Poster abstracts of the 14th Annual British Thoracic Oncology Group Conference 2016 / Lung Cancer 91, Suppl. 1 (2016) S1–S71
124
Dosimetric evaluation of adaptive planning during radiotherapy (ART) in radical lung to ensure tumour coverage and/or minimise doses to organs at risk – case discussion
A. Morenc 1 , K.A. Johnson 2 , T. Sridhar 3 , S.I. Ahmed 3 , L. Aznar-Garcia 1 . 1 Oncology, University Hospitals of Leicester, Leicester, United Kingdom; 2 Cancer Studies, University of Leicester, Leicester, United Kingdom; 3 Oncology, University of Leicester Hospitals, Leicester, United Kingdom Introduction: This case study discussion presents patients with changes in GTV volume discovered on daily CBCT scans during the course of radiotherapy; review of changes in volumes and actual dose given to tumour and organs at risk (OARs). Some people has hypothesize that first PTV should be covered with enough doses to sterilize the microscopic disease. Dose coverage with 50 Gy in areas of suspect microscopic disease (MD) as a result of adapting radiotherapy plans to changes in volumes was also analyzed. Methods: Four patients with NSCLC were treated with ART either to assure the coverage of the PTV or to reduce the doses to OARs. All patients had VMAT plans and daily CBCT that detected changes in the size of GTV. Three patients had tumour shrinkage and one patient had increase in tumour size. All patients had CT rescan, a new GTV was drawn and a new PTV created with the same margin as on the original plan. No additional volumes were used to ensure coverage of suspected MD, which in this study was defined as the volume difference between original GTV and PTV. New plans were prepared and treatment continued on old plan until new one was ready. Results: Summary of finding are given in Table 1. The idea of covering potential MD is not applicable in patient 1, it seems adequate in two cases (patient 3 and 4). Conclusion: It is important to have strategy to adapt treatment to reflect changes in tumour volume reported in CBCTs, adaptation of plan during the course of radiotherapy allows reducing dose to OARs, which could potentially allow escalation of dose to the tumour. Further studies would be needed to assess if adequate coverage of MD is needed. Disclosure: All authors have declared no conflicts of interest.
125
S45
Planning study comparing the use of photon radiation therapy to proton therapy for superior sulcus tumours (SSTs)
S. Wong 1 , J. Alshaikhi 2 , N. Lalli 3 , P. Bhudia 1 , D. D’Souza 1 , R. Amos 3 , G. Royle 4 , R. Mendes 1 . 1 Oncology, University College London Hospital, London, United Kingdom; 2 Department of Medical Physics, University College London, London, United Kingdom; 3 Medical Physics, University College London Hospital, London, United Kingdom; 4 Medical Physics and Biomedical Engineering, University College London, London, United Kingdom Introduction: SSTs represent 5% of bronchogenic carcinomas and are uniquely challenging due to characteristic local invasion of the apical chest wall and thoracic inlet. Target volume (TV) coverage using conformal photon therapy (CPT) is often compromised due to proximity of the spinal cord. Pencil beam scanning (PBS) enables target dose delivery with minimal deposition to surrounding normal tissue due to the sharp dose fall-off of its Bragg peak. The aim is to conduct a retrospective radiotherapy planning study comparing CPT with volumetric arc therapy (VMAT) and PBS for patients with SSTs to see if the therapeutic ratio improves with PBS. Methods: Patients with SSTs treated in the last 5 years are identified. Tumour motion is assessed using 4D CT planning scans. CPT, VMAT and PBS plans are generated for each and reviewed independently by physicists experienced in their respective fields. For PBS, dose distribution at extreme breathing phases are assessed on verification plans; and dosimetric uncertainty due to motion and range uncertainty is quantified by robustness analysis. TV and organ at risk (OAR) dosimetry are compared for each. Results: TV coverage with PBS is comparable to VMAT but PBS more significantly spares dose to lungs (51.00%, 25.64%, 8.25% and 25.60% reduction in lung V5, V10, V20 and mean lung dose, respectively). Achieving a low oesophageal V35 (4.99% versus 2.02%) and heart V40 (4.99% versus 4.86%) is possible with both PBS and VMAT. Verification plans show robust clinical target volume (CTV) coverage. Robustness analysis shows that in the worst case scenario, 95% CTV receives more than 95% dose and no OAR tolerances are exceeded. Conclusion: These are preliminary results based on 1 patient showing that for selected cases, PBS can achieve TV coverage comparable to VMAT whilst sparing OARs. Further cases are being planned and analysed, and a PBS planning protocol is being developed. Disclosure: All authors have declared no conflicts of interest.
Abstract 124 – Table 1
Dose* Changes in volumes and doses Change in GTV volume (%) Change in PTV volume (%) Change in dose to lung V20Gy(%) Change in dose to lung V5Gy (%) Change in mean lung dose to contra lateral lung (%) Change in oesophagus mean dose (%) Combined dose coverage from old and new plan Number of fractions treated on old plan Number of fractions treated on new plan Total dose coverage 99% volume of CTV (%) Total dose coverage 99% volume of PTV (%) Total lung dose V20Gy (%) Total lung dose V5Gy (%) Total oesophagus mean dose (Gy) Volume of MD getting 50Gy (%) Toxicity
CT local response
Patient 1 (increased GTV size ) 55Gy in 20
Patient 2 (GTV shrinkage) 60Gy in 30
Patient 3 (GTV shrinkage) 55Gy in 20
Patient 4 ) (GTV shrinkage) 60Gy in 30
165.7 85.3 38.9 145.0 18.4 −54.1
−78.1 −45.6 −14.8 −13.3 −23.1 −19.1
−62.3 −26.2 −7.8 −15.2 −42.6 −30.7
−50.7 −28.4 −3.4 −6.5 −9.2 −46.0
8 22 95.9 72.5 26.1 56.4 19.1 NA Gd 1 oesophagitis
7 23 97.8 96.0 27.5 63.3 22.3 58.2 Gd 1 pneumonitis, Gd 1 haemoptisis, Gd 1 chest pain Partial response
14 6 98.1 48.1 22.4 53.9 0.5 95.0 NA
11 19 96.8 11.2 18.2 38.5 19.8 94.7 No toxicity
NA
Recurrence
Stable
*Dose 55Gy in 20 Gy expressed as EQD2 Follow up average was 6 months for three patients.