Measuring the dose in bone for spine stereotactic body radiotherapy

Abstract

Current quality assurance of radiotherapy involving bony regions generally utilises homogeneous phantoms and dose calculations, ignoring the challenges of heterogeneities with dosimetry problems likely occurring around bone. Anthropomorphic phantoms with synthetic bony materials enable realistic end-to-end testing in clinical scenarios. This work reports on measurements and calculated corrections required to directly report dose in bony materials in the context of comprehensive end-to-end dosimetry audit measurements (63 plans, 6 planning systems). Materials and methods: Radiochromic film and microDiamond measurements were performed in an anthropomorphic spine phantom containing bone equivalent materials. Medium dependent correction factors, k med , were established using 6 MV and 10 MV Linear Accelerator Monte Carlo simulations to account for the detectors being calibrated in water, but measuring in regions of bony material. Both cortical and trabecular bony material were investigated for verification of dose calculations in dose-to-medium (D m,m ) and dose-to-water (D w,w ) scenarios. Results: For D m,m calculations, modelled correction factors for cortical and trabecular bone in film measurements, and for trabecular bone in microDiamond measurements were 0.875(±0.1%), 0.953(±0.3%) and 0.962(±0.4%), respectively. For D w,w calculations, the corrections were 0.920(±0.1%), 0.982(±0.3%) and 0.993(±0.4%), respectively. In the audit, application of the correction factors improves the mean agreement between treatment plans and measured microDiamond dose from − 2.4%(±3.9%) to 0.4%(±3.7%). Conclusion: Monte Carlo simulations provide a method for correcting the dose measured in bony materials allowing more accurate comparison with treatment planning system doses. In verification measurements, algorithm specific correction factors should be applied to account for variations in bony material for calculations based on D m,m and D w,w .

Experimental validation of absorbed dose-to-medium calculation algorithms in heterogeneous media

“…This dependency was also stressed in previous studies (Delbaere et al 2019, Younes et al 2021) where differences up to 2.6% were found between the absorbed dose D m m calculated in the Gammex cortical bone and in the ICRP-23 (ICRP 1975) cortical bone. The same findings could be observed on the ] factors and could explain the difference between our film correction factor in the Gammex cortical bone (∼0.934) and the one calculated by Shaw et al (2021) in the CIRS (Norfolk, VA, USA) cortical bone (∼0.875). In the case of equivalent lung media, the correction factors in the CIRS lung calculated by Shaw et al (2023) were in agreement with our values in the Gammex lung (∼0.970-0.980) but were not consistent with Charles et al (2021) who found a factor equal to unity for the ICRU-44 lung (ICRU 1989).…”

“…The cross-calibration coefficients were established using an aluminium-core calorimeter embedded in an aluminium phantom. Surprisingly, the authors pointed out that their film results were close to those reported by Shaw et al (2021) in the CIRS cortical bone. Nevertheless, their corrections are closer to our value in Teflon (∼0.866) than in the Gammex cortical bone.…”

A survey of compliance with stereotactic ablative body radiotherapy quality recommendations

“…• Trial credentialling (47%) • Australian Clinical Dosimetry Service (ACDS) SABR dosimetry audit 37 (35%)…”

Dosimetric accuracy of the Convolution algorithm for Leksell Gamma Plan radiosurgery treatment planning: Evaluation in the presence of clinically relevant inhomogeneities

“…Since the film dosimetry protocol followed yields dose to water within the medium where the films lay (i.e., bone and air), 20 appropriate correction factors were calculated to convert this to corresponding dose to medium values, according to 20 : $$$$\begin{equation}{K_{med}} = \;{\left( {\frac{{{D_{med}}}}{{{D_{film}}}}} \right)^{LGK}}{\left( {\frac{{{D_{film}}}}{{{D_w}}}} \right)^{Cal}}\end{equation}$$$$where, $${\left( {\frac{{{D_{med}}}}{{{D_{film}}}}}\right)^{LGK}}$$is the ratio of dose to medium to dose to film for the LGK photon fields and irradiation geometries and$${\rm{\;}}{\left( {\frac{{{D_{film}}}}{{{D_w}}}} \right)^{Cal}}$$ is the ratio of dose to film to dose to water for the film calibration photon field (i.e., the 60 Co gamma ray reference field) and calibration geometries (see also Section 2.5).…”

“…In-house software routines developed in MATLAB ® (The MathWorks, Inc., Natick, MA) were used to read the measured pixel values of each experimental film piece and convert them into dose values using a triple channel technique and the calibration data of the used film batch. 19 Since the film dosimetry protocol followed yields dose to water within the medium where the films lay (i.e., bone and air), 20 appropriate correction factors were calculated to convert this to corresponding dose to medium values, according to 20 :…”