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In-Vivo Dosimetry


3.1

Introduction

In-vivo dosimetry is used clinically to verify during treatment how accurately the planned dose is delivered to the patient. While American Association of Physicists in Medicine (AAPM) TG-62, Diode In-Vivo Dosimetry for Patients Receiving External Beam Radiation Therapy, sees in-vivo dosimetry as supplementary to a good clinical QA program, more recent developments in understanding failure modes in treatment delivery and adaptive radiotherapy have led to increased attention to the safety and quality gains an in-vivo dosimetry program can generate with relatively low cost per patient.

In-vivo dosimeters can be roughly divided into three categories:

  • 1.

    Surface dosimeters (thermoluminescent dosimeter (TLD), optically stimulated luminescent dosimeter (OSLD), diode, metal-oxide-semiconductor field-effect transistor (MOSFET)) are placed on the patient surface to measure entrance beam dose in 2D and 3D conformal plans. They can also be used more generally to verify skin dose in anatomically critical areas (e.g., above scars or on the contra­lateral breast). Sometimes these dosimeters have also been used for intracavitary applications.

  • 2.

    Implantable dosimeters were designed to verify dose in situ, both for treatment targets and critical structures. Due to their size, radiographic properties, and the invasive process of placing them, the use of implantable dosimeters has been largely abandoned in favor of transmission dosimetry.

  • 3.

    Transmission dosimeters are fluence detectors that can be placed either immediately downstream of the tertiary collimator or distal to the beam exit from the patient.

In-vivo dosimetry serves four purposes:

  • 1.

    For patient-specific QA, in-vivo dosimetry is used to confirm the treatment delivery is within the tolerance specified in the policy and procedure (P&P). In-vivo dosimetry can be performed with a diode, an implantable dosimeter, or via a transmission dosimetry system such as an EPID (Electronic Portal Imaging Device). The results of the measurements should be reviewed on a regular basis, usually during weekly chart checks, unless the results are processed and user alerts generated through software analysis. For dosimeters with instant readout (diode, MOSFET) an expected range should be given to the therapists to alert them immediately to any unexpected dose.

  • 2.

    In addition to patient-specific QA, diodes double up as delivery QA (DQA) for 2D and 3D conformal treatments. Originally, they were intended to find gross errors such as missing or incorrect wedge, setup errors involving source-to-surface distance (SSD) or couch, and errors in manually entered treatment parameters. With the implementation of accessory interlocks, couch tolerance tables, and automated R&V systems using checksum algorithms to verify data transfer as recommended in the International Electrotechnical Commission (IEC) report Medical Electrical Equipment—Requirements for the Safety of Radiotherapy Treatment Planning Systems, the DQA function of diodes has become less safety critical over the past decade.

  • 3.

    For DQA in intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) fields, in-vivo dosimetry is performed using transmission dosimetry with devices downstream of the tertiary collimator. It can be understood as a continuation of the pretreatment DQA, which is customary in IMRT. As such, its use may become more frequent as adaptive planning and rapid dose calculations move into the clinic.

  • 4.

    In-vivo dosimetry with surface dosimeters is performed where TPS calculations and/or setup uncertainties cause large uncertainties in surface dose that is relevant for clinical outcome.

    TABLE 3.1
    Characteristics of In-Vivo Dosimeters
    Type Dosimeter Dose Energy Dependence Readout Time Price Best Achievable Uncertainty Handling
    Surface Dosimeter TLD 1D No (MV therapy range)
    Yes (HDR )     		1-24 hr $ 3% + No cable
    + Small size
    OSLD 1D No (MV photons and electrons)
    Yes (kV range, LET dependent for protons/carbon)     		Immediate $ 3%
    Diode 1D Yes Immediate $$ 2%
    MOSFET 1D Yes Immediate $$ 3%
    Implantable Dosimeter 1D Yes Immediate N/A 3%
    Transmission Dosimeter EPID 2D/3D Immediate Included in linac Not ready for clinical practice
    PMH technology 2D Immediate $$$ Under development
    Others 2D Immediate $$$

The European Society for Radiotherapy and Oncology (ESTRO) booklet on methods for in-vivo dosimetry provides a thorough description and references for the surface dosimeters described in Section 2 of this chapter and some detectors used for implantable dosimeters described in Section 4. The International Atomic Energy Agency (IAEA) Human Health Report No. 8, Development of Procedures for In Vivo Dosimetry in Radiotherapy, also gives detailed description of detector technology. It also describes the process of implementing in-vivo dosimetry and reports on results of a pilot program in six countries and four continents.

3.2

Clinical Process

System Implementation

The IAEA recommends a qualified medical physicist (QMP) to be the main party responsible for setting up and supervising an in-vivo dosimetry program. To establish a quality clinical process, input from physicians as to setting appropriate clinical action levels is required. In addition, therapy staff should be involved in evaluating the in-vivo dosimetry system for ease and practicality of use in daily clinical practice. The IAEA provides a sample flow chart for setting up the process, which could be adapted into a clinical procedure.

The commissioning process of any in-vivo dosimetry system should characterize the detector performance for the range of parameters encountered in clinical use. The calibration and mea­surement of correction factors are typically performed in solid water. For the verification process, measurements of standard beams incident on an anthropomorphic phantom should be followed by verification in clinically used patient plans on an anthropomorphic phantom. Table 3.2 provides a short summary of commissioning tests.

TABLE 3.2
List of Measurements Recommended for Clinically Commissioning an In-Vivo Dosimetry System
Calibration TLD: Individual or batch calibration factors
MOSFET/OSLD: Verification of manufacturer calibration within clinically used dose range		 2-4 hours
Correction Factors Dosimeter-specific correction factors (dose linearity, fading, energy response, dose rate)
Beam dependent correction factors (SSD, field size, compensators, angle of incidence)		 4-8 hours
Verification Performed in anthropomorphic phantom
1st step: use of standard beam (e.g., 6 MV 10 cm 2 field at 100 cm SSD)
2nd step: use of patient plans		 2-6 hours

Establishing Action Levels/Statistical Process Control

Tolerance levels and action levels need to be defined so that they are clinically as well as technically meaningful. The International Commission on Radiation Units and Measurements (ICRU) defined 5% deviation from the planned dose as an acceptable action limit, which was re-emphasized by AAPM TG-40, Comprehensive QA for Radiation Oncology. With this in mind, the IAEA set 5% as the action limit for their pilot study of in-vivo dosimetry. It is established clinical practice to define a tolerance level below the action level, which serves as a signal in the QA review process that critical parameters may be trending toward the action limit. This tolerance level is different for each clinic based on the available equipment and user expertise. The standard industry method to define a site-specific appropriate tolerance level is statistical process control. Sanghangthum et al. describe how to implement this method for medical physics applications. Further information about statistical process control can be found in Chapter 12.3 .

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