Dosimetry of small megavoltage photon fields based on dose-area product
Creators: name: Kupfer, Tom
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510502
subject: Medical physics

description: Aim:The accurate dosimetry of small megavoltage photon fields is a very challenging task. Firstly, it is technically difficult to align a dosimeter precisely with the small field axis in the first place. Secondly, subsequent inference of the absorbed dose from the detector response requires special correction factors, the determination of which is often outside of the means of the clinical physicist. Although an increasing number of correction factors have been published and are now being introduced in standard measurement protocols, the need for precise alignment remains and this poses an obstacle for accurate small field dosimetry in a clinic where time and other resources are often limited. This problem has led to a considerable research interest in different and more robust measurement approaches. One such approach is to use a detector with a sensitive volume that fully encompasses the small beam. Plane-parallel large-area chamber (LACs) have circular sensitive areas with radii r_LAC of a few cm, over which they integrate the absorbed dose in a phantom. The measured quantity is ‘dose-area product in water’ (DAP_W). This quantity is not related to notions of dose-area product (DAP) and kerma-area product (KAP) used to control of exposure from diagnostic x-ray sources. A method to correct for the LAC’s energy-dependence would ideally be based on the LAC-measured tissue-phantom ratio, 𝐷𝐴𝑃𝑅_20,10𝑟𝐿𝐴𝐶. However, this approach is complicated by the little understood dependence of 𝐷𝐴𝑃𝑅_20,10,𝑟𝐿𝐴𝐶 on rLAC due to the presence of phantom scatter and collimator leakage. 

The thesis aims are to i) commission an LAC (PTW37040 Bragg-Peak Chamber, r_LAC = 4.08 cm) and use it to measure 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 of small cone-defined fields at ARPANSA’s Synergy linac, ii) investigate how DAP-based parameters can be measured in a solid phantom for subsequent comparison with a treatment planning system (TPS), using Austin Health’s clinical Elekta Agility linac and Elekta Monaco TPS. Methods: The response of the LAC was investigated for linearity, polarity and recombination effects. The spacing of its electrodes was measured using high-resolution computed tomography (CT). A calibration coefficient N_D,w,LAC was derived in a cone-defined 6 MV beam of 5 cm diameter on an Elekta Synergy linac, whose DAP_W was determined using ARPANSA’s secondary standard detector in conjunction with two-dimensional high-resolution dose maps obtained with radiochromic film (EBT3) and scanning detectors (CC13, IBA EFD). Additional calibrations in the 10 x 10 cm reference field were performed for 6, 10 and 18 MV to characterize the energy-dependence of the LAC. 

Using cone-defined 6 MV fields between 0.5 and 1.5 cm diameter, the LAC signal was obtained as a function of backup jaw field size. Assuming that DAP is proportional to the area, the signal components due to leakage radiation through the cone and a hypothetical ‘leakage-free’ primary small field could be calculated. 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 was derived separately for each component. 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 and LAC-output factors (‘modified’ output factor - MOF) were measured in a solid water-equivalent phantom on the Agility linac for 6 MV, 6 MVFFF, 10 MV &amp; 10 MVFFF for square fields with side length s between 1 and 4 cm side length, and compared to TPS. MOF are normalised to s = 4 cm, which is the largest s encompassed by the LAC. For 6 MV the comparison included absolute DAP_W measured with LAC, two-dimensional dose maps obtained with EBT3 in the same phantom, and the parameter √𝐷𝐴𝑃w𝐷CAX⁄2, where D_CAX was obtained using calibrated EBT3. 

Results: The LAC response is linear; corrections for recombination and polarity are &lt; 1%. Variations in the electrode spacing were not found. N_D,w,LAC equals 163.7 mGy cm2 nC-1 (1 SD = 1.2%). The LAC over-responds by &gt; 1 SD in 10x10 cm field. Its energy-dependence is consistent with those of air-filled ionization chambers. 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 increases with diminishing cone diameter. The trend is significantly amplified with larger backup jaw opening. The leakage component is between 1% to 30% of the total signal. Removing the leakage component yielded a self-consistent set of 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 for each cone’s ‘leakage-free’ beam, for which the aforementioned trend is much reduced. Owing to the harder leakage radiation spectrum, the 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 of the leakage component is ~5% higher than those for the ‘leakage-free’ component. For the Agility linac, measurements and TPS agreed within -2.5% to + 2.5% for MOF and within -1.3% to +1.2% for 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚. 6 MV absolute DAP_W agreed within 2.2% (LAC) and 2.7% (EBT3). EBT3 dose maps revealed small differences below the 20% penumbra. A highly linear field-size-dependence of √𝐷𝐴𝑃w𝐷CAX⁄2 has been found for field side lengths of 1 to 4 cm. This presented in both measurements and TPS data. A curve fit obtained from s = 2, 3 and 4 cm, combined with measured DAPW (s = 1 cm), yielded an extrapolated DCAX (s = 1 cm) which was within 0.2% with the measured value. 

Conclusions: Commissioning tests show that the LAC is suitable for the measurement of DAP_W in small photon beams. 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 increases with diminishing field size for all beam modalities and energies. 𝐷𝐴𝑃𝑅_20,10,4.08𝑐𝑚 is also highly dependent on the backup jaw position for cone-defined fields, however, this can be exploited to separate the leakage and the ‘leakage-free’ components. This approach could allow the LAC’s signal to be corrected for its energy-response by considering the components separately, which may be required when the leakage contribution to the total signal becomes significant. DAP-based comparison of dose measurement and TPS-calculations is possible using one convenient setup. One potential application of this method is the small field dosimetry audit. Audits are generally done under a time pressure, often prior to release of a linac into clinical service. With simplicity of setup more beams could be audited, which provides more value to the health care facility and in turn to the patient. The parameter √𝐷𝐴𝑃w(𝑟𝑙𝑎𝑐)𝐷CAX⁄2 is highly linear for 6 MV beams. This could be exploited to obtain D_CAX of small beams, or to verify that dose measurements and TPS dose calculations obey this linearity. Further data on this potentially useful parameter should be accumulated for a wide range of clinical linear accelerators, beam energies and field sizes.