In Vivo Dosimetry using Plastic Scintillation Detectors for External Beam Radiation Therapy

In vivo dosimetry, the direct measurement of dose delivered to patients during radiation therapy, has significant potential in ensuring safe and effective treatment in radiation therapy. It can serve as point-of-delivery, patient specific quality assurance and direct verification of treatment. Despite evidence that in vivo dosimetry can detect errors in patient treatment that would otherwise go undetected, it is not commonly practiced. This is due in part to a lack of available detectors ideally suited to perform in vivo dosimetry. Plastic scintillation detectors (PSDs) possess a number of dosimetric characteristics advantageous for in vivo dosimetry including water equivalence, real-time capability, small size, and energy independence. However, PSDs have not been used for in vivo dosimetry of external beam radiation therapy to date. The overall purpose of this work is to apply PSDs to in vivo dosimetry of external beam radiation therapy, and demonstrate the utility and practicality of performing in vivo dosimetry with PSDs. Three avenues of research were pursued in accordance with this purpose. First, the temperature dependence of PSDs was characterized. Prior to this work, PSDs were understood to be temperature independent detectors. However responses of PSDs constructed with BCF-60 and BCF-12, two common scintillating fibers, were demonstrated to decrease by 0.5% and 0.1% per °C increase relative to 22 °C, respectively. The spectral distribution of light was observed to change with temperature as well. This resulted in a non-negligible error in measured dose at human body temperature, requiring a temperature-specific correction factor. Next, PSDs were used for in vivo dosimetry of the rectal wall in five patients undergoing intensity modulated radiation therapy for prostate cancer. This was done as part of an Institutional Review Board approved protocol. PSDs were attached to endorectal balloons used routinely during prostate radiotherapy, positioning the detectors in close proximity with the rectal wall. Two PSDs were used for two treatment fractions each week for the duration of each patient’s treatment. The difference between the measured dose and expected dose was used to evaluate the accuracy and precision of the system. The mean difference between the measured and expected dose for the five patient population was -0.4%, with a standard deviation of 2.8%. The mean differences for individual patients fell between -3.3% and 3.3%. Finally, a thorough characterization of the response of PSDs used for absolute entrance dosimetry in proton beams was performed. Entrance dose measurements for a passively scattered proton beam performed with a PSD were compared to measurements made with an ion chamber and radiochromic film. Ionization quenching, an under-response due to densely ionizing radiation, was found to be responsible for a 7% loss of signal at the highest energy studied (250 MeV) and a 10% loss at the lowest (140 MeV). The under-response was found to be insensitive to other beam parameters, such as the width of the spread out Bragg peak

MRI radiomic features are independently associated with overall survival in soft tissue sarcoma

Purpose: Soft tissue sarcomas (STS) represent a heterogeneous group of diseases, and selection of individualized treatments remains a challenge. The goal of this study was to determine whether radiomic features extracted from magnetic resonance (MR) images are independently associated with overall survival (OS) in STS. Methods and Materials: This study analyzed 2 independent cohorts of adult patients with stage II-III STS treated at center 1 (N = 165) and center 2 (N = 61). Thirty radiomic features were extracted from pretreatment T1-weighted contrast-enhanced MR images. Prognostic models for OS were derived on the center 1 cohort and validated on the center 2 cohort. Clinical-only (C), radiomics-only (R), and clinical and radiomics (C+R) penalized Cox models were constructed. Model performance was assessed using Harrell\u27s concordance index. Results: In the R model, tumor volume (hazard ratio [HR], 1.5) and 4 texture features (HR, 1.1-1.5) were selected. In the C+R model, both age (HR, 1.4) and grade (HR, 1.7) were selected along with 5 radiomic features. The adjusted c-indices of the 3 models ranged from 0.68 (C) to 0.74 (C+R) in the derivation cohort and 0.68 (R) to 0.78 (C+R) in the validation cohort. The radiomic features were independently associated with OS in the validation cohort after accounting for age and grade (HR, 2.4; Conclusions: This study found that radiomic features extracted from MR images are independently associated with OS when accounting for age and tumor grade. The overall predictive performance of 3-year OS using a model based on clinical and radiomic features was replicated in an independent cohort. Optimal models using clinical and radiomic features could improve personalized selection of therapy in patients with STS

Dosimetric effects of bolus and lens shielding in treating ocular lymphomas with low-energy electrons

Radiation therapy is an effective treatment for primary orbital lymphomas. Lens shielding with electrons can reduce the risk of high-grade cataracts in patients undergoing treatment for superficial tumors. This work evaluates the dosimetric effects of a suspended eye shield, placement of bolus, and varying electron energies. Film (GafChromic EBT3) dosimetry and relative output factors were measured for 6, 8, and 10 MeV electron energies. A customized 5-cm diameter circle electron orbital cutout was constructed for a 6 × 6-cm applicator with a suspended lens shield (8-mm diameter Cerrobend cylinder, 2.2-cm length). Point doses were measured using a scanning electron diode in a solid water phantom at depths representative of the anterior and posterior lens. Depth dose profiles were compared for 0-mm, 3-mm, and 5-mm bolus thicknesses. At 5 mm (the approximate distance of the anterior lens from the surface of the cornea), the percent depth dose under the suspended lens shield was reduced to 15%, 15%, and 14% for electron energies 6, 8, and 10 MeV, respectively. Applying bolus reduced the benefit of lens shielding by increasing the estimated doses under the block to 27% for 3-mm and 44% for 5-mm bolus for a 6 MeV incident electron beam. This effect is minimized with 8 MeV electron beams where the corresponding values were 15.5% and 18% for 3-mm and 5-mm bolus. Introduction of a 7-mm hole in 5-mm bolus to stabilize eye motion during treatment altered lens doses by about 1%. Careful selection of electron energy and consideration of bolus effects are needed to account for electron scatter under a lens shield

Electron beam energy QA — a note on measurement tolerances

Monthly QA is recommended to verify the constancy of high-energy electron beams generated for clinical use by linear accelerators. The tolerances are defined as 2%/2 mm in beam penetration according to AAPM task group report 142. The practical implementation is typically achieved by measuring the ratio of readings at two different depths, preferably near the depth of maximum dose and at the depth corresponding to half the dose maximum. Based on beam commissioning data, we show that the relationship between the ranges of energy ratios for different electron energies is highly nonlinear. We provide a formalism that translates measurement deviations in the reference ratios into change in beam penetration for electron energies for six Elekta (6-18 MeV) and eight Varian (6-22 MeV) electron beams. Experimental checks were conducted for each Elekta energy to compare calculated values with measurements, and it was shown that they are in agreement. For example, for a 6 MeV beam a deviation in the measured ionization ratio of ± 15% might still be acceptable (i.e., be within ± 2 mm), whereas for an 18 MeV beam the corresponding tolerance might be ± 6%. These values strongly depend on the initial ratio chosen. In summary, the relationship between differences of the ionization ratio and the corresponding beam energy are derived. The findings can be translated into acceptable tolerance values for monthly QA of electron beam energies

MRI Radiomic Features Are Independently Associated With Overall Survival in Soft Tissue Sarcoma

Purpose: Soft tissue sarcomas (STS) represent a heterogeneous group of diseases, and selection of individualized treatments remains a challenge. The goal of this study was to determine whether radiomic features extracted from magnetic resonance (MR) images are independently associated with overall survival (OS) in STS. Methods and Materials: This study analyzed 2 independent cohorts of adult patients with stage II-III STS treated at center 1 (N = 165) and center 2 (N = 61). Thirty radiomic features were extracted from pretreatment T1-weighted contrast-enhanced MR images. Prognostic models for OS were derived on the center 1 cohort and validated on the center 2 cohort. Clinical-only (C), radiomics-only (R), and clinical and radiomics (C+R) penalized Cox models were constructed. Model performance was assessed using Harrell's concordance index. Results: In the R model, tumor volume (hazard ratio [HR], 1.5) and 4 texture features (HR, 1.1-1.5) were selected. In the C+R model, both age (HR, 1.4) and grade (HR, 1.7) were selected along with 5 radiomic features. The adjusted c-indices of the 3 models ranged from 0.68 (C) to 0.74 (C+R) in the derivation cohort and 0.68 (R) to 0.78 (C+R) in the validation cohort. The radiomic features were independently associated with OS in the validation cohort after accounting for age and grade (HR, 2.4; P = .009). Conclusions: This study found that radiomic features extracted from MR images are independently associated with OS when accounting for age and tumor grade. The overall predictive performance of 3-year OS using a model based on clinical and radiomic features was replicated in an independent cohort. Optimal models using clinical and radiomic features could improve personalized selection of therapy in patients with STS