Radiation Therapy Planning and Delivery

Radiation therapy (RT) aims to deliver a lethal dose of radiation to cancer (tumor) while minimizing dose to healthy tissue (normal tissue tolerance). Ionizing radiation damages DNA: high-energy photons knock electrons off atoms, creating reactive free radicals that break DNA strands. Cancer cells are more radiosensitive than normal cells because they lack efficient DNA repair mechanisms, are rapidly dividing (dividing cells are more vulnerable to radiation), and often have defective p53 tumor suppressor (which normally triggers apoptosis in damaged cells). A single photon can kill a cell, but more often, sublethal damage accumulates: cells survive 1-2 fractions but die after 5-30 fractions. Fractionation (dividing total dose into smaller daily fractions) allows normal tissue to repair between fractions while cancer cells accumulate damage. Typical breast cancer: 50 Gy in 25 fractions = 2 Gy per fraction. Head and neck cancer: 70 Gy in 35 fractions = 2 Gy per day. Prostate cancer low-dose: 64-81 Gy in 8-45 fractions depending on risk. Stereotactic radiosurgery: 1-5 large fractions (12-20 Gy) for small lesions (brain tumors, lung lesions) where the steep dose gradient allows ablation. Goal is to eradicate cancer while keeping late toxicity (years later complications) <5%.

3D conformal RT (3DCRT) shapes the beam using a multi-leaf collimator (hundreds of tungsten leaves that move in/out to conform to tumor shape) in several static beam angles. Dose distribution is calculated and verified. 2-5 beam angles typical. Advantage: better conformality than 2D RT (old box/strip technique). Limitation: dose distribution is fixed; if tumor and normal tissue overlap in the beam path, you cannot modulate dose within the beam. Intensity-modulated RT (IMRT) uses many beam angles (7-9 typical) with dynamic leaf motion (leaves move while radiation is on) to modulate dose intensity within each beam. Result: highly conformal dose distribution that hugs tumor contours and avoids normal tissue. Advantage: superior target coverage and normal tissue sparing, especially for complex tumors (head/neck, prostate) or tumors near organs at risk. Disadvantage: longer planning time, more calculations, more monitor units (higher whole-body low-dose radiation), and requires robust QA. IMRT is now standard for most cancers; 3DCRT is used for simpler cases (e.g., palliative spine RT). Volumetric modulated arc therapy (VMAT) is an advanced form of IMRT where the gantry rotates continuously while leaves move and dose rate changes, further optimizing dose distribution and reducing treatment time.

CT simulation is the process of acquiring a diagnostic CT scan of the patient in the treatment position (same position as actual treatment) to delineate the tumor and normal tissues and plan the radiation dose. Patient lies on a flat CT couch (not a diagnostic couch) immobilized in customized devices (thermoplastic mask for head/neck, vacuum bag for torso, knee sponge for leg). CT is acquired with thin slices (3-5 mm). Fiducial markers (tattoos or external marks) are placed to match daily setup. The CT DICOM images are transferred to the treatment planning system (TPS). Radiation oncologist contours the target volume (GTV = gross tumor volume, CTV = clinical target volume including microscopic disease, PTV = planning target volume accounting for setup uncertainty and organ motion). Normal tissue structures (organs at risk) are contoured to limit dose: heart, lungs, spinal cord, bowel, etc. Critical step: accurate contour delineation; errors here propagate through planning. For tumors moving with respiration (lung, liver), 4D CT (time-resolved) may be acquired to capture motion; margins are expanded to account for this. Quality of simulation directly impacts quality of plan; poor contours = poor plan.

Dose calculation is the mathematical prediction of how much radiation dose each voxel (3D pixel) in the patient will receive given the beam parameters (energy, fluence, field size) and patient anatomy. Algorithms: (1) Pencil Beam (fast, less accurate in inhomogeneous regions like lung), (2) Convolution/Superposition (accounts for dose in low-density regions, more accurate), (3) Monte Carlo (stochastic simulation, most accurate, slow). TPS calculates dose distribution. Output: isodose lines (contours of equal dose) overlaid on CT images showing coverage of target and dose to organs at risk. Plan evaluation: Is the target (PTV) covered (95-107% of prescribed dose)? Are dose constraints to organs at risk met (e.g., heart mean dose <5 Gy for breast cancer)? Verification requires: (1) point dose verification (calculate expected dose at a point and measure with ion chamber), (2) 2D dose verification (expose film or electronic portal imaging to 2D dose and compare calculated to measured), (3) 3D dose verification (use phantom with gel or dosimeter array). If calculation and measurement differ >3%, investigate (patient positioning error, ion chamber wrong setup, algorithm limitation). Acceptance criteria vary by site; most use 3%/3mm gamma analysis (95% of voxels within 3% dose or 3 mm distance). Only after passing QA does patient begin treatment.

A margin is the expansion of the clinical target volume (CTV, containing the tumor and microscopic disease) to the planning target volume (PTV, the volume to be irradiated) to account for uncertainties: (1) setup uncertainty (patient positioned slightly differently each day despite immobilization, typically 3-5 mm), (2) organ motion (tumor moves with breathing or patient position changes, 5-10 mm in lung, 0-5 mm in prostate), (3) delineation uncertainty (physician draws contour slightly differently each session, 3-5 mm). Typical PTV margins: 5-10 mm for head/neck (well-immobilized, small motion), 10-15 mm for thorax (breathing), 5-10 mm for prostate (moderate motion). Adaptive RT can reduce margins by replanning mid-course based on actual daily anatomy (cone beam CT). Margin philosophy balances target underdosage (miss tumor cells) vs. normal tissue overdosage (toxicity). If margins are too small, tumor recurs at the edge (marginal miss). If margins are too large, normal tissue gets unnecessary dose. Modern imaging (CBCT daily, MR-guided RT) is reducing margins by improving knowledge of daily tumor position.

Acute toxicities occur during treatment or within 3 months: mucositis (mouth sores), skin erythema/desquamation (burns), diarrhea (from bowel irradiation), esophagitis (chest discomfort/difficulty swallowing). Severity graded 1-5 (1 = mild, 5 = fatal). Management: supportive care (mouth rinse, topical creams, diet), and dose modification if severe. Late toxicities occur >3 months after treatment: fibrosis (tissue stiffening), telangiectasia (dilated blood vessels, cosmetic), secondary malignancy (new cancer in radiation field, years later), organ dysfunction (heart failure from cardiac RT, pulmonary fibrosis from lung RT). Late toxicity is dose-volume dependent: if mean dose to heart is <5 Gy, cardiac toxicity risk is low; >20 Gy risk rises. Tolerance doses (max dose organs can receive with acceptable risk) guide planning: spinal cord 50 Gy, brain stem 54 Gy, optic nerve 54 Gy. Fractionation matters: same total dose in fewer fractions = higher late toxicity (hypofractionation, e.g., 40 Gy in 5 fractions to lung lesion, has high late toxicity risk). Goal: maximize tumor control (cure) while keeping late toxicity risk acceptable (~5%). Patient age, comorbidities, and life expectancy factor into tolerance decisions.

Image guidance is daily imaging performed before or during RT to verify patient positioning and tumor location, reducing setup error. Cone beam CT (CBCT) is a low-dose CT acquired via the linac's imaging system (orthogonal kV X-rays or MV X-rays) before treatment. Patient lies on treatment couch; CBCT gantry rotates, acquiring a 3D image. CBCT is registered (matched) to the planning CT using anatomy landmarks (bony anatomy for head/neck, soft-tissue for prostate). If registration shows patient is off by >3-5 mm, shift is applied (couch moves, or patient repositioned). Advantages: daily verification reduces margin need, detects setup errors immediately, allows adaptive replanning if anatomy changes. Disadvantage: additional radiation dose (low-dose, but cumulative over 30+ fractions), longer treatment time. For motion-prone tumors (lung, liver), 4D CBCT or fluoroscopy shows motion, enabling gating (beam on only when tumor in correct position) or abdominal compression. Image guidance is now standard of care; improved accuracy is why modern RT outcomes are excellent.

Radiation Therapist (RTT): 2-4 year program, ARRT certification. Positions patient, operates linac, immobilizes patient, acquires daily imaging, documents treatment. Salary ~$65-75k; advancement to lead RTT, QA specialist, or educator. Medical Dosimetrist: 2-year program (some states don't require license). Assists with treatment planning, dose calculations, verifies plans. Salary ~$60-70k. Radiation Oncologist: MD/DO (4 years) + 5-year residency in radiation oncology, board certification (ABRO). Diagnoses cancer, designs treatment plans, manages side effects, conducts research. Salary $300-400k. Medical Physicist: MS/PhD in physics (2-3 years post-bachelor), board certification (ABMP). Ensures dose accuracy, QA of equipment and plans, develops protocols, conducts research. Salary $120-160k. Physicist is gateway to leadership: many radiation oncology departments are directed by physicists. All roles require continuing education and involvement in professional societies (ASTRO = American Society for Radiation Oncology, ASRT, AAPM = American Association of Physicists in Medicine). Demand is steady; aging population + cancer incidence drives demand for RT.