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Pathology-External Beam Radiotherapy (EBRT)
I. Types of EBRT
A. Superficial X-ray Therapy (80-300 kV): Used for superficial tumors (skin, ribs). Low-energy X-rays are generated by accelerating electrons in an X-ray tube, which then strike a tungsten anode causing bremsstrahlung radiation. Beam size is controlled by metal applicators.
B. Cobalt Teletherapy (Co-60): Uses gamma rays (1.25 MeV average energy) from Co-60 source for deeper tumors. Requires high source strength (around 350 TBq) for adequate dose rate.
C. Megavoltage Radiotherapy (4-20 MV): Most common method, using linear accelerators to produce high-energy X-rays. Offers advantages over Co-60: * Higher penetration * Higher dose rate * Better collimation (precise beam targeting)
D. Electron Therapy (4-20 MeV): Linear accelerators can also generate electron beams. These are suitable for superficial tumors, providing uniform treatment to a specific depth with a rapid dose fall-off beyond that depth. Depth of penetration depends on electron energy (e.g., 6 MeV ≈ 1.5 cm, 20 MeV ≈ 5.5 cm). Offers an alternative to kilovoltage X-rays for superficial tumors.
Limitations of Low-Energy X-ray Beams:
Step 1: Beam Dosimetry: Measuring the dose distribution pattern of each linear accelerator using an ionization chamber in a water tank. Calibration factors (output factors) are determined to calculate irradiation time for a specified dose.
Step 2: Planning Computer: Computer software uses measured beam data and algorithms to account for tissue density variations (often from CT scans) in dose calculations. Simpler planning can be done using tables or plots.
Step 3: Target Drawing: Defining the target volume to be irradiated. This includes: * GTV (Gross Tumor Volume): Visible tumor on imaging or clinical examination. * CTV (Clinical Target Volume): GTV plus surrounding tissue with potential microscopic tumor cells. * PTV (Planning Target Volume): CTV plus margins to account for setup uncertainties (patient positioning, organ movement, machine calibration). Critical organs (spinal cord, eyes, kidneys) are also defined.
Step 4: Dose Planning: Designing a treatment plan to uniformly irradiate the target while keeping critical organ doses within tolerance. Adjustable parameters include: * Patient position * Beam size & shape * Beam direction * Number of beams * Relative dose per beam (beam weight) * Wedging * Compensators
Step 5: Treatment Verification: Ensuring correct beam positioning and avoiding critical organ over-irradiation. Methods include: * Radiographs on a simulator. * Megavoltage radiographs or EPIDs (Electronic Portal Imaging Devices) during treatment. * In vivo dosimetry (thermoluminescence dosimeters) – the gold standard for radical treatments.
Step 6: Treatment Prescription and Delivery: The oncologist prescribes the dose, fractionation schedule, and beam configuration. This information is entered into the linear accelerator's computer system to control treatment delivery.
I. Types of EBRT
A. Superficial X-ray Therapy (80-300 kV): Used for superficial tumors (skin, ribs). Low-energy X-rays are generated by accelerating electrons in an X-ray tube, which then strike a tungsten anode causing bremsstrahlung radiation. Beam size is controlled by metal applicators.
B. Cobalt Teletherapy (Co-60): Uses gamma rays (1.25 MeV average energy) from Co-60 source for deeper tumors. Requires high source strength (around 350 TBq) for adequate dose rate.
C. Megavoltage Radiotherapy (4-20 MV): Most common method, using linear accelerators to produce high-energy X-rays. Offers advantages over Co-60: * Higher penetration * Higher dose rate * Better collimation (precise beam targeting)
D. Electron Therapy (4-20 MeV): Linear accelerators can also generate electron beams. These are suitable for superficial tumors, providing uniform treatment to a specific depth with a rapid dose fall-off beyond that depth. Depth of penetration depends on electron energy (e.g., 6 MeV ≈ 1.5 cm, 20 MeV ≈ 5.5 cm). Offers an alternative to kilovoltage X-rays for superficial tumors.
Limitations of Low-Energy X-ray Beams:
- Unsuitable for deep-seated malignancies (thorax, abdomen, pelvis).
- High skin dose.
- Rapid dose fall-off with depth.
- Higher bone dose compared to soft tissue.
- High dose delivered at depth.
- Maximum dose below the skin surface (skin-sparing effect).
- Exponential dose fall-off with depth.
- Sharp dose fall-off at beam edge (penumbra).
- Beam shape modification via metal blocks or multileaf collimators.
- Dose gradient creation using metal filters or wedges.
- Treatment from any direction is possible.
- Crossfire technique (2-4 beams) enhances target dose while sparing normal tissues.
Step 1: Beam Dosimetry: Measuring the dose distribution pattern of each linear accelerator using an ionization chamber in a water tank. Calibration factors (output factors) are determined to calculate irradiation time for a specified dose.
Step 2: Planning Computer: Computer software uses measured beam data and algorithms to account for tissue density variations (often from CT scans) in dose calculations. Simpler planning can be done using tables or plots.
Step 3: Target Drawing: Defining the target volume to be irradiated. This includes: * GTV (Gross Tumor Volume): Visible tumor on imaging or clinical examination. * CTV (Clinical Target Volume): GTV plus surrounding tissue with potential microscopic tumor cells. * PTV (Planning Target Volume): CTV plus margins to account for setup uncertainties (patient positioning, organ movement, machine calibration). Critical organs (spinal cord, eyes, kidneys) are also defined.
Step 4: Dose Planning: Designing a treatment plan to uniformly irradiate the target while keeping critical organ doses within tolerance. Adjustable parameters include: * Patient position * Beam size & shape * Beam direction * Number of beams * Relative dose per beam (beam weight) * Wedging * Compensators
Step 5: Treatment Verification: Ensuring correct beam positioning and avoiding critical organ over-irradiation. Methods include: * Radiographs on a simulator. * Megavoltage radiographs or EPIDs (Electronic Portal Imaging Devices) during treatment. * In vivo dosimetry (thermoluminescence dosimeters) – the gold standard for radical treatments.
Step 6: Treatment Prescription and Delivery: The oncologist prescribes the dose, fractionation schedule, and beam configuration. This information is entered into the linear accelerator's computer system to control treatment delivery.
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