Oncology-Recent Advances in External Beam Radiotherapy (EBRT)

Oncology-Recent Advances in External Beam Radiotherapy (EBRT)
I. Improved Accuracy and Precision
A. Volume Definition:

Past: Planning scans relied on diagnostic scanners, resulting in less precise tumor volume definition.
Present: High-resolution CT simulators are standard. 3D tumor volume definition is now accurate even for palliative treatments. Fusion of CT with MRI and PET scans further refines this for radical treatments.

B. Image-Guided Radiotherapy (IGRT):

Past: A 1cm margin was added to the Clinical Target Volume (CTV) to account for positioning errors. Megavoltage X-rays provided low-quality confirmation images.
Present:
- Electronic Portal Imaging Devices (EPIDs): Replaced port films, offering improved image quality. Online imaging corrects misalignments before treatment, while offline review identifies recurring setup errors.
- Intra-treatment Imaging: Precise tumor location is confirmed during treatment using:
  - Fiducial markers (e.g., gold seeds)
  - Ultrasound
  - Cone beam CT (mounted on the linear accelerator) for 3D online imaging.

II. Minimizing Radiation Exposure to Organs at Risk & Higher Tumor Doses
A. Inverse Planned Intensity-Modulated Radiotherapy (IMRT):

Mechanism: Computer optimization varies radiation beam intensity to meet dose constraints for healthy organs while delivering prescribed doses to the tumor (CTV and Gross Tumor Volume - GTV). This is achieved through:
- Step and shoot: Superimposing static uniform intensity segments.
- Dynamic MLC delivery: A shaped sliding window across the field.
- Tomotherapy: Multiple arcs with intensity modulated by dynamic MLCs.
Benefits: Reduced late effects on normal tissues with equivalent cancer control compared to conventional radiotherapy, particularly in prostate and head/neck cancers. Promising results in lung and gynecological cancers suggest it may replace 3D conformal radiotherapy (CRT) for many radical treatments.
Drawback: Significantly increases the workload of the physics team due to the extensive time (at least 2 hours per patient) required for meticulous volume definition.

B. Four-Dimensional (4D) Therapy Planning and Delivery:

Addresses the challenge of: Treating mobile structures (e.g., lung cancers moving with respiration).
Method: Links 4D CT planning (tumor volume defined at each respiratory phase) to treatment delivery during the expiratory phase for optimal coverage with minimal field size.

C. Stereotactic Radiotherapy:

Established for: Intracranial conditions (benign tumors, arteriovenous malformations).
Method: High precision (1-2mm) treatment of small lesions (<1cm3) using an external 3d coordinate system and stereotactic fixation. typically uses 1-3 large fractions (12-20 gy).< />pan>
Recent expansion: Successful treatment of small malignancies in the brain, lung, and liver. IGRT facilitates accuracy in sites unsuitable for localization frames.

III. Key Terms & Concepts to Remember:

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