ESTRO 2021 Abstract Book

S1429

ESTRO 2021

degree of resolution or expansion of atelectasis, and analyze the geometry changes including the center of mass(COM), boundary and volume of the tumor, and volume of ipsilateral lung. Dosimetric changes in the target and lung tissue were also quantified. Results There were 2 patients with expansion, 12 patients with resolution, and 4 patients with no change. The timing of resolution or expansion was very different, 2 patients changed in the third fraction, 5 patients changed in the middle of fractions, and 3 patients changed near the end of fractions. The tumor volume increased by 3.8% in the first 7 fractions, then decreased from the 9th fraction, and by 33.4% at the last CBCT. In the LR direction, the average COM of the tumors were gradually shifted to mediastinal, and the COM of tumors at the right side of body was much smaller than that of tumors at the left side of body. In the AP direction, the average COM of tumors were shifted slightly to posterior direction, and then gradually shifted to anterior. In the SI direction, the average COM of tumors at the right side of body was substantially shifted toward the head direction and the shift was not more than 0.2 cm. In the LR direction, the left and right borders of the tumors gradually all retract to the mediastinal, and the closer to the longitudinal boundary, the larger the retraction. In the AP direction, the anterior and posterior boundaries of tumors were all moved toward mediastinal. In the SI direction, the superior and inferior boundaries of the tumors vary greatly. The D 2 , D mean , D 98 , V 95 and V 107 of PTV were reduced during radiotherapy, and were reduced to the lowest value during the last two fractions. Compared with the original plan, the reductions were 0.27%, 3.48%, 24.56%, 8.12% and 15.75%, respectively. The V 5 , V 10 , V 20 , V 30 and V 40 of lungs gradually increased with the fraction, and reached to the highest value at the end of radiotherapy (14.23%, 19.78%, 29.79%, 42.82%, 58.51%). Conclusion For most patients, resolution of atelectasis caused COM and boundary of tumors shifted toward mediastinum center, tumor volume decreased, and introduced insufficient dose to target and over dose to lungs. Resolution or expansion may occur at any fraction, it was recommended that CBCT was scanned at least every other day. PO-1702 A simple method to measure the latency in gated proton therapy using a scintillating crystal J. Thomsen 1 , E. Worm 2 , J. Johansen 1 , P. Rugaard 3 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark; 3 Aarhus University Hospital, Danish Centre for Particle Therapy , Aarhus, Denmark Purpose or Objective Gated delivery is commonly used in radiotherapy of tumors moving with respiration. It relies on the ability to turn on the treatment beam only when the target is within a predetermined gating window. However, the latency from the target enters/exits the gating window till the beam is turned on/off affects the treatment accuracy. No standardized method for measuring the gating latency exists. We therefore propose a simple and direct method to measure the latency and demonstrate the method for a pencil-beam scanning proton system. Materials and Methods A proton pencil beam was delivered at a clinical facility (ProBeam, Varian) to a 5mm cubic scintillating ZnSe:O crystal. The crystal emitted visible light when irradiated by the beam. The beam delivery was gated by optical monitoring of a marker block (RPM, Varian) on a motion stage (Fig 1.A-B). The motion stage performed vertical sinusoidal motion with 1cm peak-to-peak amplitude and 1-8s periods. The gating window was set approximately from the middle of the motion and above giving a duty cycle close to 50%. A video camera (GoPro) acquired images at 120Hz showing both the crystal and the marker block. Post-treatment the marker block position was segmented in all video frames. A sinusoidal fit provided the times T max where the marker block was at maximum excursion. Analysis of the crystal light intensity in the video frames provided the beam- on and beam-off times for each cycle with an estimated accuracy <2ms (Fig 1C). The latencies for gate-off (t gate-off ) and gate-on (t gate-on ) were then determined as follows. If t gate-off and t gate-on were zero, the mid-time of the light signal (T light ) would coincide with the time of maximum marker block excursion T max . With finite latencies it can be shown that the light signal has a delay T light – T max = (t gate-off + t gate-on )/2. The time difference between the motion and the light signal therefore directly provided the sum of the gate-off and gate-on latency for each monitored cycle. It can furthermore be shown that the light signal duration (DT light ) increases linearly with the sinusoidal period T as follows: DT light = aT + (t gate-off - t gate-on ). Here, a is the constant fraction of time, where the RPM marker block is inside the gating window. A linear fit of the observed light signal duration as function of the sine period therefore provided the difference between the gate-off and gate-on latency. Results The mean of the summed gating latency t gate-off + t gate-on across 86 analyzed cycles was 396ms (Fig 2A). The gate-on latency was longer than the gate-off latency with t gate-off - t gate-on = -180ms (Fig 2B). The mean gating latencies were t gate-on = 288ms and t gate-off = 108ms (Fig 2C). Conclusion We propose a simple and direct method to determine the gating latency through video recording of a motion stage and a scintillating crystal placed in the beam. For the investigated proton beam, the average latency was 288 ms for beam-on and 108ms for beam-off.