ESTRO 2020 Abstract book

S1087 ESTRO 2020

organ positions and volumes change significantly during the course of treatment. PO‐1856 VMAT class solution for radiotherapy planning of esophageal carcinoma using Pinnacle Auto‐Plan L. Abbenhuis 1 , L. Tax 1 , P. Van Kollenburg 1 , M. Kunze- Busch 1 , H. Rütten 1 1 Radboud university medical center, Radiation Oncology, Nijmegen, The Netherlands Purpose or Objective The aim of the study was to find a class solution for a VMAT technique for esophageal carcinoma employing the Pinnacle Auto-Plan (AP) module. Material and Methods 22 Patients with esophageal cancer treated with a conventional technique(A-P-L-R) at our institute were re- planned with 10 MV VMAT-arcs. The treatment plans were created in Pinnacle 16.0 (Philips Radiation Oncology Systems, USA) using the Auto-Plan module. The clinical target volume was defined by adding a margin of 3 cm in superior-inferior (SI) direction and 1 cm in lateral, ventral and dorsal direction around the gross tumor volume of tumor and a margin of 1 cm for the pathological nodes. Planning target volume (PTV) was created by adding a 1 cm margin in SI direction and 5 mm margin to the lateral, dorsal and ventral direction. The following organ at risk (OARs) were contoured: lungs, heart and spinal cord. Depending on the location of the tumor, kidney and/or spleen were also contoured. Prescription dose was 41.4 Gy in 23 fractions, 5 times a week. Dose constraints to the OARs were defined according to local guidelines (table 1). The quality of the plans was evaluated by comparing the VMAT-AP plans with the conventional plans regarding target coverage, conformity index (CI) and homogeneity index (HI) of the PTV and clinical dose to the OARs.

Conclusion The use of SpaceOAR reduced rectum doses greatly, if the treatments were assumed to be localized perfectly. If the isocenter was shifted 5 mm posteriorly, rectum doses with the gel were still comparable, or even lower in high-dose area, to rectum doses without the gel but with assumed perfect treatment localization. In the light of these results, the use of SpaceOAR protects the rectum in the presence of treatment localization errors within 5 mm and in regard to rectum doses, could compensate for the lack of intrafraction motion monitoring in prostate SBRT. PO‐1855 Dosimetric effects of online adaptive replanning for prostate radiation therapy on 1.5T MR‐ Linac M.C. Lee 1 , W.M.R. Luo 1 , K.C. Chan 1 , Y.Y. Chan 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China Purpose or Objective Online 1.5T MR imaging on the MR-linac (MRL) makes it possible to acquire daily magnetic resonance imaging (MRI) onboard before, during and after each treatment session. It provides better soft tissue visualization compared to conventional imaging modalities and it facilitates online adaptive daily replanning using daily MR images. The aim of this retrospective study was to evaluate whether online adaptive replanning of prostate radiation therapy rendered beneficial dosimetric results compared to simulated patient positioning correction based on the target volume on the 1.5T MRL treatment system. Material and Methods The first prostate cancer patient treated on MRL at HKSH was used for analysis retrospectively. The eight adaptive plans (plan ATS ) generated using adapt to shape workflow were used for comparison. For positioning correction plans (plan ref ), each daily pre-treatment MR (MR pre ) and reference CT (CT ref ) were re-registered on a third party software. The registration was performed by translational correction based on the best matches of the prostate and seminal vesicle. Then reference plan dose was propagated. The volumes of planning target volume (PTV), bladder and rectum were recorded and evaluated for all plans. The dose-volume histogram (DVH) parameters of the PTV (D 98% , D 95% , D 2% , D mean ), bladder and rectum (V 60 , V 50 , V 40 , D mean , D min ) were recorded and analysed. Results With online replanning (plan ATS ) , significant dose sparing of bladder (V 50 , V 40 , D mean , D min ) (p<0.05) were obtained without compromising PTV coverage. The rectal dosimetric parameters for V 50 and V 40 were significantly higher (p<0.05) for plan ATS compared to plan ref . Significant changes (p<0.05) in volumes of bladder and rectum were noted (Fig.1).

Results The initial general beam setup consisted of 2 full arcs. During the development of the technique a second setup was found to be superior for tumors located in the proximal and mid esophagus. In this template the 2 full arcs were divided in two parts were the lateral position of the beams (54°-138°CW and 218°-306°CW) was avoided to spare lung tissue and avoid the shoulders (figure 1). All of the VMAT-AP plans had similar PTV coverage compared to the clinical conventional plan, with at least a coverage of 95% dose to 98% of PTV volume. The VMAT-AP plans yielded a significantly higher CI and lower HI than the conventional plans. The mean lung dose and the V20 significantly decreases in the VMAT-AP plan. Due to VMAT- AP a larger volume of the lungs receives 5 Gy compared to the conventional plan, however the difference was small and not significant. With VMAT-AP it is possible to reduce the heart dose (V30Gy) considerably (table 1).

Fig.1 All contours from the 8 ATS plans Conclusion

In this retrospective case study, online replanning for prostate radiation therapy treatment yielded beneficial dosimetric values for bladder. Although plan ATS did not clinically improve the rectal dose, the target volume dose retained comparable to plan ref . Since significant changes in bladder and rectum volumes were observed, online adaptive replanning is clinically advantageous when the