ESTRO 2023 - Abstract Book

S2020

Digital Posters

ESTRO 2023

Stereotactic body radiation therapy (SBRT) has become the treatment of choice in several settings. However, ionizing radiation (IR) can damage healthy tissues surrounding the tumor and the resulting toxicities depend not only on the dose, but also on the extent of the volume treated. Few approaches have been tested concerning the reduction of irradiation volumes. Several preclinical and clinical studies support the rationale of combining RT with immune checkpoints inhibitors (ICI, notably anti-PD-1), which could result in tumor regression outside of the radiation field. We hypothetized that the combination of immune modulators, especially ICI, would prompt an antitumor effect not only at distant sites but also in non-irradiated volume within the same tumor mass, inducing an “in situ abscopal effect”. In addition, there has been a growing interest for low-dose RT within the past few years, based on the immunostimulatory properties of non-lethal ionizing radiation on the tumor immune microenvironment (TIME). Thus, using low-dose RT combined with high-dose partial RT appears to be a promising approach, especially in combination with immunomodulators. We studied the effects of IR volume modulation on the tumor response to SBRT in combination with ICIs and deciphered the spatiotemporal modulation of the TIME in these conditions. Materials and Methods Using a small animal SBRT platform, we have performed partial irradiations of MC38 subcutaneous tumors, irradiating a proportion of the tumor at the high dose of 16Gy and leaving the remaining volume either non-irradiated or irradiated at low dose. We combined such approach with immune checkpoint inhibitors as anti-PD1. Treatment efficacy was assessed by tumor growth and survival monitoring, and we analyzed the TIME by single-cell RNAseq, flow cytometry, cytokinome analysis and multiparametric immunohistology, comparing the irradiated regions with those non-irradiated (or IR a low dose) within the same tumor. We also performed partial tumor irradiations of murine orthotopic lung tumors. Results Our results demonstrated a synergistic activity of partial IR and anti PD-1, which resulted in good tumor control. Partial tumor IR induced a reshaping of the TIME in both the irradiated and non-(or weakly) irradiated tumor volumes. Flow cytometry and scRNAseq analyses demonstrated a quantitative and qualitative modulation of the tumor immune populations. Conclusion We obtained the proof of concept that partial tumor IR in combination with ICIs leads to a synergistic antitumor effect. Our analyses of the TIME in the partial IR setting contribute to a better understanding of the RT-induced immune modulations prompting also to better RT-immunotherapy combinations. Our results will thus contribute to the optimization of combined RT/immunotherapy treatments, by opening up new therapeutic options for tumors currently treated in a suboptimal manner and by limiting toxicities. 1 Neuro Spinal Hospital, Department of Radiosurgery & Oncology, Dubai (UAE), United Arab Emirates; 2 Neuro Spinal Hospital, Department of Radiosurgery, Dubai (UAE), United Arab Emirates Purpose or Objective To assess the real time intrafraction motion in skull-base tumor patients treated with Cyberknife based stereotactic radiosurgery or radiotherapy using 6D-skull tracking method. Materials and Methods With 6D skull tracking, Total 1739 image datasets from 47 fractions of 15 brain tumor patients receiving radiosurgery (SRS/SRT) by Cyberknife, were analyzed. All patients were immobilized in supine position using Accu-foam pillow and frameless thermoplastic mask. The position offsets between target position and planned position were obtained in six degrees of motion and used to calculate intrafraction shifts. Mean and standard deviations were calculated for displacements in each direction, and resulting systemic and random errors were used to calculate PTV margins using Van- Herk formula. Results The mean ± 1 SD intrafraction transnationals were 0.45 ± 0.32 mm in superior-inferior direction, 0.37 ± 0.26 mm in left- right direction and 0.21 ± 0.16 mm in anterior-superior direction, while rotations were 0.21 ± 0.19 degrees roll, 0.30 ± 0.22 degrees pitch and 0.31 ± 0.25 degrees yaw. All intrafraction shifts were within the tolerance of robotic correction threshold. Calculated PTV margins were 0.7mm, 0.5mm and 0.4mm in supero-inferior, left-right and anterior-posterior directions, respectively. Conclusion Intrafraction movements are definitely exist to a finite degree during stereotactic radiosurgery/radiotherapy in intracranial tumor patients. Cyberknife based 6D skull tracking using frequent imaging and correction in real time can help mitigate the effect of these movements. Poster (Digital): Patient care, preparation, immobilisation and IGRT verification protocols PO-2246 Real-time motion during 6D-Skull tracking: An Initial study from the first Cyberknife Center of UAE T. Singh 1 , S. Yanek 2 , S. Murphy 2 , C. Antypas 2

PO-2247 Impact of SGRTsetup over three-point localization in pelvic patients for two different IGRT workflow

S. Manna 1 , S. Singh 2 , P.K. Gupta 3 , R. T 4