Permit to enter the “no-fly-zone”: SBRT for oligometastases in the central chest appears safe and effective R Cooke1,2, DLP Holyoake1, K Chu1,2, A Buckle2, MA Hawkins1
Introduction
Conclusions
Stereotactic body radiation therapy (SBRT) to oligometastases within the central chest “no-fly-zone” (2cm around proximal bronchial tree [PBT], to second division of bronchi) must be undertaken with caution due to the risks of severe toxicity that may be observed with extreme hypofractionation schedules. A risk-adapted approach uses moderated dose-fractionation schedules and intensity modulated radiation therapy (IMRT) to meet tolerance constraints of critical normal tissues, even at the cost of reduced coverage of the planning target volume (PTV), aiming to achieve disease control with an acceptable safety profile.
Using moderated dose-fractionation schedules and IMRT to meet constraints of normal tissues appears to enable safe and effective delivery of SBRT to central chest oligometastatic disease. Treatment resulted in very low incidence of toxicity and excellent rates of local control.
Figure 1 Illustration of location and size of GTV structures for patients
Demand will continue to increase in line with increasing rates of resections of pulmonary lesions for patients with oligometastastic disease and our centre has registered three further patients in the month since these data were collated. It will be crucial to undertake ongoing follow-up to detect late toxicity and record long-term survival outcomes.
Results
treated within this cohort in relation to the proximal bronchial tree.
Dosimetry Median PTV size was 50.1 cc (range 10.7-106.4 cc); two patients underwent simultaneous treatment of two separate lesions within the “no-fly-zone”.
R
For eleven patients the PTV overlapped with the PBT; of these three also overlapped with the heart, and three with both the heart and oesophagus. The PTV of the remaining patient came to within 0.4 cm of the PBT and overlapped the heart and chest wall.
L
The portion of PTV overlapping with organs at risk (OARs) ranged from 0.1 to 25.1 cc (median 2.7 cc). Mean D95 for the PTV was 80.8% of prescribed dose (range 64.3-98.6%). For the portion of PTV not overlapping OARs, mean D95 was 85.7% of prescribed dose (range 69.6-99.0%), and minimum dose to this volume was between 56.4-86.8% of prescribed dose (mean 69.1%). All mandatory dose constraints for oesophagus, heart, PBT, lungs and spinal canal were met; however the optimal constraint for the PBT could not be met for any patient with PTV overlapping PBT (Dmax 0.5 cc < 32.0 Gy).
Methods Twelve patients were treated with radiotherapy for central oligometastases in Oxford between November 2014 and October 2016. Most primary tumours were colorectal (8) or renal (2), and mean age was 68 years (range 38-89 years). Nine patients received 60 Gy in 8 fractions delivered on alternate days, and three patients received 45-50 Gy in 10 daily fractions. All patients were treated on a conventional linear acccelerator, with static IMRT (3) or volumetric modulated arc therapy (9). Eight-fraction radiotherapy plans were evaluated against the target coverage recommendations and normal-tissue constraints of the NHS Commissioning through Evaluation SBRT programme (NHS England, 2015); for 10 fraction schedules locally defined constraints were used. Follow-up data on disease status and toxicity were collated from imaging and clinical assessment records at Oxford or the referring hospital.
Table 1 Details of dose to GTV and PTV. GTV
10 fractions
8 fractions
ID R03 R05 R06 R07 R08 R09 lesion 1 R09 lesion 2 R10 R11 R12 R01 R02 R04 lesion 1 R04 lesion 2 Mean Median
PTV
Volume V100% Volume (cc) (%) (cc) 2.4 3.9 14.2 14.5 30.8 5.2 1.2 21.0 48.3 9.4 48.2 21.0 21.1 25.9 19.1 17.8
88.3 97.8 82.6 39.3 86.5 100.0 49.5 78.7 98.2 71.9 78.7 100.0 54.7 83.7 79.3 83.2
22.5 10.7 42.6 51.6 65.7 30.5 11.8 44.7 106.4 48.5 81.1 57.9 59.7 51.6 49.0 50.1
Non-overlap PTV
D95% (%)
D99% (%)
Volume (cc)
D95% (%)
D99% (%)
68.9 91.5 73.8 75.8 67.5 96.1 64.3 71.7 98.6 71.6 71.8 97.3 87.2 95.3 80.8 74.8
66.1 79.9 64.8 70.6 61.7 88.7 62.8 65.8 95.6 69.0 65.8 94.0 84.9 92.5 75.9 69.8
18.1 10.6 40.2 26.5 57.1 30.4 9.9 38.2 94.3 45.6 79.4 56.7 48.3 50.5 43.3 42.9
76.6 93.1 84.0 97.7 75.4 96.0 69.6 78.0 99.0 72.9 72.9 97.8 90.8 95.5 85.7 87.4
71.5 82.1 75.6 85.0 67.0 89.4 66.1 70.1 95.4 70.5 68.3 94.5 88.3 92.8 79.8 78.9
Clinical Outcomes After a median follow-up of 218 days (range 14-563 days), in-field progressive disease has only been seen in one patient, who subsequently died of metastatic disease. All other patients have shown partial response or stable disease, with one having complete response on CT at 6 months. Five patients in all had distant progressive disease, one of whom died but with local disease control at 6 months. No patients suffered acute toxicity affecting delivery of radiotherapy. One patient developed grade 2 pneumonitis which resolved with steroids.
Figure 2 Radiotherapy plan showing 20 Gy dose wash and demonstrating compromise to coverage of PTV (blue outline) due to overlap with PBT (orange) and oesophagus (pink).
References NHS England (2015) Commissioning Through Evaluation: Standards for the Provision of Stereotactic Ablative Radiotherapy. Available from http://www.swscn.org.uk/wp/wpcontent/uploads/2014/11/SABR-CtE-Service-Specification-2nd-Sept-2015-final.pdf
Affiliations 1CRUK/MRC
Oxford Institute for Radiation Oncology, Dept of Oncology, University of Oxford, Oxford. 2Oxford University Hospitals NHS Foundation Trust, Department of Clinical Oncology, Oxford. Contact:
[email protected]
