Optically stimulated luminescence dosimeters for simultaneous measurement of point dose and dose-weighted LET in an adaptive proton therapy workflow

Abstract

PurposeTo demonstrate the suitability of optically stimulated luminescence detectors (OSLDs) for accurate simultaneous measurement of the absolute point dose and dose-weighted linear energy transfer (LETD) in an anthropomorphic phantom for experimental validation of daily adaptive proton therapy.MethodsA clinically realistic intensity-modulated proton therapy (IMPT) treatment plan was created based on a CT of an anthropomorphic head-and-neck phantom made of tissue-equivalent material. The IMPT plan was optimized with three fields to deliver a uniform dose to the target volume covering the OSLDs. Different scenarios representing inter-fractional anatomical changes were created by modifying the phantom. An online adaptive proton therapy workflow was used to recover the daily dose distribution and account for the applied geometry changes. To validate the adaptive workflow, measurements were performed by irradiating Al2O3:C OSLDs inside the phantom. In addition to the measurements, retrospective Monte Carlo simulations were performed to compare the absolute dose and dose-averaged LET (LETD) delivered to the OSLDs.ResultsThe online adaptive proton therapy workflow was shown to recover significant degradation in dose conformity resulting from large anatomical and positioning deviations from the reference plan. The Monte Carlo simulations were in close agreement with the OSLD measurements, with an average relative error of 1.4% for doses and 3.2% for LETD. The use of OSLDs for LET determination allowed for a correction for the ionization quenched response.ConclusionThe OSLDs appear to be an excellent detector for simultaneously assessing dose and LET distributions in proton irradiation of an anthropomorphic phantom. The OSLDs can be cut to almost any size and shape, making them ideal for in-phantom measurements to probe the radiation quality and dose in a predefined region of interest. Although we have presented the results obtained in the experimental validation of an adaptive proton therapy workflow, the same approach can be generalized and used for a variety of clinical innovations and workflow developments that require accurate assessment of point dose and/or average LET.

Multi-institutional experimental validation of online adaptive proton therapy workflows

“…Similar to clinical fractionated treatment, where patients are consistently positioned in alignment with the treatment planning CT, a moulage consisting of a dedicated head support and a thermoplastic mask was prepared for the anthropomorphic phantom. With this positioning equipment, sub-millimeter positioning accuracy between the treatment planning CT and subsequent fractions was achievable, as shown in our previous study (Bobić et al 2024).…”

“…In addition to film dosimetry, we also added two sets of OSLDs to measure the point dose at two different locations in the target volume. The first set (denoted OSLDs 1) was placed as indicated on the sagittal CT slice: seven 3 mm diameter OSLDs were fixed in a circular arrangement on a tissue-equivalent rod, which was inserted into the phantom, as described in detail in (Bobić et al 2024). In addition, a second circular arrangement of seven OSLDs (denoted OSLDs 2) was added in the coronal plane behind slab 2, as indicated in figure 1(a).…”

“…The dose calibration was obtained at the Center for Proton Therapy at PSI, where the OSLDs were irradiated at various doses at a depth of 2 cm using a monoenergetic 240 MeV beam. The OSLD dose calibration is traceable to an ionization chamber that was placed in the same position as the OSLDs to measure and verify the delivered doses, ranging from 0.1 to 5 Gy (Bobić et al 2024). More details can be found elsewhere (Christensen et al 2021(Christensen et al , 2023.…”

“…Figure 4. Point dose OSLD measurements for fraction 1.For OSLDs 1, a single-slice contour was used to evaluate the corresponding point dose by averaging the calculated voxel-wise dose as in(Bobić et al 2024). For OSLDs 2, only the measured dose is evaluated.…”

Assessment of fluence- and dose-averaged linear energy transfer with passive luminescence detectors in clinical proton beams

“…Recently, the LET determination with OSLDs relevant to protons was improved by incorporating corrections based on reference irradiations (Christensen et al 2022). That approach was used to simultaneously determine LET and dose with OSLD measurements in an anthropomorphic head phantom to validate a treatment plan for adaptive proton therapy (Bobić et al 2024). The measurement technique has been extended to helium and carbon ions with both Al 2 O 3 :C and Al 2 O 3 :C,Mg OSLDs (Christensen et al 2023).…”