ESTRO 38 Abstract book

S710 ESTRO 38

While it is still early to assess differences in toxicity we are continuing follow up of patients in this cohort and hypothesise reduction in cardiotoxicity in the DIBH group. EP-1298 MR-guided delineation of target volumes in internal mammary lymph node radiotherapy M. Groot Koerkamp 1 , A.C. Houweling 1 , M.E.P. Philippens 1 , M. Albregts 1 , I.E. Van Dam 1 , J.M. Tijink- Felderhof 1 , E.M. Aalbers 1 , G.G. Sikkes 1 , J.J.W. Lagendijk 1 , H.J.G.D. Van den Bongard 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective In breast cancer patients with tumorpositive internal mammary lymph nodes (IMN) internal mammary chain irradiation is indicated. Target delineation requires visualization of the internal mammary (IM) vessels. On standard planning-CT (pCT) scans without intravenous (IV) contrast these vessels can be difficult to distinguish. The purpose of this evaluation was to assess if additional non- contrast MR imaging facilitates delineation of IM vessels. Material and Methods Since July 2017, all breast cancer patients with an indication for IMN irradiation at our department underwent 1.5T MR imaging. Patients were scanned in supine RT position, on a 5° wedge and an MR compatible arm support. The anterior receive coil was placed upon height adjustable coil bridges to avoid body contour deformation (Figure 1). Three fat-suppressed MR scans were acquired: a 3D T 1 -weighted spoiled gradient echo (T1w), a T 2 -weighted spin echo (T2w) and a respiratory gated and cardiac-triggered 3D balanced gradient echo (3Dcor) scan. The scans were registered to the pCT based on the region around the manubriosternal joint. Breast radiation oncologists (ROs) and an RO in-training supervised by a breast RO delineated the IM vessels on the pCT. The use of the MR scans was up to the RO. In September 2018, the ROs filled in a short questionnaire to evaluate the delineation process using the MR images in addition to the pCT. Furthermore, we evaluated the workflow with RTTs that performed the MR scanning and MR-to-pCT registration. Results Eighteen breast cancer patients underwent MR imaging for IM vessel visualization. MR scanning took ±30 minutes per patient, including patient set-up. Scan quality was high in all patients, except in one patient due to patient motion caused by pain. According to the RTTs, the 3Dcor scan was most difficult to acquire due to gating and triggering settings to be entered manually. Registration of the T1w and T2w scans to the pCT went well, whereas the 3Dcor scan could not always be matched because the sternum is not clearly visible in this scan. Six breast ROs and one RO in-training who delineated the scans of these patients evaluated their delineation process. All of them used the T1w scan and four ROs also used the T2w and/or 3Dcor scans. Five ROs (in training) reported that the MR scans (Figure 2) facilitated delineation of the IM vessels, provided that the scans were correctly matched to the pCT. However, three ROs reported no additional value of the MR scans when the IM vessels were already clearly visible on the pCT. One RO

Conclusion This hypofractionated regimen confirmed to be a feasible and safe treatment in reconstructed patients with both techniques. HT was associated with best target dose coverage, but at the cost of a greater OAR exposure to low doses and to higher mean doses.A longer follow-up is needed to assess the impact of low doses to healthy tissues. Both techniques allowed to achieve a good outcome, with a low failure rate. EP-1297 Heart of the Matter: A study of 112 left breast cancer patients treated with DIBH J. Windsor 1 , T. Ramanarasiah 1 , M. Burke 2 , J. Mitchell 2 1 Genesis CancerCare Queensland, Gold Coast, Tugun, Australia ; 2 Genesis CancerCare Queensland, Brisbane, Brisbane, Australia Purpose or Objective To quantitatively compare the dose cardiac structures, including the Left Anterior Descending (LAD) artery, receive when Deep Inspiration (DIBH) technique is employed compared with Free Breathing (FB) technique in a cohort of 112 patients with left sided breast cancer. Material and Methods In 2015, DIBH was first implemented across the 5 centres in our hospital network located in South-East Queensland, Australia. To assess the efficacy of the DIBH method, the first 112 patients with left sided breast cancer were analysed to quantitatively compare cardiac dose between FB and DIBH techniques. Each patient underwent two CT scans, one using standard FB technique and the second scan using Varian Real-time Position Management (RPM) respiratory gating system for DIBH technique. Treatment planning was performed per departmental protocol with tangential fields on each CT scan, using either IMRT or 3D conformal technique and dose fractionation at the discretion of the treating Radiation Oncologist (RO). Subsequent dose-volume histograms were calculated and compared using a paired t-test to evaluate mean dose to heart and Left Anterior Descending (LAD) coronary artery. All doses were compared at 2Gy per fraction equivalent. Results 112 patients were assessed for suitability to employ the DIBH technique. 10 patients were excluded from the study due to inability to breath hold or other technical reasons. A total of 102 patients, 101 female and 1 male, between the ages of 36 and 80 (mean 59) with left sided breast cancer were included for comparison. DIBH showed significant reduction in maximum cardiac dose (25.82Gy vs 38.29Gy, p<0.001), mean cardiac dose (1.21Gy vs 1.92Gy, p<0.001), cardiac V25Gy (1.78cc vs 8.46cc, p<0.001) and maximum LAD dose (15.8Gy vs 27.05Gy, p<0.001) compared to FB. A statistically significant increase in volume of lung receiving > 50% of the prescribed dose in patients treated with DIBH (168.54cc vs 106.3cc, p<0.001) was noted compared to FB however all DIBH lung doses were within acceptable dose limitations for Organs at Risk (OAR).

Conclusion DIBH provides a significant reduction in the mean cardiac dose and LAD dose in patients undergoing left sided breast irradiation. It was possible to implement this technique in majority of the patients and this technique has now been successfully integrated into routine clinical practice.