ESTRO 2020 Abstract book

S534 ESTRO 2020

population (p=0.982). RT toxicities were not overall more frequent in the HIV positive population (p=0.567), except for fatigue (p<0.05) and hematological toxicities

PO-0916 Dose-rate dependence in haematological recovery following total marrow irradiation A. Haraldsson 1 , W. Stina 2 , J. Engellau 2 , C. Ceberg 3 , S. Bäck 1 , S. Ceberg 3 , S. Engelholm 2 , S. Warsi 2 , P.E. Engström 1 1 Skåne University Hospital, Radiation physics- Department Hematology- Oncology and Radiation Physics, Lund, Sweden ; 2 Skåne University Hospital, Department Hematology- Oncology and Radiation Physics, Lund, Sweden ; 3 Lund University, Medical Radiation Physics- Department of Clinical Sciences, Lund, Sweden Purpose or Objective We retrospectively evaluate dose-rate dependent differences in haematological recovery and engraftment after allogenic stem-cell transplantation with organ sparing total marrow irradiation (TMI), as compared to historically data from our clinic for patients treated with total body irradiation (TBI). The effect of dose rate on engraftment has previously been studied in pre-clinical trials (Down et al. 1991, Glass et al 2013), but is here to the authors´ knowledge for the first time investigated in a clinical setting. Material and Methods We evaluated 30 patients that received TMI between 2014- 2019 with 33 patients that received TBI between 2009- 2014. Two patients were excluded from the TMI group, one due to early transplant-related-mortality, and one due to extremely low amount of CD34 cells available for transplantation. The source of CD34+ stem cells for most of the patients was peripheral blood stem cell transplantation (PBSCT), but we included 5 respective 4 children for the TMI and TBI group with bone marrow transplantation (BMT) source. Prescribed dose was 12 Gy in 6 fractions given twice daily, TMI was delivered with helical tomotherapy with organ sparing to lungs, kidneys, liver, and bowel. TBI were delivered AP-PA using lead lung shielding with the patient at 4.5 m source to skin distance. Doses to organs at risk for TBI was estimated from central doses and anterior posterior measurements. Dose rate was <30 cGy/min at dose maximum for the TBI treatments, and approximately 850 cGy/min for the TMI treatments. Time to engraftment was assessed by thrombocytes and neutrophils count. Graft versus host disease (GVHD) can be caused by remaining host stem cells not completely eradicated by the pre-transplantation treatment and was also scored. Significance testing was done using Mann Whitney U-test and the impact of covariates using Cox’s proportional hazard model. Results For patients receiving PBSCT, mean time to engraftment as measured by thrombocytes over 50 [K/µL], was 19 and 16 days for TBI and TMI respectively, a significant difference (p=0.0006). Cox proportional model revealed an impact of umber of transplanted CD34-cells, radiotherapy type and stem cell transplantation type, figure 1. Further, there was a significant shorter time to thrombocytes >20 (p=0.0016) for the TMI cohort. Mean time to a neutrophil count of >0.5 [K/µL], was 19.7 and 20.4 for the TBI and TMI cohort, and to >1.0 [K/µL] it was 18.4 and 17.2, which was not significantly different (p=0.30 and 0.15). moderate to severe chronic GVHD was seen in 5 TBI patients versus 1 of the TMI patients, 6 patients treated with TBI had acute GVHD versus 1 treated with TMI. The median CD34 count was similar, 6.0 [10 6 /kg].

(p=0.022). Conclusion

HIV status did not impact on survival when patients are treated with consolidative radiotherapy. For HIV positive patients it seems to be an increase in fatigue and hematological toxicities of any grade with radiation. PO-0915 Clinical Activation Of An Efficient Low-Dose Total Body Irradiation Using The Tomodirect Approach A. Chiara 1 , S. Broggi 2 , G. Salvadori 1 , J. Peccatori 3 , A. Assanelli 3 , S. Piemontese 3 , M. Pasetti 1 , S. Selli 1 , R. Calandrino 2 , F. Ciceri 3 , C. Fiorino 2 , N. Di Muzio 1 1 Ospedale San Raffaele IRCCS, Radiation Oncology, Milan, Italy ; 2 Ospedale San Raffaele IRCCS, Medical Physics, Milan, Italy ; 3 Ospedale San Raffaele IRCCS, Haematology and Bone Marrow Transplantation Unit, Milan, Italy Purpose or Objective The feasibility of a low dose sub-mieloablative total body irradiation (TBI) technique using TomoDirect approach was investigated. The definition of a semi-automatic planning optimization procedures was explored and clinically implemented. First clinical results were also reported. Material and Methods TBI delivery was split in two parts: an upper plan from the head to half leg and a lower part, including all legs. The PTVs (PTV_upper and PTV_lower) were defined as the whole body, cropped by 3 mm from the external body contour. Four patients were selected for an in-silico investigation to optimize the irradiation geometry and the optimal plan parameters. For the upper plan a field width equal to 5 cm and a pitch equal to 0.5 were set for all simulations: five different modulation factor (MF) values and 3 different ballistic (6, 8 and 12 fields) were considered. For the PTV_lower plan two anterior/posterior beams with a field width of 5 cm, a pitch of 0.5 and a modulation factor equal to 1.5 were used. For each optimization an automatic process with 300 and 100 iterations was used, respectively for the upper and the lower plan. Between May 2018 and July 2019 twenty patients (13 male and 7 female) were treated by applying the optimal automatic solution assessed by the in-silico investigation. All patients, candidate to allogenic transplant after conditioning regimen with TBI and high dose chemotherapy, were treated with a low-dose TBI regimen: 12 patients with 4 Gy (2 Gy/day;2 fraction) and 8 with a single 2 Gy session. Results For the upper plan, the optimal compromise between treatment time and PTV coverage/ homogeneity was found for MF equal to 1.5 and 8 equidistant beams. All clinical plans were automatically optimized with the found optimal solutions: a good PTV coverage (PTV95%>98.5%) with a good PTV homogeneity (median sigma 0.04) were found with a median beam-on time of 17 minutes for the upper plan and around 9 minutes for the lower one. All patients underwent the transplant in the planned timeframe; no treatment interruption and no effects were registered during treatment. None of the patients experienced acute side effects correlated with TBI. Two patients experiencing a G1 and G2 itchy rash, respectively at three weeks and two months from the transplant. No cGvHD were actually registered. Conclusion TBI with Tomodirect approach was feasible and well tolerated by all patients. The chosen planning optimization technique was found to be efficient, fast and easy to implement in our clinical practice; all patient’s treatment plans were automatically optimized reaching optimal PTV coverage and homogeneity.