ESTRO 37 Abstract book

S1223

ESTRO 37

planning is usually limited to varying the treatment beam angle, energy and field shape. An additional mechanism to modify the treatment plan involves the placement of uniform thickness bolus on the patient surface which increases the dose to the superficial part of the tumour volume and helps reduce the dose to the underlying normal tissues. However, the degree to which the dose conforms to the deeper section of the target is often poor. The consequence is that electron therapy often delivers an unnecessarily high dose to immediately underlying structures and tissues. Improving plan quality by overcoming these ERT limitations can be achieved by implementing modulated electron radiation therapy (MERT). Implementing MERT would typically involve modifying the linac head which is expensive and disruptive to introduce. An emerging alternative offering a cost-effective means to implement MERT is to produce modulated thickness bolus created using a 3D printer with specialised materials specifically designed for radiotherapy treatments. Material and Methods To implement MERT using 3D printed bolus, a new software package called 3DBolus was used. Bolus thickness was optimized by considering the shape of the PTV and the range of electron beam, inhomogeneities within the calculation volume and hotspots due to electron scatter.Two MERT test plans and a number of clinical cases were generated and compared with plans created by independent planners. Plans were compared by evaluating dose conformity, target homogeneity, and the dose to any underlying OAR. For the clinical cases, a Radiation Oncologist was asked to choose between the 3D printed MERT bolus plans and the standard clinical electron plans. Results Dose conformity improvements were found in all MERT cases and each printed boluses conformed well to the patients surface. For all cases, the target d max increased but was found to be an acceptable trade-off considering the improved conformity and underlying OAR sparing. Compared to other bolus fabrication techniques, the patient was not required to attend for the bolus fabrication offering practical advantages in the clinic. Lastly, the benefits of using modulated bolus decreased with increasing PTV depth due to the extra scattered electrons generated by the modulated bolus. Conclusion We found that using 3D printed technology was a cost- effective approach for clinics wanting to implement MERT. Modulated bolus improved our electron planning capabilities and provided preferable plans compared to our standard electron planning approach. The result is a customized dose distribution within the patient that both conforms to all parts of the target volume and further minimizes dose to any distal organs at risk compared to standard electron planning.

EP-2209 Results of a multicentre dosimetry audit using a respiratory phantom within the EORTC Lungtech trial M. Lambrecht 1 , J.J. Sonke 2 , U. Nestle 3,4 , M. Guckenberger 5 , H. Peulen 1 , D. Weber 6 , M. Verheij 2 , C.W. Hurkmans 1 1 Catharina Hospital, Department of Radiation Oncology, Eindhoven, The Netherlands 2 The Netherland Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands 3 Kliniken Maria Hilf, Department of Radiation Oncology, Mönchengladbach, Germany 4 University Hospital Freiburg i. Br., Department of Radiation Oncology, Freiburg, Germany 5 University Hospital Zürich, Department of Radiation Oncology, Zürich, Switzerland 6 Paul Scherrer Institute- ETH Domain, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective In 2014 the EORTC launched the phase II 22113-08113— LungTech trial to assess safety and efficacy of SBRT for centrally located NSCLC. To improve treatment safety, a comprehensive RTQA procedure has been implemented including end-to-end tests investigating the dosimetric accuracy of IMRT and VMAT under static and respiratory conditions. Material and Methods Eleven centres were audited using a CIRS008A phantom. The phantom was successively scanned using two spherical target of 15mm and 25mm diameter. For each insert a 3DCT was performed in static condition and for the 15mm insert a 4DCT was acquired with the phantom following a cos 6 function (3s period/15mm amplitude). The CT scans were acquired according to the institutions local protocols. A treatment plan was made for all three scans according to the trial guidelines (Figure 1). Prior to phantom measurements a beam output check was performed in water under reference conditions. The plans were then measured both using EBT3 films (12,5x5 cm) placed in the sagittal plane, and a 0.04cc ionization chamber (Scanditronix/Wellhoffer Inc.) traceable to a secondary standard dosimetry laboratory. The film analysis was done in RIT113, version6.3 (RIT Inc). Global gamma analyses were performed using film dose as reference, a normalisation at the centre of the sphere, a dose threshold at 20%Dmax and 3%dose/3mm deviation as agreement criteria. A wilcoxon test was performed to assess the difference between the dose deviations found using the 15mm insert both in static and in dynamic conditions.