eContour

76-year-old woman with oligometastatic lung adenocarcinoma, s/p primary lung tumor treated with SBRT several years ago, subsequently developed biopsy-proven metastatic disease now with two adjacent sclerotic bony lesions in the right ilium, associated with pain.

Zoom:
Image:

Rx: The patient was treated with definitive radiation using SBRT, delivered by three volumetrically modulated arcs.

  • The high-dose PTV received 20 Gy in one fraction.
  • The low-dose PTV received 16 Gy in one fraction.
  • This was delivered via a simultaneously integrated boost.

Though this patient was treated with single fraction SBRT, there is considerable variability in dosing schemes in clinical practice, with minimal data to guide selection of a particular regimen (Nguyen PRO 2020). Reasonable options include: 

  • 18-24Gy in 1Fx
  • 24Gy in 2Fx
  • 28-30Gy in 3Fx
  • 30-50Gy in 5Fx

Sim/Planning:

  • Immobilization: BodyFix or similar rigid immobilization.
  • Positioning: Head first, supine, arms on chest.
  • Imaging: CT scan without contrast with 1 – 1.25 mm slices, musculoskeletal MRI pelvis with and without contrast in treatment position. No motion management.

 

Case contributed by Mayo Clinic Rochester

Contours per Nguyen IJROBP 2022:

  • GTV: Tumor visible on CT (MRI may be more informative in some cases).
  • PTV_high: Equal to GTV.
  • CTV_low = GTV + 1 cm, anatomically constrained to bone (unless soft tissue involvement is evident).
  • PTV_low = CTV_low + 2 mm, unconstrained

OAR’s:

  • Dose will only distribute to about 5mm inferior and superior to the PTV_low, but we contour 2cm here for completeness.
  • The lumbosacral plexus may be contoured per Yi IJROBP 2012, though in cases of bone SBRT near the plexus, we include individual nerves as the intraosseous component is particularly important for dose optimization.

Dose Constraints (per TG101):

  • The cauda equina and sacral plexus should not receive doses exceeding 16 Gy for single-fraction courses.
  • When utilizing SBRT, organs at risk are priority 1. Use the T2 sequence to delineate the cauda equina.
  • The sacral plexus is often seen well on T1 as well as CT.

  • Typically, SBRT candidates have a life expectancy of at least one year and five or fewer sites of metastases.
  • Recent evidence (Nguyen JAMA Oncol 2019) shows that for non-spinal bone metastases, single-fraction SBRT may produce better pain relief and local control than traditionally fractionated EBRT
  • On-board imaging is essential for SBRT.
    • Use CBCT: bony match to the area of interest.
    • ExacTRAC SNAP imaging may be used to assess the patient’s position between each arc.
    • If tolerance is exceeded, return to CBCT before proceeding.

  • Counsel patients about pain flare: risk estimates range from 10% to >50%.
  • SBRT is safe to use in most cases of re-irradiation (Ogawa Acta Oncol 2018).
    • Multi-fraction regimens are more appropriate in this setting.
    • Calculate previous dose to OAR’s and adjust dose constraints accordingly.