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Objective: We evaluated the radiobiological effect of the irradiation time with the interruption time of stereotactic radiosurgery (SRS) using CyberKnife ® (CK) systemfor brain metastases., Methods: We used the DICOM data and irradiation log file of the 10 patients with brain metastases from non-small-cell lung cancer (NSCLC) who underwent brain SRS. We defined the treatment time as the sum of the dose-delivery time and the interruption time during irradiations, and we used a microdosimetric kinetic model (MKM) to evaluate the radiobiological effects of the treatment time. The biological parameters, i.e. α 0 , β 0 , and the DNA repair constant rate ( a + c), were acquired from NCI-H460 cell for the MKM. We calculated the radiobiological dose for the gross tumor volume (GTV bio ) to evaluate the treatment time's effect compared with no treatment time as a reference. The D95 (%) and the Radiation Therapy Oncology Group conformity index (RCI) and Paddick conformity index (PCI) were calculated as dosimetric indices. We used several DNA repair constant rates ( a + c) (0.46, 1.0, and 2.0) to assess the radiobiological effect by varying the DNA repair date ( a + c) values., Results: The mean values of D95 (%), RCI, and PCI for GTV bio were 98.8%, 0.90, and 0.80, respectively, and decreased with increasing treatment time. The mean values of D95 (%), RCI, and PCI of GTV bio at 2.0 (a+c) value were 94.9%, 0.71, and 0.49, respectively., Conclusion: The radiobiological effect of the treatment time on tumors was accurately evaluated with brain SRS using CK., Advances in Knowledge: There has been no published investigation of the radiobiological impact of the longer treatment time with multiple interruptions of SRS using a CK on the target dose distribution in a comparison with the use of a linac. Radiobiological dose assessment that takes into account treatment time in the physical dose in this study may allow more accurate dose assessment in SRS for metastatic brain tumors using CK., Competing Interests: Conflict of Interest Statement: The authors have no conflict of interest to declare., (© 2022 The Authors. Published by the British Institute of Radiology.)

Objectives: We evaluated the radiobiological effectiveness based on the yields of DNA double-strand breaks (DSBs) of field induction with flattening filter (FF) and FF-free (FFF) photon beams., Methods: We used the particle and heavy ion transport system (PHITS) and a water equivalent phantom (30 × 30 × 30 cm 3 ) to calculate the physical qualities of the dose-mean lineal energy (y D ) with 6 MV FF and FFF. The relative biological effectiveness based on the yields of DNA-DSBs (RBE DSB ) was calculated for standard radiation such as 220 kVp X-rays by using the estimating yields of SSBs and DSBs. The measurement points used to calculate the in-field y D and RBE DSB were located at a depth of 3, 5, and 10 cm in the water equivalent phantom on the central axis. Measurement points at 6, 8, and 10 cm in the lateral direction of each of the three depths from the central axis were set to calculate the out-of-field y D and RBE DSB ., Results: The RBE DSB of FFF in-field was 1.7% higher than FF at each measurement depth. The RBE DSB of FFF out-of-field was 1.9 to 6.4% higher than FF at each depth measurement point. As the distance to out-of-field increased, the RBE DSB of FFF rose higher than those of FF. FFF has a larger RBE DSB than FF based on the yields of DNA-DSBs as the distance to out-of-field increased., Conclusions: The out-of-field radiobiological effect of FFF could thus be greater than that of FF since the spreading of the radiation dose out-of-field with FFF could be a concern compared to the FF., Advances in Knowledge: The RBE DSB of FFF of out-of-field might be larger than FF., (© 2021 The Authors. Published by the British Institute of Radiology.)

We aimed to compare the outcomes of high-dose-rate brachytherapy (HDR-BT) boost and external beam radiation therapy (EBRT) alone for high-risk prostate cancer. This was a single-center, retrospective and observational study. Consecutive patients who underwent initial radical treatment by HDR-BT boost or EBRT alone from June 2009 to May 2016 at the Niigata University Medical and Dental Hospital, Japan were included. A total of 96 patients underwent HDR-BT boost, and 61 underwent EBRT alone. The prescription dose of HDR-BT boost was set to 18 Gy twice a day with EBRT 39 Gy/13 fractions. The dose for EBRT alone was mostly 70 Gy/28 fractions. The high-risk group received >6 months of prior androgen deprivation therapy. Overall survival, biochemical-free survival, local control and distant metastasis-free survival rates at 5 years were analyzed. The incidence of urological and gastrointestinal late adverse events of Grade 2 and above was also summarized. In the National Comprehensive Cancer Network (NCCN) high-risk calssification, HDR-BT boost had a significantly higher biochemical-free survival rate at 5 years (98.9% versus 90.7%, P = 0.04). Urethral strictures were more common in the HDR-BT boost group. We will continuously observe the progress of the study patients and determine the longer term results., (© The Author(s) 2021. Published by Oxford University Press on behalf of The Japanese Radiation Research Society and Japanese Society for Radiation Oncology.)

There are no quantitative selection criteria for identifying high-grade glioma (HGG) patients who are suited for volumetric-modulated arc therapy (VMAT). This study aimed to develop selection criteria that can be used for the selection of the optimal treatment modality in HGG. We analyzed 20 patients with HGG treated by 3D conformal radiotherapy (3DCRT). First, VMAT plans were created for each patient retrospectively. For each plan, the normal tissue complication probability (NTCP) for normal brain was calculated. We then divided the patients based on the NTCPs of the 3DCRT plans for normal brain, using the threshold of 5%. We compared the NTCPs of the two plans and the gross tumor volumes (GTVs) of the two groups. For the GTVs, we used receiver operating characteristic curves to identify the cut-off value for predicting NTCP < 5%. We determined the respective correlations between the GTV and the GTV's largest cross-sectional diameter and largest cross-sectional area. In the NTCP ≥ 5% group, the NTCPs for the VMAT plans were significantly lower than those for the 3DCRT plans (P = 0.0011). The NTCP ≥ 5% group's GTV was significantly larger than that of the NTCP < 5% group (P = 0.0016), and the cut-off value of the GTV was 130.5 cm3. The GTV was strongly correlated with the GTV's largest cross-sectional diameter (R2 = 0.82) and largest cross-sectional area (R2 = 0.94), which produced the cut-off values of 7.5 cm and 41 cm2, respectively. It was concluded that VMAT is more appropriate than 3DCRT in cases in which the GTV is ≥130.5 cm3., (© The Author(s) 2019. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology.)

We investigated the outcomes of treatment for patients with localized prostate cancer (PCa) treated with 3D conformal radiation therapy (3D-CRT) followed by two-fraction high-dose-rate brachytherapy within a single day (2-fr.-HDR-BT/day) at a single institution. A total of 156 consecutive Asian males (median age, 67 years) were enrolled. To compare our findings with those of other studies, we analyzed our results using the D'Amico classification, assigning the patients to low- ( N =: 5; 3.2%), intermediate- ( N =: 36; 23.1%) and high-risk ( N =: 115; 73.7%) groups (Stage T3 PCa patients were classified as high-risk). One patient in the D'Amico low-risk group (20%), 13 intermediate-risk patients (36.1%) and 99 high-risk patients (86.1%) underwent androgen deprivation therapy. We administered a prescription dose of 39 Gy in 13 fractions of 3D-CRT combined with 18 Gy of HDR-BT in two 9-Gy fractions delivered within a single day. We did not distinguish between risk groups in determining the prescription dose. The median follow-up period was 38 months. Of the 156 patients, one died from primary disease and five died from other diseases. The 3-year overall survival (OS) rates were 100%, 100% and 93.7%, and the 3-year 'biochemical no evidence of disease (bNED)' rates were 100%, 100% and 96.9% for the D'Amico low-, intermediate- and high-risk groups, respectively. No patient developed ≥ Grade 3 early toxicity. The Grade 3 late genitourinary toxicity rate was 2.6%, and no ≥ Grade 3 late gastrointestinal toxicity occurred. The efficacy and safety of this study were satisfactory, and longer-term follow-up is necessary., (© The Author 2016. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology.)