G. Heilemann

Multidimensional dosimetry of 106Ru eye plaques using EBT3 films and its impact on treatment planning

PURPOSE The purpose of this study was to establish a method to perform multidimensional radiochromic film measurements of (106)Ru plaques and to benchmark the resulting dose distributions against Monte Carlo simulations (MC), microdiamond, and diode measurements. METHODS Absolute dose rates and relative dose distributions in multiple planes were determined for three different plaque models (CCB, CCA, and COB), and three different plaques per model, using EBT3 films in an in-house developed polystyrene phantom and the mcnp6 MC code. Dose difference maps were generated to analyze interplaque variations for a specific type, and for comparing measurements against MC simulations. Furthermore, dose distributions were validated against values specified by the manufacturer (BEBIG) and microdiamond and diode measurements in a water scanning phantom. Radial profiles were assessed and used to estimate dosimetric margins for a given combination of representative tumor geometry and plaque size. RESULTS Absolute dose rates at a reference depth of 2 mm on the central axis of the plaque show an agreement better than 5% (10%) when comparing film measurements (mcnp6) to the manufacturers data. The reproducibility of depth-dose profile measurements was <7% (2 SD) for all investigated detectors and plaque types. Dose difference maps revealed minor interplaque deviations for a specific plaque type due to inhomogeneities of the active layer. The evaluation of dosimetric margins showed that for a majority of the investigated cases, the tumor was not completely covered by the 100% isodose prescribed to the tumor apex if the difference between geometrical plaque size and tumor base ≤4 mm. CONCLUSIONS EBT3 film dosimetry in an in-house developed phantom was successfully used to characterize the dosimetric properties of different (106)Ru plaque models. The film measurements were validated against MC calculations and other experimental methods and showed a good agreement with data from BEBIG well within published tolerances. The dosimetric information as well as interplaque comparison can be used for comprehensive quality assurance and for considerations in the treatment planning of ophthalmic brachytherapy.

Pushing the boundaries of spatial resolution in dosimetry using polymer gels and radiochromic films

Advanced radiotherapy and brachytherapy techniques are raising the bar for detectors with respect to high spatial resolution. Dosimetry based on most point-like dosimeters, e.g. diamond detectors or small volume ionization chambers cannot be used efficiently and accurately for detecting 2 or 3D-dose variations at millimeter scale. Hence radiochromic films and polymer gels with high two/three-dimensional resolution provide a good verification tool for measuring dose distributions of very small collimated beams. In this study the performance of film and gel detectors in detecting the very fine dose distributions generated from collimation holes of four different sizes is investigated. Pencil beams with diameters down to 0.455 mm could be resolved by both detector types comparably.

Treatment plan optimization and robustness of 106Ru eye plaque brachytherapy using a novel software tool

BACKGROUND AND PURPOSE To analyze treatment plan robustness and plan optimization strategies of 106Ru eye plaque brachytherapy using a novel software tool. MATERIALS AND METHODS A treatment planning software was developed that allows to calculate dose-volume metrics. Plaque misplacements were simulated and evaluated with respect to the effect on tumor coverage and dose changes in critical structures. Two treatment plan optimization approaches were analyzed: (a) reducing plaque size and (b) shifting the plaque away from organs-at-risk (OAR). RESULTS Maximum tumor sizes were identified which can be covered by the prescribed dose for different robustness levels (0-2mm). For an apex height of 5mm a 1mm uncertainty yielded changes in D2% to the lens of up to ±13Gy in anterior and ±20Gy to the optic nerve in posterior tumors. By reducing the plaque size Dmean and D2% to lens, optic nerve and macula were decreased by >60% for most simulated cases. Similarly, by shifting the plaque away from the lens dose reductions of 15%/mm in anterior and even 30%/mm in central tumors were achieved. CONCLUSION Critical structures in the treatment of uveal melanomas with 106Ru plaques can benefit from the proposed, computational treatment plan optimization.

Retina dose as a predictor for visual acuity loss in 106Ru eye plaque brachytherapy of uveal melanomas

BACKGROUND AND PURPOSE To evaluate the retina dose as a risk factor associated with loss of visual acuity (VA) in 106Ru plaque brachytherapy. MATERIAL/METHODS 45 patients receiving 106Ru plaques brachytherapy (median follow-up 29.5 months) were included in this study. An in-house developed treatment planning system with Monte Carlo based dose calculation was used to perform treatment planning and dose calculation. Risk factors associated with loss of VA were evaluated using the Cox proportional hazards models, Kaplan-Meier estimates and Pearson correlation coefficients. RESULTS A significant correlation was found between VA loss and mean (r = 0.49, p = 0.001) and near maximum (r = 0.47, p = 0.001) retina dose D2% and tumor basal diameter (r = 0.50, p < 0.001). The Kaplan-Meier and Cox proportional hazards model yielded a significantly higher risk for VA loss (>0.3Snellen) for patients receiving a maximum dose of >500 Gy (p = 0.002). A Cox multivariate analysis including the macula dose (p = 0.237) and basal diameter (p = 0.791) showed that a high maximum retinal dose is the best risk factor (p = 0.013) for VA loss. CONCLUSION The study showed that retina dose (D2% and Dmean) is a suitable predictor for VA loss.

Characteristic of EBT-XD and EBT3 radiochromic film dosimetry for photon and proton beams

Recently, a new type of radiochromic film, the EBT-XD film, has been introduced for high dose radiotherapy. The EBT-XD film contains the same structure as the EBT3 film but has a slightly different composition and a thinner active layer. This study benchmarks the EBT-XD against EBT3 film for 6 MV and 10 MV photon beams, as well as for 97.4 MeV and 148.2 MeV proton beams and 15-100 kV x-rays. Dosimetric and film reading characteristics, such as post irradiation darkening, film orientation effect, lateral response artifact (LRA), film sensitivity, energy and beam quality dependency were investigated. Furthermore, quenching effects in the Bragg peak were investigated for a single proton beam energy for both film types, in addition measurements were performed in a spread-out Bragg peak. EBT-XD films showed the same characteristic on film darkening as EBT3. The effects between portrait and landscape orientation were reduced by 3.1% (in pixel value) for EBT-XD compared to EBT3 at a dose of 2000 cGy. The LRA is reduced for EBT-XD films for all investigated dose ranges. The sensitivity of EBT-XD films is superior to EBT3 for doses higher than 500 cGy. In addition, EBT-XD showed a similar dosimetric response for photon and proton irradiation with low energy and beam quality dependency. A quenching effect of 10% was found for both film types. The slight decrease in the thickness of the active layer and different composition configuration of EBT-XD resulted in a reduced film orientation effect and LRA, as well as a sensitivity increase in high-dose regions for both photon and proton beams. Overall, the EBT-XD film improved regarding film reading characteristics and showed advantages in the high-dose region for photon and proton beams.