Population Random Errors Research Articles

Comparative study of setup errors between new and reused thermoplastic masks in irradiated head and neck cancer patients

Background: The accuracy and precision of patient setup are important in radiotherapy. The thermoplastic mask is used to immobilize head and neck cancer patient. However, the mask is reused for low-income patients. Therefore, the setup error should be evaluated to approve that these patients remain in the exact position during treatment. Objective: To investigate setup errors with the use of thermoplastic masks in head and neck cancer patients and to compare setup errors of new and reused thermoplastic masks. Materials and Methods: Eighty patients who underwent volumetric modulated arc radiotherapy (VMAT) for head and neck cancer lesions were retrospectively evaluated. The setup error, population systematic error, and population random error were calculated. Subsequently, setup errors in patients using the new and reused thermoplastic masks were compared. Results: The population systematic error in the vertical, longitudinal, and lateral directions for new masks was 2.02, 2.27, and 2.13 mm, respectively, and that for reused masks was 2.37, 1.96, and 2.33 mm, respectively. The population random error in the vertical, longitudinal, and lateral direction for new masks was 1.46, 1.54, and 1.57 mm, respectively, and that for reused masks was 1.65, 1.63, and 1.87 mm, respectively. The results showed no statistically significant difference supported by p value > 0.05 in the setup error between using the new and reused thermoplastic masks in all directions. Conclusion: For head and neck radiotherapy, the population setup errors were < 3 mm in all directions. Moreover, thermoplastic masks can be reused in patients with head and neck cancer.

Evaluation of set-up errors and estimation of set-up margin during external beam radiation therapy of prostate cancer using electronic portal imaging device (EPID)

AbstractAim:In radiation therapy, accurate dose distribution in target volume requires accurate treatment setup. The set-up errors are unwanted and inherent in the treatment process. By achieving these errors, a set-up margin (SM) of clinical target volume (CTV) to planning target volume (PTV) can be determined. In the current study, systematic and random set-up errors that occurred during prostate cancer radiotherapy were measured by an electronic portal imaging device (EPID). The obtained values were used to propose the optimum CTV-to-PTV margin in prostate cancer radiotherapy.Materials and methods:A total of 21 patients with prostate cancer treated with external beam radiation therapy (EBRT) participated in this study. A total of 280 portal images were acquired during 12 months. Gross, population systematic (Σ) and random (σ) errors were obtained based on the portal images in Anterior–Posterior (AP), Medio-Lateral (ML) and Superior–Inferior (SI) directions. The SM of CTV to PTV were then calculated and compared by using the formulas presented by the International Commission on Radiation Units and Measurements (ICRU) 62, Stroom and Heijmen and Van Herk et al.Results:The findings showed that the population systematic errors during prostate cancer radiotherapy in AP, ML and SI directions were 1·40, 1·95 and 1·94 mm, respectively. The population random errors in AP, ML and SI directions were 2·09, 1·85 and 2·29 mm, respectively. The SM of CTV to PTV calculated in accordance with the formula of ICRU 62 in AP, ML and SI directions were 2·51, 2·68 and 3·00 mm, respectively. And according to Stroom and Heijmen, formula were 4·23, 5·19 and 5·48 mm, respectively. And Van Herk et al. formula were 4·96, 6·17 and 6·45 mm, respectively.Findings:The SM of CTV to PTV in all directions, based on the formulas of ICRU 62, Stroom and Heijmen and van Herk et al., were equal to 2·73, 4·98 and 5·86 mm, respectively; these values were obtained by averaging the margins in all directions.

Evaluation of Six-Dimensional Cranial Target Positioning Accuracy in Two Different Immobilization Methods Using Exactrac System

Introduction: The aim of this study was to determine the accuracy of two different immobilization methods in patient positioning in cranial radiotherapy. The six-dimensional (6D) target localization accuracy of using a dedicated stereotactic mask was compared with that of a conventional head mask by the ExacTrac system. Material and Methods: A total of 56 patients with cranial lesions were included in this study (26 patients with a dedicated stereotactic mask and 30 subjects with a conventional head mask). The ExacTrac image-guided positioning system was utilized to obtain daily translational and rotational patient positioning displacement from the intended position. The 6D setup data was analyzed to obtain population mean, systematic and random errors, and three-dimensional (3D) vector shifts in all the patients. Results: The population mean values of setup errors were comparable with both immobilization systems; however, the spread as indicated by population systematic and population random errors was more in the use of a conventional head mask. The mean values of the 3D vector shifts were 2.09±1.00 and 4.51±3.38 mm with the use of a dedicated stereotactic mask and conventional head mask, respectively. The frequency distribution of maximum rotational deviation and statistical analysis demonstrated a significant difference in immobilization accuracy between stereotactic immobilization and 3-clamp immobilization (p <0.05). Conclusion: The results revealed that there was a significant reduction in target positioning errors with a dedicated stereotactic mask, compared to that with a conventional cranial mask. Furthermore, a dedicated stereotactic mask is required to keep rotational deviations within system correctable limits.

Set-up errors analyses in IMRT treatments for nasopharyngeal carcinoma to evaluate time trends, PTV and PRV margins

Introduction. The aims of this study were to analyze the systematic and random interfractional set-up errors during Intensity Modulated Radiation Therapy (IMRT) in 20 consecutive nasopharyngeal carcinoma (NPC) patients by means of Electronic Portal Images Device (EPID), to define appropriate Planning Target Volume (PTV) and Planning Risk Volume (PRV) margins, as well as to investigate set-up displacement trend as a function of time during fractionated RT course. Material and methods. Before EPID clinical implementation, an anthropomorphic phantom was shifted intentionally 5 mm to all directions and the EPIs were compared with the digitally reconstructed radiographs (DRRs) to test the system's capability to recognize displacements observed in clinical studies. Then, 578 clinical images were analyzed with a mean of 29 images for each patient. Results. Phantom data showed that the system was able to correct shifts with an accuracy of 1 mm. As regards clinical data, the estimated population systematic errors were 1.3 mm for left-right (L-R), 1 mm for superior-inferior (S-I) and 1.1 mm for anterior-posterior (A-P) directions, respectively. Population random errors were 1.3 mm, 1.5 mm and 1.3 mm for L-R, S-I and A-P directions, respectively. PTV margin was at least 3.4, 3 and 3.2 mm for L-R, S-I and A-P direction, respectively. PRV margins for brainstem and spinal cord were 2.3, 2 and 2.1 mm and 3.8, 3.5 and 3.2 mm for L-R, A-P and S-I directions, respectively. Set-up error displacements showed no significant changes as the therapy progressed (p>0.05), although displacements >3 mm were found more frequently when severe weight loss or tumor nodal shrinkage occurred. Discussion. These results enable us to choose margins that guarantee with sufficient accuracy the coverage of PTVs and organs at risk sparing. Collected data confirmed the need for a strict check of patient position reproducibility in case of anatomical changes.