ESTRO 2020 Abstract book

S1098 ESTRO 2020

fields (left).

Events (version 5.0). In addition, the positioning accuracy of all treated targets was evaluated. Results For all patients, the median dose was 22 Gy prescribed at 80%; median gross total volume (GTV) and planning target volume (PTV) were 0.30 and 0.48 cm 3 , respectively. Median V95 (the volume of the PTV covered by 95% of the prescription dose) was 98.5%. With a median follow-up of 11 months, 1-year survival and local control rates were 66% and 87%. The treatment was safe, with Grade 2 or 3 toxicity occurring in five patients. Neurocognitive function remained relatively stable after SRS. A significant decline of neurocognitive functioning (assessed by HVLT-R) occurred in 11% and 6% of patients after 6 and 12 months, respectively. All but two patients were independent at 12- month follow-up. A significant variation of V95 (>5% or resulting in a V95<95%) in at least one lesion was observed in 27% of patients, although GTV coverage was maintained for all treated lesions using a 1 mm GTV-to-PTV margin. Conclusion SIMT DCA SRS represents an effective and safe approach for patients with 10 or more brain metastases able to maintain the pretreatment neurocognitive function in the majority of patients. A 1 mm GTV-to-PTV expansion is recommended to ensure dose coverage of all treated lesions. PO‐1874 Improving Treatment Accuracy of Cranial Boost Radiotherapy (RT) with Total Body Irradiation (TBI) I.Z. TAN 1 , J. Francis 2 , G. Smyth 2 , H. Burland 2 , S. Eagle 2 , H. Mandeville 1 1 The Royal Marsden, Pediatrics/ TYA/ Hemato-Oncology, Sutton, United Kingdom ; 2 The Royal Marsden, Radiotherapy, Sutton, United Kingdom Purpose or Objective Patients with relapsed acute lymphoblasticleukaemia (ALL) with central nervous system (CNS) involvement receive additional cranial boost radiotherapy prior to TBI as part of their conditioning regimen for haematopoetic stem cell transplant (HSCT). RTT-led virtual simulation and treatment with lateral opposed fields has traditionally been used. We decided to evaluate whether a physician- led contouring of the whole brain as the clinical target volume (CTV) improves coverage of critical areas such as the cribriform plate (CP) and optic nerves (ON), compared to 2D virtual simulation using bony landmarks. Material and Methods Planning CT brain (1mm slices) were performed with the patient supine in a 3 point thermoplastic head mask. For the physician-led contoured whole brain CTV (including the CP and ON) were manually contoured; a 3 mm geometric margin applied from CTV to PTV. A further 6 mm geometric margin PTV to field edge to account for penumbra was done using multileaf collimator shaping for lateral opposed fields. For the bony landmarks referenced field, the outer canthus of the eyes, the CP and C2/3 cervical vertebral junction were used. The 95% isodose coverage of the PTV at the CP and ON were visually assessed for both. Results Retrospective data collection from Jan 2015 - Oct 2018 yielded 15 patients who received cranial boost RT in addition to TBI. Of these only 9 patients plans were available for this planning study. For the physician-led contoured PTV, the 95% isodose coverage of both the ON and CP was improved in 6 patients (67%) and CP or ON coverage alone was better in 2 patients (22%). In 8 out of 9 patients (89%) evaluated, physician-led contoured PTV fields (right) resulted in an improvement in one or both CP and ON coverage compared to bony landmarks referenced

Conclusion We conclude that physician-led contoured PTV for cranial boost improved the 95% isodose coverage of the PTV at the ON and CP compared to bony landmarks referenced PTV. Further work is planned to develop autocontouring of structures to streamline this process and to incorporate this in a RTT–led clinical workflow, including methods to optimise the coverage of the upper cervical spinal canal and adjacent vertebrae which is crucial in growing children. PO‐1875 Is a single anterior field sufficient to treat regional nodes in breast cancer? H. Almasri 1 , P. Jain 1 , S. Pace 1 , S. Allen 1 , S. Kumar 1 1 Leeds teaching hospitals NHS trust, Radiation Oncology, Leeds, United Kingdom Purpose or Objective The use of axillary radiotherapy in adjuvant treatment of breast cancer has been recently increasing mainly as an alternative to axillary dissection after positive sentinel lymph nodes in early stages. However, as contouring nodal volumes can be time consuming, most centres in the UK still treat with standard fields based on bony landmarks. This is usually done utilizing a single anterior field. The dose distribution resulting from this technique might not be always adequate. The aim of this study is to identify a subset of patients in which nodal irradiation using a single field would produce insufficient dose coverage even when high photon energy is used. Material and Methods CT data sets of thirty patients who were treated with 10 MV single anterior field were retrieved. The nodal target volume was retrospectively contoured according to the ESTRO consensus guideline. A sub-volume consisting of the PTV encompassed within the anterior field was created for the purpose of dose evaluation. DVH statistics were generated to evaluate the dose coverage from existing fields that were used in patients’ treatment where a dose of 40 Gy was prescribed to 95% (approximately 4 cm depth). The maximum axillary separation was measured consistently as the anteroposterior diameter just medial to the humeral head on CT slices. The maximum depth of PTV was measured from the skin to the deepest point of PTV along the beam axis. Coverage was considered adequate if at least 90% of the target volume received at least 90% of the prescribed dose (V36>90%). Results Inadequate PTV coverage (V36<90%) was found in 15/30 patients (50%). In 14 out of these 15 patients (93%), the measured separation was ≥18 cm, (median=19.4,