ESTRO 38 Abstract book

S615 ESTRO 38

Conclusion Because of lower doses in the organs at risk, hybrid DMLC plans are preferred over hybrid VMAT plans. The differences between three or four DMLC beams were small in this study. During the clinical implementation we found that in some individual cases the use of four DMLC beams resulted in a better plan quality, in particular the conformality. Therefore, the use of four DMLC beams is preferred and currently used in our clinic. PO-1107 Institutional experience of adaptation from IMRT to VMAT in post-operative cases of carcinoma tongue K. Jain 1 , P. Patel 2 1 GCRI- civil hospital - Ahmedabad, Radiotherapy, Ahme, India ; 2 GCRI, Radiotherapy, Ahmedabad, India Purpose or Objective intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) are being used for treatment of head and neck cancers with discrepancy regarding superiority of one technique over the other. Our aim was to analyse various dosimetric parameters of IMRT and VMAT plans , to compare between the two techniques for treatment of operated cases of Ca.Tongue Material and Methods the study included 50 patients of IMRT and 50 patients of VMAT with patients matched with respect to the pathological stage and volume of PTV to minimise selection bias. Following parameters were assessed for comparison : Target coverageDose to Organ at Risk (OAR), conformity index (CI)Homogeneity index (HI)Monitor Unit (MU) deliveredTreatment time Results There was no significant difference regarding PTV coverage , HI and CI in both plans (p>0.05).Significantly more MUs were delivered in IMRT with average difference of about 30% more MUs as compared to VMAT (p<0.001). Treatment delivery time was reduced by around 8.5+/-4 minute for VMAT. No significant difference was found in dose to OAR like larynx, Parotid, brain stem and pharyngeal constrictor muscles. Spinal cord maximum dose was about 1+/- 0.4 Gy less in VMAT plans. Conclusion VMAT is emerging as mainstream treatment option for various sites like pelvis,lung and head and neck. No sufficient data is available for a specific sub site in head and neck cancers. In our study, we found that both VMAT and IMRT plans had comparable coverage of PTV . Sparing of OAR was marginally better with VMAT. But there was marked reduction in treatment delivery time with VMAT . Risk of intrafraction motion was reduced with VMAT with improved patient compliance . VMAT has become an institutional policy for patients of Ca. Tongue receiving adjuvant radiotherapy at our centre. We are exploring role of VMAT in other sub sites too. PO-1108 Comparison of VMAT plans for spine SABR according to optimization algorithm PRO and PO S. Son 1 1 Seoul national university hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To evaluate the quality of volumetric modulated arc therapy (VMAT) plans for stereotactic ablative radiotherapy (SABR) by optimization algorithm of progressive Resolution Optimizer (ver.13. 7.16, PRO) and Photon Optimizer (ver.13.7.16, PO). Material and Methods 20 cases of VMAT plans for spine SABR were retrospectively selected for this study. For each case, 2 kinds of VMAT plans with coplanar 2 full arcs were generated using Eclipse (Ver.13.7) and optimized with 2 different optimization algorithms the PRO and PO

interobserver variation in RTQA feedback and allow meaningful interpretation of RTQA results with associated outcomes. RTQA should implement prospective TVD review to identify deviations that require modification prior to treatment delivery.

Poster: RTT track: Treatment planning and dose calculation / QC and QA

PO-1106 Comparison of hybrid IMRT techniques for breast SIB irradiation C. Van den Beemd 1 , J. Penninkhof 1 , N. Holtzer 1 , M. Baaijens 1 , B. Heijmen 1 1 Erasmus MC, Radiotherapy, Rotterdam, The Netherlands Purpose or Objective Intensity modulated radiotherapy (IMRT) significantly reduces target dose heterogeneity in curative breast treatments. However, it is associated with an increase in whole body exposure to low doses from scattered and leakage radiation, and the reproducibility of the planned dose distributions during delivery (robustness) may become more challenging. To mitigate these problems, hybrid IMRT has been proposed, combining open fields with an overall low dose contribution of IMRT. This study compares four different techniques for hybrid planning of simultaneous integrated boost (SIB) treatment, using either a static-gantry Dynamic Multi-Leaf Collimation (DMLC) or VMAT beams in combination with two open beams. Material and Methods Plans were created for a dose prescription of 45.57 Gy to the whole breast and 55.86 Gy to the tumor bed volume in 21 fractions. All hybrid techniques employed two open tangential beams encompassing the whole breast, together contributing at least 70-80% of the prescribed dose. This dose was used as bias dose in optimization of the full dose while adding 1) three DMLC beams (hDMLC3), 2) four DMLC beams (hDMLC4), 3) one partial arc of 200 degrees (hVMAT1), or 4) three partial arcs (hVMAT3: 30, 200, 30 degrees). See Figure 1 for graphical presentations of the four treatment approaches. Figure 1: Beam arrangements of the DMLC/VMAT beams in the four investigated hybrid techniques. Five patients were included with scans both in breath hold and in free breathing. Target coverage, dosimetric parameters for organs at risk, conformity indexes and delivered Monitor Units are evaluated. All plans were created using Monaco version 5.11.01 (Elekta AB, Stockholm). Results Relative rankings of the hybrid approaches were similar for free breathing and breath hold planning CT-scans. For all patients, target coverage and homogeneity in the 4 hybrid approaches were clinically acceptable and comparable. The best conformality was always observed with VMAT. For the organs at risk hybrid plans with DMLC fields were superior to VMAT. Particularly the mean dose to the contralateral lung and contralateral breast was higher for VMAT plans with average differences of 0.6 Gy and 0.5 Gy, respectively. Differences between three or four DMLC beams were clinically insignificant. The hybrid DMLC technique with three beams (hDMLC3) resulted in the lowest amount of MU (374 MU), compared to 408 MU for hDMLC4, 426 MU for hVMAT1 and 593 MU for hVMAT3, respectively. 1. 2. 3. 4.