ESTRO 2020 Abstract Book

S489 ESTRO 2020

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. 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].

Conclusion The difference in time to engraftment and occurrence of acute and chronic GVHD could be attributed to the difference in dose rate between the treatment cohorts. To increase the understanding of dose rate effects in haematological recovery, a larger cohort should be investigated. PO-0917 Radiotherapy-based approach for the treatment of solitary plasmacytoma A. Alghisi 1 , P. Borghetti 1 , M. Maddalo 1 , A.M. Roccaro 2 , A. Tucci 3 , S.M. Magrini 1 , A. Lo Casto 4 , M.L. Bonù 1 , D. Tomasini 1 , A. Baiguini 1 , G. Peretto 1 , M. Buglione 1 , L. Triggiani 1 1 Spedali Civili di Brescia, Department of Radiation Oncology- Brescia University, Brescia, Italy ; 2 Spedali Civili di Brescia, CREA Laboratory, Brescia, Italy ; 3 Spedali Civili di Brescia, Division of Haematology- Department of Clinical Oncology, Brescia, Italy ; 4 University of Palermo, Radiation Oncology, Palermo, Italy Purpose or Objective Solitary plasmacytoma (SP) is characterized by a single mass of clonal plasma cells, without any evidence of systemic involvement or bone marrow (BM) plasmacytosis. It can present either as solitary bone (SBP) or as extramedullary (EMP) plasmacytoma. Definitive RT can result in long-term local control of the SP. Due to the small number of patients (pts) and narrow range of doses, phase III randomized trials are lacking and the optimal dose of radiation for treating SP is not clearly established. The aim of this study is to further support for the potential use of RT for the treatment of SP. Material and Methods Clinical data of all pts treated for SP at our Institution between 1992 and 2018 were reviewed. Pts with a diagnosis of SBP or EMP based on a biopsy showing features characteristic of SP along with a normal BM biopsy, and without evidence of systemic disease attributing to multiple myeloma (MM) were considered eligible. Results A total of 42 consecutive pts were analyzed. The median follow-up was 84.8 months (range 6.2-265.7 months). Details and characteristics of pts are shown in Table 1. EBRT was used with all pts. The prescribed radiation dose ranged from 20 Gy to 50 Gy, with a median of 46 Gy and a dose per fraction ranging from 1.4 to 4 Gy. The equivalent dose (EQD2) ranged from 23.3 to 50 Gy, with a median of 46 Gy, calculated by using a ratio of α/β=10. Radiation dose did not differ significantly as a function of sex, type of SP, tumor size; conversely differs significantly as a