Evaluation of IGRT-Induced Imaging Doses and Secondary Cancer Risk for SBRT Early Lung Cancer Patients In Silico Study.

OBJECTIVES: This study performed dosimetry studies and secondary cancer risk assessments on using electronic portal imaging device (EPID) and cone beam computed tomography (CBCT) as image guided tools for the early lung cancer patients treated with SBRT. METHODS: The imaging doses from MV-EPID and kV-CBCT of the Edge accelerator were retrospectively added to sixty-one SBRT treatment plans of early lung cancer patients. The MV-EPID imaging dose (6MV Photon beam) was calculated in Pinnacle TPS, and the kV-CBCT imaging dose was simulated and calculated by modeling of the kV energy beam in TPS using Pinnacle automatic modeling program. Three types of plans, namely PlanEPID, PlanCBCT and Planorigin, were generated with incorporating doses of EPID, CBCT and no imaging, respectively, for analysis. The effects of imaging doses on dose-volume-histogram (DVH) and plan quality were analyzed, and the excess absolute risk (EAR) of secondary cancer for ipsilateral lung was evaluated. RESULTS: The regions that received less than 50 cGy were significantly impacted by the imaging doses, while the isodose lines greater than 1000 cGy were barely changed. The DVH values of ipsilateral lung increased the most in PlanEPID, followed by PlanCBCT. Compared to Planorigin on the average, the estimated EAR of ipsilateral lung in PlanEPID increased by 3.43%, while the corresponding EAR increase in PlanCBCT was much smaller (about 0.4%). Considering only the contribution of the imaging dose, the EAR values for the ipsilateral lung due to the MV-EPID dose in 5 years,10 years and 15 years were 1.49 cases, 2.09 cases and 2.88 cases per 104PY respectively, and those due to the kV-CBCT dose were about 9 times lower, correspondingly. CONCLUSIONS: The imaging doses produced by MV-EPID and kV-CBCT had little effects on the target dose coverage. The secondary cancer risk caused by MV-EPID dose is more than 8.5 times that of kV-CBCT.

Individualized Fraction Regimen of SBRT Patients With Non-Small Cell Lung Cancer Based on Uncomplicated and Cancer-Free Control Probability.

INTRODUCTION: Stereotactic body radiotherapy (SBRT) currently adopts non-discriminative prescription regimen. This study attempts to investigate an individualized fraction regimen (IFR) method for SBRT patients with non-small cell lung cancer (NSCLC) based on Uncomplicated and Cancer-free Control Probability (UCFCP). METHODS: Twenty patients with NSCLC were retrospectively prescribed with 40 regimens, ranging from 8Gy×5f to 12Gy×5f in step of 0.1 Gy. Taking into consideration of the age and the BMI index of each patient as well, the tumor control probability (TCP), the normal tissue complication probability (NTCP) of the total lung, chest wall and rib, and the secondary cancer probability (SCP) of the total lung were calculated for each plan of the patients. For the 40 regimens, the UCFCP was calculated and the maximum value of UCFCP was the IFR of the specified patient. Besides, IFR of UCP approach which only took account of the TCP and NTCP was also derived and to be compared with the IFR based on the UCFCP method. RESULTS: For all the patients, the UCFCP value showed a bell-shaped trend with the change of prescription dose. Among the 20 patients, the IFRs of 16 patients were different from the original fixed regimen. Of the 16 patients, the IFR of 5 patients exhibited slight changes between UCP and UCFCP methods. CONCLUSION: The method based on the maximum value of UCFCP function may be helpful to provide IFR for specific SBRT patients with NSCLC, differentiating the patient specific characteristics such as anatomical structures and locations.

Intensity Modulated Radiation Therapy for Pleural Recurrence of Thymoma: A Prospective Phase 2 Study.

PURPOSE: This study aimed to evaluate the efficacy and safety of intensity modulated radiation therapy (IMRT) for pleural recurrence of thymoma that was not suitable for surgery and had progressed after chemotherapy. MATERIALS AND METHODS: From February 2012, consecutive patients with pleural recurrence of thymoma were prospectively enrolled. Due to dose restrictions to normal tissue (lung, liver, and kidney), 3 different levels of radiation doses (30 Gy, 40 Gy, and 50 Gy) were prescribed for pleural lesions of different sizes and locations, with a daily fraction dose of 2 Gy. The objective response rate, local control time (LCT), overall survival time, and toxicity were recorded, respectively. RESULTS: By August 2016, 31 patients had completed the IMRT treatment. There were 21 male and 10 female patients, with a median age of 49 (range, 22-70) years. B3 thymoma was the major (62%) tumor subtype observed. During the median follow-up of 48 (24-70) months, the objective response rate was 97%, and the median LCT was 49 (95% confidence interval, 40.4-58.1) months. However, 29 (93.5%) patients developed out-of-field recurrence, among whom 10 (32%; 30 Gy, n = 7; 40 Gy, n = 3) developed both out-of-field and in-field recurrence. The median progression-free survival was 19 months, and no in-field recurrence occurred in the 50 Gy group. Moreover, a higher dose was related to a longer LCT. No toxicities higher than a grade 4 occurred after IMRT within the normal-tissue dose limitation. The 5-year overall survival of the patients was 81%. CONCLUSIONS: IMRT for pleural recurrence may act as an alternative treatment when surgery is not feasible, with a higher dose resulting in a longer LCT. In this study, out-of-field recurrence was considerably common, but repeated IMRT for new recurrence should be cautiously carried out due to the high risk of radiation-induced pneumonitis.

Analyses of distribution and dosimetry of brain metastases in small cell lung cancer with relation to the neural stem cell regions: feasibility of sparing the hippocampus in prophylactic cranial irradiation.

BACKGROUND: This work aims to assess the feasibility of selectively sparing the hippocampus during prophylactic cranial irradiation (PCI) for small cell lung cancer (SCLC). METHODS: SCLC patients with brain metastases (BMs) diagnosed with MRI were enrolled. Lesions localized to the neural stem cell (NSC) compartments [subventricular zone (SVZ) or hippocampus] were analyzed. Patients were categorized by the total number of intracranial metastases, the therapy processes and the symptoms. Hippocampi and enhanced lesions within 15 mm from the hippocampus were contoured. IMRT treatment plans were generated for hippocampal avoidance (HA)-PCI (25Gy in 10 fractions). RESULTS: From Jan 2011 to Oct 2014, 1511 metastases were identified in 238 patients. The overall ratio of metastatic lesions located in NSC regions was 2.0% in the 1511 total metastases and 9.7% in the 238 overall patients. Among the NSC region metastases, 15 (1.0%) lesions involved the HA region of 14 (5.9%) patients and another 15 (1.0%) involved the SVZ of 15 (6.3%) patients. The involvement of HA region or SVZ was significantly different between patients with oligometastatic and non-oligometastatic BMs (P < 0.05). Based on the dosimetric analysis, 26 (10.9%) patients with 41 (2.7%) metastases within 15 mm from the hippocampus had inadequate dosage in case that HA-PCI was applied. CONCLUSIONS: Our retrospective review of 1511 metastases in 238 patients (among whom 89.5% were male) suggests that the metastatic involvement of the NSC regions (especially hippocampus) is unusual and limited primarily to patients with non-oligometastatic disease in SCLC. Also, dosimetric analysis shows that about 10% of patients may have adequate dosage due to HA-PCI treatment. But we believe that this is still an acceptable clinical treatment strategy for SCLC.