ESTRO 2020 Abstract Book

S1091 ESTRO 2020

3 University of Manchester, Division of Cancer Sciences- Faculty of Biology Medicine & Health, Manchester, United Kingdom ; 4 University College London Hospital, Radiotherapy, London, United Kingdom Purpose or Objective INTERLACE (NCT01566240) is an international phase III multicentre trial of weekly induction chemotherapy followed by standard chemoradiation versus standard chemoradiation alone in patients with locally advanced cervical cancer. As well as a trial protocol, detailed radiotherapy guidelines were produced with a quality assurance (QA) programme to ensure compliance at the 37 participating centres. The guidelines mandated GEC- ESTRO recommendations for image-guided adaptive brachytherapy (IGABT) and prescribing to the high-risk CTV (HRCTV). This study assesses adherence to these recommendations. Material and Methods As part of the trial QA programme brachytherapy centres were required to submit a brachytherapy dummy run (a patient already treated at their centre) for central review prior to trial participation. Both contouring and planning were reviewed to assess their standard brachytherapy technique. The following parameters have been evaluated: use of MR and whether scanned with implant in situ; HRCTV and intermediate-risk CTV (IRCTV) definition; and dose conformity (HRCTV D 90 , HRCTV V 100 , and V ref ). Results Of 34 institutions providing a brachytherapy service, 23 stated they performed IGABT, prescribing and optimising to HRCTV D90. However, 3 of these centres did not use MR in this process and 1 performed MR without the implant in situ . 8 centres did not follow GEC-ESTRO HRCTV delineation recommendations (see Figure 1). In 2 of the 11 centres defining an IRCTV this was not a direct expansion of the HRCTV and in a further 2 the vagina was not edited out of the expansion. The plan was assessed for dose conformity around the HRCTV. Only 9 centres did this adequately. Most of the uterus was included within the prescription isodose in 14 centres and a large section of the vagina in 7 centres, instead of conforming to the HRCTV definition (see Figure 2).

shielding (figure 1B, E). It was also found that care should be taken in the shielding design as shadowing effects are set by the shape of the shielding and cannot be easily modulated out. The shielded applicator designs investigated here, however, can provide radially symmetric dose with isodose line (IDL) deviations less than 0.5 mm from circular (figure 1A, C). The rounded design type was also further investigated with the option of a modular second shielded region that can be placed at several angles allowing further modulation (figure 2). These designs were also chosen with Treatment delivery time in mind. Treatment times for these shielded designs were found to be only 1.25 to 1.35 times longer than a six- channel unshielded cylinder (when irradiating the full circumference to the prescription dose).

Fig. 1 A-D The shielded regions (white) are 8g/cc, elsewhere is water (1 g/cc). IDL ‘s calculated using AXB. The calculations were for 12 dwell positions spaced 0.5 cm over 6 cm. E-F are dose profiles calculated from the 2D analytical model. The 0.1 mm resolution of the 2D model in E, F makes the heterogeneity more visible than the AXB calculations, which have a 1 mm dose grid.

Fig 2 A-D Plan with rectum and urethra structures. E-G show other ways the shielding can be used. Conclusion The 2D calculation methods provide a simple way to rapidly evaluate shielding designs. Verification with TOPAS Monte Carlo and AXB calculations demonstrate good agreement with both calculations showing that significant dose shaping, and organ at risk (OAR) sparing can be achieved without compromising the plan in regions that require full dose. OC-1037 Real world interpretation of GEC-ESTRO image-guided brachytherapy recommendations for cervix cancer P. Diez 1 , J. Bourner 2 , A. Sharp 2 , P.J. Hoskin 1,3 , M. McCormack 4 1 National Radiotherapy Trials Quality Assurance Group, Mount Vernon Cancer Centre, Northwood, United Kingdom ; 2 Cancer Research UK & UCL Cancer Trials Centre, Lung & Gynae Group, London, United Kingdom ;