRT/VMAT: verify fluence, MLC leaf positions, and dose rate; patient-specific QA (gamma passing ≥ 90–95% at 3%/3 mm, 10% threshold) precedes first treatment.

SRS/SBRT: use stereotactic localization, immobilization (frame or mask), and verify with CBCT or orthogonal kV imaging; intrafraction motion monitored per protocol.

Monitor unit check: independent second calculation or measurement; MU = prescribed dose / (dose rate × output factor × depth factor × Sc × Sp × wedge × tray).

Emergency during treatment: pause beam, remove patient, assess; keep a written procedure for source stuck (brachytherapy) and linear accelerator interlocks.

Setup errors: random vs systematic; systematic errors corrected via offline review, random errors reduced with imaging and immobilization.

Acute radiation effects by week: fatigue and skin erythema (weeks 2–4), mucositis (week 2+), esophagitis, diarrhea, and cytopenias; grade with CTCAE criteria.

Skin care: wash with mild soap and water, pat dry, no deodorant/deodorant-free per institutional policy, avoid sun and friction; moist desquamation may need dressings.

Radiation dermatitis grading: grade 1 faint erythema, grade 2 moderate to brisk erythema, grade 3 moist desquamation, grade 4 life-threatening skin necrosis.

Myelosuppression: monitor CBC; neutropenic precautions when ANC < 500/µL; thrombocytopenia precautions when platelets < 50,000/µL.

Nutrition: assess weight weekly; head-and-neck patients need calorie/protein support and swallowing exercises to prevent stenosis.

Patient education: explain what to expect, when to call (fever ≥ 38°C, uncontrolled pain, new neurologic symptoms), and the importance of skin marking preservation.

Late effects: fibrosis, xerostomia, radiation pneumonitis (1–6 months), radiation myelopathy (dose-dependent), and second malignancy risk; document baseline and follow-up.

Pregnancy and fertility: screen for pregnancy before pelvic/abdominal RT; discuss fertility preservation and gonadal shielding where feasible.

Fractionation: conventional 1.8–2 Gy/day, 5 days/week; radiobiology 4 R's - repair, repopulation, redistribution, reoxygenation.

Linear-quadratic model: surviving fraction = e^(−αD−βD²); α/β ratio ≈ 10 Gy for tumors and early effects, ≈ 3 Gy for late effects.

BED = nd[1 + d/(α/β)]; EQD2 = BED/[1 + 2/(α/β)]; useful to compare fractionation schemes.

Tumor control probability rises with dose; normal tissue complication probability rises with dose and volume; therapeutic ratio compares the two.

Cell cycle: M phase most radiosensitive, late S phase most resistant; oxygen enhancement ratio ≈ 2.5–3 for photons.

Brachytherapy dose rate: LDR < 2 Gy/h, MDR 2–12 Gy/h, HDR > 12 Gy/h; HDR commonly 10–12 Gy/h with Ir-192.

Staging and treatment intent: definitive, adjuvant, neoadjuvant, palliative; palliative doses (e.g., 8 Gy × 1 or 30 Gy/10) relieve symptoms with less toxicity.

Combined modality: concurrent chemoradiation improves local control in many sites but increases mucositis, dermatitis, and hematologic toxicity.

Daily QA: output constancy, laser alignment, door interlock, audiovisual monitoring, and beam-on indicator; record results before first patient.

Monthly QA: output calibration, energy constancy, flatness/symmetry, light-radiation field coincidence, and mechanical checks (gantry, collimator, couch).

Annual QA: full calibration per TG-51 or TRS-398, MLC positional accuracy, imaging system performance, and dosimetry audit.

TG-51: absorbed dose to water calibration with ion chamber; N_D,w and k_Q factors; measure at 10 cm depth in water at reference conditions.

CT simulator QA: CT number accuracy, laser alignment, slice thickness, and image quality monthly; annual full calibration.

Brachytherapy source QA: source strength assay, well-chamber constancy check, and source exchange verification; Ir-192 half-life 73.8 days.

Linear accelerator safety: interlocks, beam-off on door open, collision avoidance, and emergency stop; test interlocks monthly and after service.

MLC QA: leaf position accuracy (typically ±1 mm), leaf speed, and interleaf leakage; test with film or EPID per TG-142.

Ion chamber calibration: N_D,w from ADCL, corrected for temperature and pressure (P_tp = (273.15+T)/295.15 × 101.33/P), and polarity/ion recombination.

Point dose verification: measure with ion chamber in phantom at the planned isocenter; agreement typically within 3% for conventional plans.

Gamma analysis: 3%/3 mm, 10% low-dose threshold; ≥95% passing is common for IMRT/VMAT patient-specific QA; report global vs local normalization.

In-vivo dosimetry: diodes or TLDs on skin, entrance dose check within ±5%; useful for TBI, TSEB, and complex setups.

Image guidance: kV orthogonal, CBCT, or MV portal imaging; verify setup to ≤3 mm for most sites, ≤1–2 mm for SRS/SBRT.

Dose distribution verification: film (radiochromic), 2D arrays, or EPID; calibrate film with a known dose series and scan at fixed time post-exposure.

Independent MU calculation: second algorithm or hand calculation; flag discrepancies >5% for review before treatment.

Record and verify: treatment management system checks field parameters and accumulated dose; document all overrides and corrections.

Scope of practice: perform only tasks within your certification and institutional policy; escalate tasks outside scope to a qualified professional.

Informed consent: verify the patient understands the procedure, risks, alternatives, and has signed consent before simulation or treatment.

Patient privacy: HIPAA/privacy rules apply to images, treatment records, and conversations; share only with the care team on a need-to-know basis.

Error disclosure: report errors and near-misses through the institutional incident system; disclose to the patient per policy and supervisor guidance.

Documentation: record setup, dose, and interventions accurately and contemporaneously; never alter records retroactively without an addendum.

Conflict of interest and gifts: decline gifts that could influence clinical decisions; follow institutional policy on vendor relationships.

Cultural competence: respect language, religion, and modesty preferences; use qualified interpreters rather than family members for consent.

Continuing education: maintain certification through approved CE credits and stay current with practice standards and safety alerts.