ESTRO 2021 Abstract Book

S687

ESTRO 2021

chemotherapy, 1 patient required adjuvant inguinal irradiation. Results The median number of catheters used was 7(4-15) in 2-4 planes. The median dosimetric values for CTV V 100 93(90 – 98), V 150 58(50 – 79); Urethra V 95 36 Gy (35-41) and V 115 36(21 – 45). In the acute phase all the patients had grade 2 skin toxicity. At a median follow up of 13 months all patients have maintained sexual function, report no urinary toxicity and have no skin necrosis with the exception of 1 that is related to inguinal external beam radiotherapy. Conclusion HDR interstitial brachytherapy is an attractive organ preservation option for treatment of penile cancer in resource-constrained environments with the view to reduce the burden on external beam radiotherapy (6 days treatment versus 33 days). There is need for a cost effective study and continued prospective data collection on the clinical and patient reported outcomes in resource-constrained environments. PD-0854 Deep inspiration breath hold in radiation therapy of gastric lymphomas: a phase 2 trial. P. Petersen 1 , L.A. Rechner 1 , L. Specht 1 1 Rigshospitalet, University of Copenhagen, Department of Oncology, Copenhagen, Denmark Purpose or Objective Radiotherapy is an important part of treatment of gastric lymphomas. Survival after radiotherapy is long, so prevention of long-term adverse effects is important. Patients with gastric lymphomas are usually older than 60 years of age, and second cancers are of less concern than cardiac late effects. The aim of this study was to assess the possibility to reduce the dose to the heart without compromising the dose to the clinical target volume (CTV) or increasing the risk of other late effects. Materials and Methods Patients diagnosed with gastric lymphoma, referred to our institution 2015 - 2019 were eligible for the study. Treatment planning scans were done in free breathing (FB) and in deep inspiration breath hold (DIBH). CTV encompassed the stomach with 1 cm margin, modified to account for solid surrounding organs, but not for mobile structures. Heart, lungs, kidneys, pancreas, spleen, duodenum, liver and bowel were contoured. RT plans were made for all patients using the DIBH and FB scans, respectively. The optimal plan in terms of target coverage (i.e. PTV) and sparing of the heart in DIBH or FB IMRT was chosen for RT delivery. Wilcoxon signed rank test for non-parametric paired data was used to compare differences for the dependent variables from the FB and DIBH RT plans with a two-tailed significance level of 0.05 Results 15/19 screened patients completed all procedures in the study. 2 patients declined to participate and 2 could not give informed consent. All 15 patients tolerated and complied with the procedures and were treated with volumetric arc therapy technique in DIBH, 24-35 Gy, 2 Gy/fraction. The resulting dose estimates with FB and DIBH, respectively, are presented in table 1. There was no statistically significant difference in either the sizes of CTV or PTV, or in estimated PTV coverage (V95%), between FB and DIBH. Figure 1 illustrates the dose distribution for a patient in FB (right) and DIBH (left). A statistically significant reduction in favour of DIBH was demonstrated of the estimated mean doses to the heart and the absolute and relative volumes of the heart receiving ≥20 Gy (V 20 ), ≥10 Gy (V 10 ) and ≥5 Gy (V 5 ). The estimated absolute and relative volumes of the heart receiving the following dose levels: ≥D80%, ≥D60% and ≥D40% were also lower with DIBH than in FB. The estimated mean doses to the liver, duodenum, pancreas and spinal cord were statistically increased in DIBH, however the dose differences were small. Poster discussions: Poster discussion 22: Managing intra-fraction motion

FB Median (range

DIBH Median (Range)

P (Wilcoxons signed rank test)

Table 1

PTV_D 95 (%) 96.3(99.7-93.8) 96.1(98.6-92.6) 0.49 Heart Mean dose (Gy) 5.54(10.7-4.44) 3.2(7-1.17) 0.001 V20 (ccm) 54(106-16) 15(78-0) 0.001 V10 (ccm) 124(360-73) 46(214-0) 0.001 V5 (ccm) 205(538-125) 93(316-2) 0.001 BowelBag Mean dose (Gy) 12.2(19.7-3.91) 14.2(18.1-4.63) 0.36