ESTRO 38 Abstract book

S998 ESTRO 38

Conclusion It is possible to develop cardiac avoidance treatment planning for NSCLC patients undergoing VMAT treatment (55Gy in 20#) without significantly compromising other OARs or PTV coverage. EP-1840 comparing the dosimetric impact of omentum spacer in carbon-ion, proton and photon radiotherapy M. Yamada 1 , H. Sato 1 , T. Ono 2 , Y. Ieko 1 , T. Kanai 1 , N. Yano 1 , H. Akamatsu 1 , M. Harada 1 , M. Ichikawa 1 , Y. Kikuchi 2 , K. Nemoto 1 1 Yamagata University Faculty of Medicine, Department of Radiation Oncology, Yamagata, Japan ; 2 Southern Tohoku Proton Therapy Center, Department of Radiation Oncology, Koriyama, Japan Purpose or Objective This study was an in silico planning study to compare carbon-ion (C-ion), proton and photon radiotherapy (RT) plans regarding dose reduction of the gastrointestinal (GI) tract by the greater omentum spacer (GO spacer). Material and Methods A total of 10 patients who received surgical spacer placement for abdominal and pelvic tumors were included in this study. In all patients, the tumor was adjacent to the GI tract, and GO spacer was inserted between the tumor and the surrounding GI tract. Simulation plans were created on pre-spacer CT and post-spacer CT for C-ion, proton and photon RT respectively. All plans prescribed 70 Gy (relative biological effectiveness (RBE) equivalent) delivered in 35 fractions to the PTV. They were normalized so that at least 95% of the PTV received the prescribed dose. The dose constrain for the spinal cord was maximum dose (Dmax) < 45Gy. All plans were created with the dose to the GI tract reduced as much as possible under the condition of meeting the dose constrains for the PTV and spinal cord. The GI tract to be evaluated was defined as that most adjacent to the PTV. We aimed to satisfy the dose constraints of the minimum dose received by the most exposed 2 cc volume of the organ (D2 cc) of the GI tract <50 Gy. C-ion RT plans and proton RT plans were calculated by a spot scanning technique and photon RT plans were calculated employing fixed-field intensity- modulated radiation therapy (IMRT). We tested the difference in mean values of the D2cc, the volume receiving 10 Gy or more (V10), V15, V20, V25, V30, V35, V40, V45, V50, V55, V60, V65 and V70 of the GI tract on pre-spacer CT and post-spacer CT for all three RT modalities. Statistical analysis was performed by the paired t-test. Results GO spacer significantly reduced D2 cc of the GI tract for C-ion RT (mean±SD: 65.4±5.2 vs 6.4±2.4 Gy, p<0.0001), proton RT (63.3±6.1 vs 6.4±2.9 Gy, p<0.0001) and photon RT (58.8±4.5 vs 24.4±5.2 Gy, p<0.0001). Reduction of D2 cc of the GI tract by GO spacer was more effectively achieved by C-ion RT and proton RT than by photon RT (C- ion vs photon p=0.001, proton vs photon p=0.002). There was no significant difference between C-ion RT and proton RT in reduction of D2 cc of the GI tract by GO spacer (C- ion vs Proton p=0.992). In one patient on photon RT plan, D2 cc of the GI tract did not meet the constrain. The GTV of that patient was the left acetabular and sacral bone metastasis of uterine body cancer. In that patient, the GI tract most adjacent to the PTV on post-spacer CT was sigmoid colon and the separation distance from the PTV to the GI tract was 0.43 cm and D2 cc of the GI tract of C-ion RT, proton RT and photon RT were 22.4, 29.2 Gy and 61.3 Gy respectively. Conclusion GO spacer significantly reduced D2 cc of the GI tract for C-ion RT, proton RT and photon RT. Reduction of D2 cc of the GI tract by GO spacer was more effectively achieved by C-ion RT and proton RT than by photon RT.

MLC plans (1,31), but the difference was not significant. The MLC plans had significantly (p=0,02) lower average maximum doses for the shell (1282 cGy) compared to IRIS TR plans (1473 cGy), while IRIS plans (1281 cGy) showed similar maximum dose in the lower dose region. The D 0,04ccm , D 0,2ccm for Optic Pathway and the D 0,04ccm and D 0,5ccm values for Brainstem were 733 cGy, 491 cGy, 761 cGy, 593 cGy with MLC and 744 cGy, 549 cGy, 828 cGy, 630 cGy with IRIS and 785 cGy, 578 cGy, 817 cGy, 633 cGy with IRIS TR, without significant differences. Conclusion The InCise 2 MLC offers shorter treatment time, less MU, higher conformality for low doses with same or better organs at risk protection. However, the IRIS plans had a slightly better homogeneity and conformity index values. The treatment time of IRIS TR plans are still more than MLC plans’ if we insist on clinically acceptable plans. The further reduction of treatment time of IRIS TR plan results in a much worse plan quality, except with large spherical targets where the treatment time of IRIS plans could be reduced to the level of MLC plans, maintaining the same plan quality. EP-1839 Development of Cardiac Avoidance Treatment Planning for Non-Small Cell Lung Cancer Patients S. De Vos 1 , C. Rowbottom 1 , M. Gilmore 1 1 Clatterbridge Cancer Centre, Radiotherapy, Birkenhead, United Kingdom Purpose or Objective Lung cancer is the deadliest type of cancer, and eight in 10 lung cancers are non-small cell lung cancers (NSCLC). There is evidence that cardiac dose contributes to reduced overall survival in NSCLC patients. If cardiac dose can be minimised in the treatment of these patients, without impacting on planning target volume (PTV) dose, or other organs at risk (OARs), this may improve their prognosis. This study will aim to develop cardiac avoidance treatment planning techniques for NSCLC patients. Material and Methods Twenty-three patients who have received the standard Clatterbridge Cancer Centre (CCC) volumetric modulated arc therapy (VMAT) NSCLC protocol of 55Gy in 20 fractions (#) were included in the study. Three new plans were created retrospectively: a second plan (B) was morphed from the original delivered plan (A), where the cardiac dose objectives (mean heart dose (MHD), V5Gy and V30Gy) were pushed harder in the optimiser. For the third plan (C), the full or half arcs were replaced by partial arcs, with cardiac avoidance sectors, in a coplanar orientation. The fourth plan (D) has the same partial arcs as the third plan, but in a noncoplanar orientation, to further drive heart dose down. For a small subset of patients, a more extreme noncoplanar arc configuration (seven partial arcs) was created. Plans were compared and evaluated with statistical analysis, using a range of PTV and OAR dosimetric parameters. Results MHD, V5Gy and V30Gy were improved for all three new plans, compared with the clinical plan (A). The largest improvements in heart dose were observed for the noncoplanar partial arc geometry (D), followed by the coplanar full/half arc geometry with tighter planning objectives (B). MHD reduced by 32.4% for Plan D, compared with Plan A (P < 0.001), whereas heart V5Gy and V30Gy saw a reduction of 26.1% (P < 0.001) and 38.8% (P < 0.001) respectively. For Plan B, compared to A, the mean reduction in MHD, heart V5Gy and V30Gy was 25.7% (P = 0.001), 17.6% (P = 0.002) and 35.7% (P < 0.001) respectively. Oesophagus dose increased significantly for Plan B (2.2%, P = 0.009), and Paddick Conformity Index (CI) decreased significantly for Plan D (3.3%, P = 0.02) but on the whole OAR doses and PTV coverage either improved or remained similar between plans.