Photon and proton small field dosimetry based on dose-area product

Fulco Alexandro

Promoter

Sterpin Edmond, (KULeuven), edmond.sterpin@kuleuven.be

SCK•CEN Mentor

Mihailescu Cristian
cristian.mihailescu@sckcen.be
+32 14 33 23 89

SCK•CEN Co-mentor

de Freitas Nascimento Luana
luana.de.freitas.nascimento@sckcen.be
+32 14 33 27 19

Expert group

Dosimetric and Calibration Services

PhD started

2017-10-01

Short project description

Small field dosimetry has historically focused on the determination of the absorbed dose to water at a point (Dw). This approach leads to relatively large uncertainties, as most radiation detectors available in the clinic perturb significantly the measurement. Here, we propose a new approach to small field dosimetry based on the concept of dose-area product (DAPw)—the integral of the absorbed dose over the plane perpendicular to the beam direction. Instead of determining Dw with a small detector, it consists in determining the dose-area product using a large detector, such as a large diameter plane-parallel ionization chamber. This approach has the advantage of removing the perturbation introduced by the detector. 

Objective

The PhD project would consist of three parts:

  1. the development of detector calibration procedures in terms of DAPw at the laboratory for nuclear calibrations of SCK•CEN

  2. the application to photon therapy, namely the design of a novel approach to calibrate treatment planning systems for stereotactic treatments based on DAPw measurements

  3. the application to proton therapy, namely the development of a new approach to reference dosimetry of single proton pencil beams based on a dose-area product formalism—in contrast to the current approach based on composite fields made of several pencil beams.

 

Part I: Development of detector calibration procedures in terms of DAPw

The goal of this part is to obtain a calibration coefficient in terms of DAPw (NDAP,w) for a large diameter plane-parallel chamber. Different approaches can be envisioned and compared.
To maintain the traceability to a primary (or secondary) standard of absorbed dose to water, two approaches are possible, both of them requiring a 2D-mapping of the radiation field produced by the 60Co source at SCK-CEN.

  1. Broad field approach: To calibrate the large diameter plane-parallel chamber in a broad 60Co beam. This approach requires an accurate knowledge of the sensitive area of the ionization chamber.

  2. Small field approach: To calibrate the large diameter plane-parallel chamber in a narrow 60Co beam. This approach requires an optimal intermediate field size which is large enough for a secondary standard (typically a graphite-walled Farmer chamber) and small enough for a large diameter plane-parallel chamber.

Also the influence of the variation of response over any large area detector needs to be investigated.

Part II: Application to photon therapy

Stereotactic radiosurgery is gaining importance in the field of external radiotherapy because the high doses delivered to small tumor volumes enable excellent local control rates [Simonova1999]. Small fields (< 3x3 cm2) used in typical radiosurgery treatments are difficult to characterize experimentally using commercial detectors because of the interference of the latter with the measurements. Moreover, the modeling of such fields in treatment planning systems is more challenging than with conventional fields and relies on the aforementioned measurements that are coming with large uncertainties. Therefore, experimental verification of the treatment planning system (TPS) is essential to ensure the accuracy of the delivered treatments. However, the experimental verification also attempts to measure small fields, which inevitably involves the same dosimetric issues than the measurements performed to tune the treatment planning system. This situation is typical of a closed-loop where an independent and trustworthy experimental or computational reference is lacking to validate the entire treatment preparation and execution chain.

We propose to escape the trap of a closed-loop through this novel approach that associates both an iterative optimization of treatment modeling parameters and reference measurements that are not affected by the uncertainties specific to small fields.

II.1. Acquisition of experimental reference data

The key quantity to acquire when characterizing small fields is the output factor, which is defined as the ratio between the point dose at the center of a given field and the point dose at the center of a reference field (typically 10x10 cm2). Measurements of point output factors in small fields are challenging and correction factors have to be applied to take into account the size of the detector (volume effect) and the non-water equivalence of the detector (density/composition effect).

In our approach, volume and density/composition effects are estimated and eliminated by measuring the dose-area product with different detectors with limited energy-spectrum dependence:

  1. a large diameter plane-parallel chamber (e.g. PTW Bragg Peak chamber) that would encompass the field size entirely.

  2. a high-resolution planar detector (e.g. EBT Gafchromic field or 2D luminescent dosimeters) with subsequent integration over an area of interest.

  3. an intermediate solution using a conventional ion chamber (e.g. Farmer chamber) scanning along the small opening of a field that is completely opened in the other direction (dose-line product). This strategy has the advantage to take into account potential asymmetry of the output factors.

II.2 Optimization of beam modeling parameters

Measured point output factors are used as input to tune the photon beam models in commercial TPS. We propose here a novel approach where point output factors in the TPS are optimized iteratively in order to reproduce accurately the dose-area product measurements described above.  

Dose engines based on Monte Carlo methods will also be considered. However, the key parameter to tune in a Monte Carlo model is the size of the photon source. We propose two approaches to determine the source size:

  1. an iterative optimization of the source size following a similar process than with a conventional TPS

  2. a direct measurement of the source shape and size using a properly collimated camera [Chen2011] (a first camera prototype has already been developed in a previous master thesis work at UCL).

Part III: Application to proton therapy

Proton therapy delivered with the superposition of many small fields (“pencil beams”) with different intensities and energies provides unparalleled dose conformation capabilities. However, to circumvent the issues related to a direct dosimetric characterization of a single proton pencil beam, reference dosimetry is currently performed in a composite (msr) field [IAEA2000]. The aim of this work package is to develop a new formalism for the reference dosimetry of proton pencil beams also based on dose-area product. Here, Monte Carlo calculations of beam quality correction factors (kQ), in terms of DAPw, are required. The new approach will be validated against the standard msr-field approach based on Dw.

References

[IAEA2000] Andreo et al. Absorbed dose determination in external beam radiotherapy. An international code of practice for dosimetry based on standards of absorbed dose to water (Technical Reports Series No. 398). International Atomic Energy Agency.

[Chen2011] Chen et al. A slit method to determine the focal spot size and shape of TomoTherapy system

 [Simonova1999] Simonova G et al. Radio- surgery with the Leksell gamma knife in the treatment of solitary brain metastasis—5-year results. Vnitr Lek 45:284–290