Dosimetry of Small Static Fields Used in External Beam Radiotherapy

Dosimetry of Small Static Fields Used in External Beam Radiotherapy


External beam radiotherapy is a common treatment modality for cancer patients. It involves the use of high-energy radiation beams to target and destroy cancer cells. The accuracy and precision of dose delivery are crucial in ensuring effective treatment outcomes. One aspect of radiotherapy dosimetry that requires special attention is the measurement and determination of dose in small static fields.

Importance of Dosimetry in Small Static Fields

Small static fields are commonly used in radiotherapy for various treatment techniques, such as stereotactic radiosurgery and intensity-modulated radiation therapy. These fields have dimensions smaller than the conventional treatment fields and require accurate dosimetry to ensure proper dose delivery to the target volume while minimizing dose to surrounding healthy tissues.

International Code of Practice

The International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) have jointly developed an international code of practice for the reference and relative dose determination in small static fields. This code provides guidelines and recommendations for the measurement and calculation of dose in these fields, aiming to standardize dosimetry procedures and ensure consistency in dose reporting.

Key Guidelines and Recommendations

  1. Measurement Techniques: The code outlines various measurement techniques, including ionization chambers, diode detectors, and film dosimetry, for accurate dose determination in small static fields.
  2. Correction Factors: It provides correction factors for different detectors and measurement conditions to account for the perturbation effects caused by the small field size.
  3. Uncertainty Analysis: The code emphasizes the importance of uncertainty analysis in dose measurements, considering factors such as detector response, positioning accuracy, and beam quality.
Frequently Asked Questions
  1. Why is accurate dosimetry important in small static fields?
  2. Accurate dosimetry ensures that the prescribed dose is delivered to the target volume, minimizing the risk of underdosing or overdosing the tumor.

  3. What are the challenges in measuring dose in small static fields?
  4. Small fields exhibit significant dose perturbations due to the presence of high-density materials, such as collimators and applicators, which can affect the accuracy of dose measurements.

  5. How does the international code of practice help in improving dosimetry?
  6. The code provides standardized guidelines and recommendations for dose measurement and calculation in small static fields, promoting consistency and accuracy in dose reporting across different radiotherapy centers.


The dosimetry of small static fields used in external beam radiotherapy plays a crucial role in ensuring accurate and precise dose delivery. The international code of practice developed by the IAEA and AAPM provides guidelines and recommendations for reference and relative dose determination in these fields, aiming to improve the quality and consistency of radiotherapy treatments worldwide.