Relative Dosimetry for MV Beams

(1)

Accurate Technical Functionality

Relative dosimetry is very sensitive to technical changes in the accelerator itself. For example, changes in the steering coils or ion chamber can cause the beam symmetry to deviate above the limits specified by acceptance testing and American Association of Physicists in Medicine (AAPM) recommendations. Relative dosimetry is used during installation and acceptance testing to fine-tune accelerator technical parameters. Subsequently, relative dosimetry measurements are used as part of periodic accelerator QA to verify accurate and consistent functioning of the accelerator. AAPM Task Groups (TG)-40 and TG-142 provide detailed tables of the relative dosimetry measurements, frequencies, and tolerances recommended for clinical practice in standard linear accelerators. AAPM TG-135 and TG-148 provide additional information for CyberKnife and TomoTherapy, respectively.

(2)

Characterizing Beams for TPS

Measurements of relative dosimetry data during commissioning are used as input in TPS and secondary monitor unit (MU) calculation. It is therefore essential for the data to be of high quality, because they provide the basis for treating thousands of patients during the lifetime of the treatment delivery machine . The relative dosimetry data set taken during annual QA serves two purposes. First, it is a more stringent check on accelerator technical performance than the monthly or daily relative dosimetry measurements. Second, the annual dosimetry data are used to confirm that the machine is still accurately characterized by the data set used in the TPS. Therefore, it is important to use the baseline TPS data after commissioning as the basis for comparison. For example, if a measured value was originally 2% different from the TPS value and changed to 3.5% different in subsequent years, a comparison to measurement baseline would show only a 1.5% difference, which occludes the actual mismatch between TPS and beam data.

Vendors of TPS generally do provide a list of required relative dosimetry measurements for TPS commissioning. There are several main approaches to commissioning TPS:

• 1.

  The TPS requires a set of relative dosimetry data to be measured and entered by the user. Some of these types of TPS do provide standard beam data sets for delivery machines for comparison purposes but use the measured data set for treatment planning. An example for this approach is the CyberKnife MultiPlan TPS.

• 2.

  The TPS is delivered with a standard data set for a delivery machine. The measured relative dosimetry data set could be used for verification that the machine is “sufficiently matched” to the standard data set to be used for treatment planning. An example of this approach is the Varian Eclipse TPS. There are currently no manufacturer-independent recommendations on the tolerance for the measured versus standard data set match. While this use of pre-existing beam data is not in agreement with the recommendations of AAPM TG-106 , it follows established clinical practice for TomoTherapy and Gamma Knife (see item 4 below). If this approach is taken, the vendor-provided data must be very carefully compared against measured data and any deviations thoroughly assessed.

• 3.

  The relative dosimetry measurement data are used to generate parameters for one or more beam models. Examples of this approach are the Philips Pinnacle, Elekta XIO, and RaySearch RayStation planning systems. Modeling beam sizes from 40 cm × 40 cm maximum down to small fields of 0.5 cm × 0.5 cm size with one parameter set may require compromise on model accuracy that may not be acceptable in clinical practice. In those cases, it may be prudent to generate two beam models for one beam energy to match the relative dosimetry across all field sizes.

• 4.

  The TPS is part of a treatment delivery system and comes precommissioned from the manufacturer. The relative dosimetry measurements are used only to verify the manufacturer-generated data. An example for this approach is the TomoTherapy system and Leksell Gamma Knife.

The rapid development of technology has had an impact on the requirements for relative dosimetry measurements as well. Accelerator dose rates have increased by a factor of 10 or more due to improvements in accelerator design and the adoption of flattening filter free (FFF) beam delivery across almost all delivery systems. The introduction of intensity-modulated radiation therapy (IMRT) and volumetric modulated arc treatment (VMAT) has heightened the importance of accurate penumbra dosimetry measurements to correctly model steep dose gradients at the field edge in the TPS. Improvements in multi-leaf collimator (MLC) design have reduced the leaf width to 2.5 mm in some cases, allowing the treatment of targets as small as 5 mm using the rule of thumb of 2× the minimum leaf width. This development requires the use of small-field relative dosimetry techniques, which previously were restricted to dedicated stereotactic radiosurgery/stereotactic body radiotherapy (SRS/SBRT) devices. The increased use of SRS, SBRT, and hypofractionated treatment regimens has increased the demands on relative dosimetry accuracy, because the impact on systematic errors introduced by uncertainties in relative dosimetry on clinical plan delivery accuracy is increased compared to conventional fractionation schemes. The Institute of Physics and Engineering in Medicine (IPEM) report on small field dosimetry provides a very comprehensive introduction to relative dosimetry in small megavoltage (MV) photon beams. In this chapter, we follow the nomenclature of AAPM TG-106 to define “small field” as any field equal to or less than 4 cm × 4 cm equivalent square. “Very small field” denotes fields equal to or smaller than 2 cm × 2 cm equivalent square.

Chapter 4 deals in more depth with the topic of commissioning and QA of new treatment equipment (a TPS being one of them). Standard references are AAPM TG-53 (QA for Clinical Radiotherapy Equipment) and International Atomic Energy Agency (IAEA) Report 430 (Commissioning and QA of Computerized Planning Systems).