Improving normal tissue sparing using scripting in endometrial cancer radiation therapy planning

The aim of this study was to improve the protection of organs at risk (OARs), decrease the total planning time and maintain sufficient target doses using scripting endometrial cancer external beam radiation therapy (EBRT) planning. Computed tomography (CT) data of 14 endometrial cancer patients were included in this study. Manual and automatic planning with scripting were performed for each CT. Scripts were created in the RayStation™ (RaySearch Laboratories AB, Stockholm, Sweden) planning system using a Python code. In scripting, seven additional contours were automatically created to reduce the OAR doses. The scripted and manual plans were compared to each other in terms of planning time, dose-volume histogram (DVH) parameters, and total monitor unit (MU) values. While the mean total planning time for manual planning was 368 ± 8 s, it was only 55 ± 2 s for the automatic planning with scripting (p < 0.001). The mean doses of OARs decreased with automatic planning (p < 0.001). In addition, the maximum doses (D2% and D1%) for bilateral femoral heads and the rectum were significantly reduced. It was observed that the total MU value increased from 1146 ± 126 (manual planning) to 1369 ± 95 (scripted planning). It is concluded that scripted planning has significant time and dosimetric advantages over manual planning for endometrial cancer EBRT planning.

Effects of reconstruction methods on dose distribution for lung stereotactic body radiotherapy treatment plans

The aim of the present study was to investigate the effect of tumour motion on various imaging strategies as well as on treatment plan accuracy for lung stereotactic body radiotherapy treatment (SBRT) cases. The ExacTrac gating phantom and paraffin were used to investigate respiratory motion and represent a lung tumour, respectively. Four-dimensional computed tomography (4DCT) imaging was performed, while the phantom was moving sinusoidally with 4 s cycling time with three different amplitudes of 8, 16, and 24 mm. Reconstructions were done with maximum (MIP) and average intensity projection (AIP) methods. Comparisons of target density and volume were performed using two reconstruction techniques and references values. Volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) were planned based on reconstructed computed tomography (CT) sets, and it was examined how density variations affect the dose-volume histogram (DVH) parameters. 4D cone beam computed tomography (CBCT) was performed with the Elekta Versa HD linac imaging system before irradiation and compared with 3D CBCT. Thus, various combinations of 4DCT reconstruction methods and treatment alignment methods have been investigated. Point measurements as well as 2 and 3D dose measurements were done by optically stimulated luminescence (OSL), gafchromic films, and electronic portal imaging devices (EPIDs), respectively. The mean volume reduction was 7.8% for the AIP and 2.6% for the MIP method. The obtained Hounsfield Unit (HU) values were lower for AIP and higher for MIP when compared with the reference volume density. In DVH analysis, there were no statistical differences for D_95%, D_98%, and D_mean (p > 0.05). However, D_2% was significantly affected by HU changes (p < 0.01). A positional variation was obtained up to 2 mm in moving direction when 4D CBCT was applied after 3D CBCT. Dosimetric measurements showed that the main part of the observed dose deviation was due to movement. In lung SBRT treatment plans, D_2% doses differ significantly according to the reconstruction method. Additionally, it has been observed that setups based on 3D imaging can cause a positional error of up to 2 mm compared to setups based on 4D imaging. It is concluded that MIP has advantages over AIP in defining internal target volume (ITV) in lung SBRT applications. In addition, 4D CBCT and 3D EPID dosimetry are recommended for lung SBRT treatments.

Transit dosimetry of stereotactic body radiotherapy treatments with electronic portal dosimetry device in patient with spinal implant

In recent years, the use of the Electronic Portal Imaging Device (EPID) as an in vivo dosimeter has become widespread. However, reports of EPID for stereotactic body radiotherapy (SBRT) applications is scarce. There is no data on this topic especially when there are high-density materials in the radiation field. In this study, we aimed to investigate the dose distributions of SBRT treatment plans in patients with spinal implants by transit EPID dosimetry. Implants were inserted in phantoms that mimic the vertebrae, and VMAT plans were created on the phantoms to deliver 16 Gy radiation doses to the target in 1 fraction. Transit EPID measurements were performed for each irradiation. The results were compared with the treatment planning system using the gamma analysis method. According to the gamma analysis results, while the non-implant model met the acceptance criteria with a rate of 95.4%, the implanted models did not pass the test with results between the rates of 70% to 73%. In addition, while the dose difference in the isocenter was 1.3% for the non-implanted model, this difference was observed to be between 7 and 8% in the implanted models. Our study revealed that EPID can be used as transit dosimetry for the VMAT-SBRT applications. However, unacceptable dose differences were obtained by transit EPID dosimetry in the VMAT-SBRT applications of patients with an implant. In the treatment of such patients, alternative treatment methods should be preferred in which the interaction of the implants with radiation can be prevented.

Factors affecting post-treatment radiation-induced lung disease in patients receiving stereotactic body radiotherapy to lung

The aim of the study is to investigate factors that may cause radiation-induced lung disease (RILD) in patients undergoing stereotactic body radiotherapy (SBRT) for lung tumors. Medical records of patients treated between May 2018 and June 2019 with SBRT were retrospectively evaluated. All patients should have a diagnosis of either primary non-small cell lung cancer (NSCLC) or less than three metastases to lung from another primary. The median treatment dose was 50 Gy in 4-5 fractions. Tumor response and RILD were evaluated in thoracic computer tomography (CT) using RECIST criteria. 82 patients with 97 lung lesions were treated. The median age was 68 years (IQR = 62-76). With a median follow-up of 7.2 months (3-18 months), three patients had grade 3 radiation pneumonitis (RP). RILD was observed in 52% of cases. Patients who had RILD had a higher risk of symptomatic RP (p = 0.007). In multivariate analyses older age, previous lung radiotherapy history, and median planning treatment volume (PTV) D95 value of ≥ 48 Gy were associated with RILD. Local recurrence (LR) was observed in 5.1% of cases. There was no difference in overall survival and LR with the presence of RILD. Older age, previous lung radiotherapy history, and median PTV D95 value of ≥ 48 Gy seems to be associated with post-SBRT RILD.

Feasibility of novel in vivo EPID dosimetry system for linear accelerator quality control tests

The main aim was to validate the capability of a novel EPID-based in vivo dosimetry system for machine-specific quality control (QC) tests. In current study, two sets of measurements were performed in Elekta Versa HD linear accelerator using novel iViewDose™ in vivo dosimetry software. In the first part, measurements were carried out to evaluate the feasibility of novel in vivo system for daily dosimetric QC tests including output constancy, percentage depth dose (PDD) and beam profile measurements. In addition to daily QC tests, measured output factor as a function of field size, leaf transmission and tongue and groove effect were compared with calculated TPS data. In the second part of the measurements, detection capability of iViewDose software for basic mechanical QC tests were investigated for different setup conditions. In dosimetric QC tests, measured output factor with changing field size, PDD, beam profile and leaf transmission factors were found to be compatible with calculated TPS data. Additionally, the EPID-based system was capable to detect given dose calibration errors of 1% with ± 0.02% deviation. In mechanical QC tests, it was found that iViewDose software was sensitive for catching errors in collimator rotation (≥ 1°), changes in phantom thickness (≥ 1 cm) and major differences in irradiated field size down to 1 mm. In conclusion, iViewDose was proved to be as useful EPID-based software for daily monitoring of linear accelerator beam parameters and it provides extra safety net to prevent machine based radiation incidents.

Evaluation of NanoDot Optically Stimulated Luminescence Dosimeter for Cone-shaped Small-field Dosimetry of Cyberknife Stereotactic Radiosurgery Unit: A Monte Carlo Simulation and Dosimetric Verification Study

Aim

The aim of this study was to investigate the adequacy of nanoDot optically stimulated luminescence (OSL) dosimeter for small field dosimetry before its in vivo applications in CyberKnife SRS unit.

Materials and Methods

A PTW 60018 SRS Diode, 60019 microDiamond, and Gafchromic EBT3 films were used along with a nanoDot carbon-doped aluminum oxide OSL dosimeter to collect and compare beam data. In addition, the EGSnrc/BEAMnrc code was employed to simulate 6-MV photon beams of CyberKnife SRS system.

Results

All detectors showed good consistency with each other in output factor measurements for cone sizes of 15 mm or more. The differences were maintained within 3% for these cones. However, OSL output factors showed higher discrepancies compared to those of other detectors for smaller cones wherein the difference reached nearly 40% for cone size of 5 mm. Depending on the performance of OSL dosimeter in terms of output factors, percentage depth doses (PDDs) were only measured for cones equal to or larger than 15 mm. The differences in PDD measurements were within 5% for depths in the range of 5-200 mm.

Conclusion

Its low reliable readings for cones smaller than 15 mm should be considered before its in vivo applications of Cyberknife system.