SRS rounds

Here are notes of papers and issues brought up in SRS rounds today. Discussion centered around a 42 yo patient with breast cancer treated 3 years ago, with a single symptomatic met close to the motor strip.

The standard of care for oligometastases is consideration of surgery if feasible - Patchell study demonstrating survival advantage to surgery in addition to WBRT.

Plenty of data on SRS and WBRT combinations, none compare directly to surgery. SRS doses are defined by RTOG 9005. <20mm 24Gy, 20-30mm 18Gy, 30-40mm 15Gy.

Conformality Index is volume encompassed by the prescription isodose line divided by the volume of the PTV. Goal of Rx is to be less than 2, though this can be impossible for very small lesions, and a CI of 3-4 would be acceptable.

GTV to PTV expansions are 1mm as per Baumert BG, Int J Radiat Oncol Biol Phys. 2006 Sep 1;66(1):187-94 - pathologic study of 45 pts, 76 mets. demonstrating that for most histologies microscopic extension was <1mm, except for melanoma and SCLC which were greater.

Process at Duke is to use a non-invasive frame for immobilization. With this scans can be performed prior to SRS delivery and the patient does not have to wait all day in the department with a head frame on. The exceptions are for very small targets near critical structures - trigeminal neuralgia patients and acoustic neuromas for example. In these cases we will still use an invasive frame.

Thin cut SRS protocol MRI with contrast is obtained and fused with the planning image. Often for small convex tumors, 4-5 dynamic conformal arcs offer a simple solution. Remember that the MLC shapes must include a dosimetric margin. For larger concave targets, such as AVMs, IMRS (intensity modulated radiosurgery) may be utilized for better conformality and for sparing normal brain.

With this approach, patients can come in at a scheduled treatment time, and complete treatment in a few hours.

All plans are reviewed with a Neurosurgeon, and if there is concern about proximity to the motor strip, we will ofter back off the dose. For the lesion presented, measuring 1.4cm, we backed the dose off to 20Gy.

Limit on the brainstem is a conservative 12Gy in a single fraction, and for optic apparatus our limit is 8Gy.

Quick rundown of SRS doses:
Mets <2cm 24Gy
Mets 2-3cm 18Gy
Mets 3-4cm 15Gy
Acoustic Neuroma 12.5Gy (54Gy fractionated)
Meningioma 15-16Gy (50.4Gy fractionated benign, 54-60Gy fractionated malignant)
AVM 20 Gy marginal dose (keep V12 less than 26cc for most sites, refer to this Flickinger paper)