Analysis of Individualized Silicone Rubber Bolus Using Fan Beam Computed Tomography in Postmastectomy Radiotherapy: A Dosimetric Evaluation and Skin Acute Radiation Dermatitis Survey.

Objective: To investigate the dosimetric effects of using individualized silicone rubber (SR) bolus on the target area and organs at risk (OARs) during postmastectomy radiotherapy (PMRT), as well as evaluate skin acute radiation dermatitis (ARD). Methods: A retrospective study was performed on 30 patients with breast cancer. Each patient was prepared with an individualized SR bolus of 3 mm thickness. Fan-beam computed tomography (FBCT) was performed at the first and second fractions, and then once a week for a total of 5 times. Dosimetric metrics such as homogeneity index (HI), conformity index (CI), skin dose (SD), and OARs including the heart, lungs, and spinal cord were compared between the original plan and the FBCTs. The acute side effects were recorded. Results: In targets' dosimetric metrics, there were no significant differences in Dmean and V105% between planning computed tomography (CT) and actual treatments (P > .05), while the differences in D95%, V95%, HI, and CI were statistically significant (P < .05). In OARs, there were no significant differences between the Dmean, V5, and V20 of the affected lung, V5 of the heart and Dmax of the spinal cord (P > .05) except the V30 of affected lung, which was slightly lower than the planning CT (P < .05). In SD, both Dmax and Dmean in actual treatments were increased than plan A, and the difference was statistically significant (P < .05), while the skin-V20 and skin-V30 has no difference. Among the 30 patients, only one patient had no skin ARD, and 5 patients developed ARD of grade 2, while the remaining 24 patients were grade 1. Conclusion: The OR bolus showed good anastomoses and high interfraction reproducibility with the chest wall, and did not cause deformation during irradiation. It ensured accurate dose delivery of the target and OARs during the treatment, which may increase SD by over 101%. In this study, no cases of grade 3 skin ARD were observed. However, the potential of using OR bolus to reduce grade 1 and 2 skin ARD warrants further investigation with a larger sample size.

[Influence of patient setup error on planning risk volumes for nasopharyngeal carcinoma patients receiving intensity-modulated radiation therapy].

BACKGROUND & OBJECTIVE: Determination of planning risk volumes (PRVs) for an organ at risk greatly affects dose optimization in designing the intensity-modulated radiation therapy (IMRT) regimen. Patient setup errors have been found to closely correlate to the definition of PRVs. This study was to investigate the safety margin for the organ at risk during IMRT planning for nasopharyngeal carcinoma (NPC) patients. METHODS: Nineteen NPC patients (stage T1-2N0M0) who received IMRT for the first time were studied. Repeated computed tomography (CT) scans were performed for the patients once a week during the whole treatment course. A total of 85 CT scan reports were obtained. Differences between patient positioning of each time and first treatment setup were caluculated by comparing the anatomical landmarks (that is, optical nerve, pituitary, spine, and parotid) on each CT scan image using Osiris software. RESULTS: The displacement of optical nerve and pituitary in X, Y, and Z directions were, in absolute values, (0.86+/-0.53) mm, (0.84+/-0.68) mm, and (0.93+/-1.02)mm, respectively. The standard deviations (SDs) of systematic errors for the axial vector displacement were 0.83 mm, 1.08 mm, and 1.21 mm, while the SDs of random errors were 0.85 mm, 0.83 mm and 1.14 mm. The displacement of spine and parotid in X, Y, and Z directions were, in absolute values, (0.98+/-0.74) mm, (1.25+/-0.88) mm, and (1.43+/-1.02) mm, respectively. The SDs of systematic errors for axial vector displacement were 0.98 mm, 1.35 mm, and 1.87 mm, while the SDs of random errors were 1.02 mm, 1.46 mm, and 1.54 mm. CONCLUSION: It is feasible to determine the size of a safety margin of IMRT for organs at risk using repeated CT scans for NPC patients.

[Design planning target volumes (PTVs) in intensity-modulated radiotherapy for nasopharyngeal carcinoma].

BACKGROUND & OBJECTIVE: Intensity-modulated radiotherapy (IMRT) may help to diminish radiation-induced normal tissue damage and improve quality of life of nasopharyngeal carcinoma (NPC) patients. However, highly conformal treatment techniques commonly establish steep dose gradients between tumor and normal tissues, therefore, daily setup variations can significantly compromise the ultimate precision of idealized IMRT delivery. This study was to investigate the setup accuracy of thermoplastic masks used for immobilizing NPC patients treated by simultaneous integrated boost IMRT, and to determine adequate margins to account for those uncertainties. METHODS: Nineteen patients with early stage (T1-2N0M0) NPC received CT scan weekly during their 6-week treatment course of fractionated IMRT. A total of 85 scanning parameters were obtained. The differences in anatomic landmark coordinates in 3-dimensional directions between daily setup and the first day setup were calculated through comparing the CT images with Osiris software. RESULTS: Mean target isocenter translation was (0.89+/-0.69) mm in x-direction, (0.82+/-0.79) mm in y-direction, and (0.95+/-1.24) mm in z-direction. The systematic errors were 0.94 mm, 1.00 mm, and 1.32 mm. The random errors were 0.87 mm, 0.80 mm, and 1.04 mm. The mean total magnitude vector of isocenter motion was 1.87 mm; the 95% confidence interval (CI) was 2.03-7.24 mm. CONCLUSION: During IMRT for early stage nasopharyngeal carcinoma, setting appropriate margin of planning target volume (PTV) by widening 3 mm from clinical target volume (CTV) in x, y, z directions will be sufficient to compensate for the dosimetric uncertainty of target areas due to patient setup error. The measured data in the present study should enable the user of this kind of thermoplastic mask to assign appropriate margins for the generation of planning target volumes (PTVs).