Dosimeter calibration for radiotherapy is a critical quality requirement in medical physics. In Portugal, radiotherapy services in public and private hospitals are required to maintain metrological traceability chains for all dose measurement instruments — ensuring that every radiation field delivered to the patient is accurate, reproducible and safe.
This guide is intended for medical physicists, hospital radiophysicists and radiotherapy technicians who need to understand regulatory requirements, available instrument types and reference equipment accessible in the national market.
Why calibrate dosimeters in medical physics?
Radiation dosimetry in radiotherapy has direct implications for patient safety. A systematic error of only 5% in delivered dose can compromise tumour control or increase toxicity risk to adjacent healthy tissues. For this reason, international standards such as IAEA TRS-398 and the guidelines of the Directorate-General of Health (DGS) require that:
- Reference ionization chambers be calibrated at an accredited primary or secondary laboratory;
- Calibration be renewed at defined intervals (typically every 2 years);
- Calibration factors be determined at beam quality relevant to clinical use (60Co, high-energy X-rays, proton beams);
- A documentary quality assurance system records all measurements and deviations.
Without a traceable calibration chain, any dosimetric measurement loses metrological validity — and, consequently, clinical and regulatory value.
Types of dosimeters used in radiotherapy
The choice of dosimeter depends on the clinical application, treatment modality and required level of accuracy. In modern radiotherapy, the main types are:
Ionization chambers
These are the primary reference instruments for absolute dosimetry in external beam radiotherapy (EBRT). They measure electric charge produced by ionization of the gas inside the chamber when exposed to radiation. Farmer-type chambers (cylindrical, volume ~0.6 cm³) are the standard for high-energy photon and electron beams, while parallel-plate chambers are preferred for low energies and electron beams.
Thermoluminescent dosimeters (TLD)
Based on materials such as LiF:Mg,Ti (TLD-100) or LiF:Mg,Cu,P (TLD-100H), TLDs are widely used in in vivo dosimetry and postal dose audits. They enable measurements in small volumes and body cavities, being essential for independent dose verification in brachytherapy and intensity-modulated radiotherapy (IMRT).
MOSFET dosimeters
MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) dosimeters offer real-time readout and extremely small size, making them suitable for in vivo surface dosimetry and in hard-to-reach cavities. Their sensitivity and dose-rate response must be verified through prior calibration.
Diode dosimeters
Silicon diodes are solid-state sensors with high sensitivity and excellent spatial resolution. They are frequently integrated into two-dimensional arrays (such as the MapCHECK®3 or ArcCHECK®) for verification of dose distributions in IMRT/VMAT. They require energy- and dose-rate-dependent correction factors.
Radiochromic films (EBT)
EBT3/EBT-XD radiochromic films enable 2D dosimetry with high spatial resolution. They are used in stereotactic radiotherapy field verification (SRS/SBRT), dose audits and treatment planning system commissioning.
Calibration standards: IAEA TRS-398 and Portuguese DGS
Dosimeter calibration for radiotherapy in Portugal falls within a hierarchical normative system:
IAEA TRS-398 — Absorbed Dose Dosimetry Protocol
The IAEA Technical Report Series No. 398 (2000, updated) is the international reference protocol for external beam radiotherapy dosimetry. It establishes the formalism for determining absorbed dose to water from charge readings in a calibration ionization chamber in terms of ND,w (absorbed dose to water calibration factor). It applies to photon beams (60Co and linear accelerators), electrons, protons and heavy ions.
National regulations (DGS)
The Directorate-General of Health establishes minimum requirements for clinical dosimetry, including mandatory calibration traceable to the International System of Units (SI). The IST Radiation Standards Laboratory (IST-LPSR) is the secondary reference laboratory in Portugal for ionization chamber calibration.
AAPM TG-51 and IPEM Report 103
For medical physicists familiar with the American (AAPM TG-51) or British (IPEM Report 103) protocols, these are functional equivalents to TRS-398 and are accepted in international collaboration contexts and external audits.
Reference facilities available in Portugal
Metrological traceability for radiotherapy ionization chambers in Portugal can be ensured through:
- IST-LPSR (Instituto Superior Técnico — Radiation Standards Laboratory): accredited secondary laboratory for calibration in 60Co and X-ray beams;
- PTB (Germany), NPL (UK), BIPM: internationally accepted primary laboratories for high-level reference chambers;
- IAEA SSDL network: secondary standards dosimetry laboratories supporting member countries including Portugal;
- ESTRO/IAEA audits: independent dose verification programmes (postal TLD/OSLD) for hospitals performing radiotherapy.
Ionization chambers, electrometers and dosimetry systems — available products
Interphysix provides a selection of instruments and dosimetry systems for medical physics in radiotherapy, suitable for reference dosimetry, quality control and dose verification:
- PC Electrometer™ — high-precision electrometer for charge and current measurement, compatible with Farmer chambers and parallel-plate chambers. Essential for TRS-398 reference dosimetry on linear accelerators.
- MapCHECK®3 — 2D diode array for real-time IMRT/VMAT field verification with gamma analysis.
- ArcCHECK® — cylindrical phantom with 1386-diode array for VMAT/RapidArc arc verification.
- SRS MapCHECK® — high-resolution solution for stereotactic radiotherapy (SRS/SBRT) field verification.
- SNC600c™ — Farmer-type cylindrical ionization chamber for high-energy photon beam reference dosimetry.
- SNC350p™ — parallel-plate ionization chamber for electron and low-energy photon beam dosimetry.
- SNC125c™ — compact small-volume ionization chamber for small-field dosimetry (SRS/SBRT).
- SunSCAN™ 3D — 3D water tank scanning system for linear accelerator commissioning and beam data acquisition.
Interphysix — specialist supplier for medical physics
Interphysix is a specialist supplier of equipment for medical physics, radiation dosimetry and quality control in radiotherapy, nuclear medicine and diagnostic radiology. With presence in Portugal and Spain, Interphysix provides specialist technical support, training and access to the most advanced systems available for medical physicists and hospital radiophysicists.
For more information on dosimeter calibration for radiotherapy, technical assistance or to request a demonstration of available equipment, contact our specialist team.
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Última actualização: 23/02/2026