Abstract Book

S811

ESTRO 37

1 National Cancer Center, Proton Therapy Center, Goyang-si- Gyeonggi-do, Korea Republic of 2 Asan Medical Center, 3Department of Oncology, Seoul, Korea Republic of 3 National Cancer Center, Center for Colorectal Cancer, Goyang-si- Gyeonggi-do, Korea Republic of Purpose or Objective To compare the patterns of failure between rectal cancer patients with negative and positive circumferential resection margin (CRM) after surgery following preoperative chemoradiation. Material and Methods Of 944 stage II–III rectal cancer patients treated with radical surgery following preoperative concurrent chemoradiation, 74 patients (7.8%) showed positive CRM. Each 72 patients from the negative and positive CRM groups were identified by propensity score matching and compared in terms of clinical outcomes, including the local control and patterns of failure. Local failure was defined as recurrence at the anastomosis site or adjacent to the mesorectal fascia. Results The median follow-up duration was 46 months (range 4– 155) for the population with propensity score matching. There were 30 initial recurrences in the negative CRM group and 43 in the positive CRM. The 5-year overall survival with positive CRM was inferior to those with negative CRM without statistical significance (55.8% vs . 67.5%, p = 0.186). No difference was seen in 5-year local recurrence-free survival (89.6% vs . 93.4%, p = 0.442), but statistically significant difference in relapse-free survival (39.1% vs . 53.9%. p = 0.042) and distant metastasis-free survival (43.3% vs . 59.4%, p = 0.069). As the initial failure pattern, distant metastasis was dominant in both groups, and isolated local recurrence was observed in none with positive CRM and in two with negative CRM ( p = 0.497). Conclusion Considering that distant metastasis was the major pattern of failure regardless of CRM involvement, the additional radiotherapy would not be necessary in rectal cancer patient with positive CRM after surgery following preoperative chemoradiation. EP-1495 Cone beam computed tomography for movement evaluation of GTV and mesorectum in rectal cancer. C. ROSA 1 , L. Caravatta 1 , M. Trignani 1 , F. Patani 1 , C. Di Carlo 1 , C. Vecchi 2 , S. Marcucci 1 , D. Genovesi 1 1 SS. Annunziata Hospital- “G. D’Annunzio” University, Department of Radiation Oncology, Chieti, Italy 2 Tecnologie Avanzate, Srl, Torino, Italy Purpose or Objective A dose response relationship in rectal cancer radiotherapy has been reported, with a greater interest in dose intensification on small boost volume. When conformational techniques, as IMRT and VMAT, are used an accurate delineation of gross tumor volume (GTV) and an appropriate evaluation of target movement are recommended. MRI and online cone-beam CT (CBCT) could represent two valid methods for these purposes. This study aimed to use CBCT for the evaluation of internal movement (IM) of GTV and mesorectum, in patient treated with neoadjuvant radio-chemotherapy for rectal cancer. Material and Methods Seventeen patients, (M:12, F:5), with II-III stage rectal cancer, underwent CT scan simulation in prone position

with controlled bladder filling. GTV (tumor site and corresponded rectum) and mesorectum (from the sacral promontory to the level where the levator ani muscle inserts into the rectal wall) were delineated on MRI imaging co-registrated with CT scan simulation. CBCTs were performed once a day during the first 5 fractions, then once or twice a week during the all treatment, by Elekta X-Ray volume imaging system (XVI). The IM was estimated for GTV on all CBCTs co-registrated with CT scan simulation. Bladder was also delineated to evaluate the impact of its volume on GTV and mesorectal IM. Co- registrations were performed on RayStation platform (RaySearch Laboratories, Stockolm, Sweden) by bone landmarks and corrected for set-up error. IM evaluation was obtained as mean shift in left and right (L-R), postero- anterior (P-A) and cranio-caudal (C-C) directions and volumes variability were calculated by DICE index (Figure1).

Results A total of 158 CBCTs were performed and retrospectively analyzed. Detailed values are reported in Table 1. Measured movements of the GTV were -0.13cm (+/- 0.09) and 0.14cm (+/- 0.08) for L-R direction. In P-A direction, mean shifts were 0.28cm (+/- 0.24) and -0.32cm (+/- 0.24), in C-C direction were 0.10cm (+/-0.10) and - 0.17cm (+/- 0.13). Mesorectal IM resulted in -0.11cm (+/- 0.07) and 0.13cm (+/- 0.04) for L-R direction, 0.29cm (+/- 0.19) and -0.24cm (+/- 0.14) in P-A direction, and 0.19cm (+/- 0.15) and -0.19cm (+/- 0.09) in C-C direction. Concerning bladder, movements were less than 0.6cm, except for caudal shift calculated as -1.05cm (+/- 0.54). Mean DICE index for GTV, mesorectum and bladder was 0.59, 0.75, 0.50, respectively.

Conclusion In our study, GTV and mesorectum IM, evaluated in patients in prone position, were less than 4mm in all directions. Although bladder volume variability, as shown by a worst DICE index, this could be not related to an increased movement of target or mesorectum. CBCT resulted effective for IM assessment and could represent a valid method for appropriate treatment intensification. EP-1496 The natural history of pulmonary micronodules evidenced at initial CT scan in pts affected by LARC S. Montrone 1 , F. Pasqualetti 1 , R. Morganti 2 , B. Manfredi 1 , C. Laliscia 1 , A. Gonnelli 1 , F. Orlandi 1 , A. Cristaudo 1 , D. Baldaccini 1 , A. Molinari 1 , A. Sainato 1 , F. Paiar 1 1 OSPEDALE SANTA CHIARA, Radiotherapy, PISA, Italy 2 OSPEDALE SANTA CHIARA, Biostatistical Consulting, PISA, Italy