ESTRO 2021 Abstract Book

S502

ESTRO 2021

was escalated using a simultaneous integrated boost (SIB) to the BTV, defined as the pixels for which the expected pre-boost TCP was in the lower quartile of the TCP range for each patient. A SIB dose was applied so that the average TCP in the BTV increased to match the average TCP of the whole tumour.

Results The

GTVs were 470 to 1660 mm 2 /s, corresponding

ADC

values inside

the

to tumour cell densities of 1.88x10 5 to 3.15x10 5 cm -2 with of 2.67x10 5 cm -2 . This resulted in pixelated TCP values ranging from 0.6433 to 0.9363 with a median of 0.8176. The pixelated TCP map of an example patient is shown in Fig.1 (a). Fig.1 (b) shows the BTV for the same patient. The ADC values within BTV range from 478 to 958 mm 2 /s for this patient. By applying a SIB of between 1.60Gy and 8.02Gy isotoxically to the BTV, we increased the cohort’s TCP by a mean of 8.44% (range 7.19% to 16.84%). As expected, the required SIB doses were larger for patients with lower pixelated TCP values at the 60Gy dose. Patients with larger tumours tended to have lower mean average TCP. a median

Conclusion We have demonstrated a novel methodology that yields ADC-driven SIB dose painting prescriptions for GBM patients based on cellular density and TCP modeling. Future work is required to validate the conversion of ADC to cellular density, but the inclusion of DW-MRI information has been shown to be a feasible path towards treatment personalization in RT of GBM. This offers the prospect of improved tumour control, delivered by SIB to a heterogeneous tumour target, a possibility that requires testing in future, prospective clinical trials.

Proffered papers: Proffered papers 39: Outcome modelling 2

OC-0637 Thoracic dose patterns associated with radiation induced lymphopenia in patients treated for NSCLC S. Monti 1 , L. Cella 1 , T. Xu 2 , R. Mohan 3 , Z. Liao 2 , G. Palma 1 1 National Research Council, Institute of Biostructures and Bioimaging, Napoli, Italy; 2 The University of Texas MD Anderson Cancer Center, Department of Radiation Oncology, Houston, USA; 3 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, USA Purpose or Objective Radiation-induced lymphopenia (RIL) may diminish the efficacy of chemo-radiotherapy (RT) and hence patients’ survival [Xie et al, Radiother Oncol 2020]. The goal of our research is to identify thoracic regions contributing most significantly to RIL in patients enrolled in a randomized trial of Intensity Modulated RT (IMRT) versus Passive Scattering Proton Therapy (PSPT) for locally advanced Non-Small-Cell Lung Cancer (NSCLC). Materials and Methods We analyzed 164 patients treated with PSPT (62 patients) or IMRT (102 patients) for NSCLC in a prospective study. Patients (median age: 66 yr – [33-85] yr) were treated to a prescribed dose of 66 or 74 Gy in conventional 2 Gy daily fractionation with concurrent chemotherapy. According to CTCAE v. 4.0, 147 patients (90%) developed grade ≥3 lymphopenia (absolute lymphocyte count – ALC<0.5*10 9 cells/l) and 59 patients (36%) developed grade ≥4 lymphopenia (ALC<0.2*10 9 cells/l) during RT treatment [1]. Median ALC value at nadir during RT and median ALC value at baseline (RT start) were 0.26*10 9 cells/l ([0.04-0.88]*10 9 cells/l) and 1.78*10 9 cells/l ([0.30-4.00]*10 9 cells/l), respectively. Median time-to-nadir ALC was 41 days (range: [0-58] days). Planning CTs and dose maps were spatially normalized to a common anatomical reference after masking the gross tumor volume. The tumor-subtracted dose maps were converted into biologically effective dose maps (BED – α/β=10 Gy). A Voxel-Based Analysis (VBA) was performed to assess voxel-wise the relationship between the dose accumulated at the time-to-nadir ALC and RIL, defined as the logarithm of nadir ALC to baseline ALC ratio. The generalized linear model (GLM) was designed to include dose maps and each non-dosimetric variables significantly correlated with RIL [Palma et al, Phys Med 2020]. A non-parametric permutation test of the maximum threshold-free cluster-enhanced statistic accounted for multiple comparisons. The significance map of correlation between accumulated dose and RIL was generated.