We report on a patient with metastatic extraskeletal myxoid chondrosarcoma (EMC), who was treated with palliative high-dose-rate (HDR) interstitial brachytherapy (ISBT). The patient was an 87-year-old woman who underwent an amputation for a bulky tumor in her right ankle joint. EMC was histopathologically confirmed. She presented with a large right inguinal lymph node metastasis of EMC 16 months after surgery. Palliative HDR-ISBT (30 Gy/2 fractions in 1 day) for the right inguinal lymph node metastasis was administered to shorten treatment period. Additional HDR-ISBT (30 Gy/2 fractions in 1 day) was administered for the management of subsequent subcutaneous metastasis to the right breast and right popliteal fossa. HDR-ISBT provided significant long-term control of the recurrent tumor in all three sites, without severe acute and late toxicity. Thus, HDR-ISBT regimen of 30 Gy/2 fractions in 1 day can be a suitable option for both palliation and long-term local control for patients with metastatic EMC.

We measure the dose distribution of gated delivery for different target motions and estimate the gating latency in a magnetic resonance-guided radiotherapy (MRgRT) system. Method: The dose distribution accuracy of the gated MRgRT system (MRIdian, Viewray) was investigated using an in-house-developed phantom that was compatible with the magnetic field and gating method. This phantom contains a simulated tumor and a radiochromic film (EBT3, Ashland, Inc.). To investigate the effect of the number of beam switching and target velocity on the dose distribution, two types of target motions were applied. One is that the target was periodically moved at a constant velocity of 5 mm/s with different pause times (0, 1, 3, 10, and 20 s) between the motions. During different pause times, different numbers of beams were switched on/off. The other one is that the target was moved at velocities of 3, 5, 8, and 10 mm/s without any pause (i.e., continuous motion). The gated method was applied to these motions at MRIdian, and the dose distributions in each condition were measured using films. To investigate the relation between target motion and dose distribution in the gating method, we compared the results of the gamma analysis of the calculated and measured dose distributions. Moreover, we analytically estimated the gating latencies from the dose distributions measured using films and the gamma analysis results. Results:The gamma pass rate linearly decreased with increasing beam switching and target velocity. The overall gating latencies of beam-hold and beam-on were 0.51 ± 0.17 and 0.35 ± 0.05 s, respectively. Conclusions: Film measurements highlighted the factors affecting the treatment accuracy of the gated MRgRT system. Our analytical approach, employing gamma analysis on films, can be used to estimate the overall latency of the gated MRgRT system.