Stereotactic Treatments with Small Photon Beams-ICRU 91

Which systems (besides MV-CBCT or kV-CBCT) can used for IGRT during SRS/SBRT treatments?

• surface detection

• radioactive source-based tracking

• electromagnetic frequency-based

• ultrasound

• MRI guidance

The overall uncertainty of an SRT system is the

combination of uncertainties resulting from several

components, which mainly consist of uncertainties

in the:

• target location

• dosimetry in the TPS

• radiation treatment delivery system;

• IGRT system

Daily checks for GammaKnife include:

• testing the functionality of the unit on a short test run

• checking the daily decayed estimate of standard

• absorbed dose rate computed by the TPS

The typical tolerance for isocenter verification is typically <1 mm and <1 degree of rotation. When the couch position is modified as a result IGRT shifts, it is essential that:

the patient position be re-verified before the radiation is delivered

The overall uncertainty of an SRT system is composed of uncertainties associated with:

1. CT planning equipment

2. dosimetry and TPS

3. machine delivery system

4. IGRT system

How is absorbed dose prescribed in SRT?

Absorbed dose is prescribed to the isodose surface (DV) that covers an optimal percentage of the PTV volume, while still optimally restricting dose to the PRV.

Level 2 dose reporting should include:

• Brief clinical history (ie. description of clinical examination, location, diagnostic technique, histopathological eval, staging, prior treatment, performance status)

• Treatment intent (i.e., palliative, curative)

• Patient simulation (i.e., immobilization devices, accessories, planning image acquisition, & protocols)

• Target volumes and OAR selection and delineation

1. Target volumes (cm³)

• GTV

• CTV

• ITV, PTV

2. Normal tissues

• (i) OAR (cm3)

• PRV (cm3)

• RVR (cm3)

• Planning aims and DV constraints

• Description of treatment planning system (i.e. algorithm, voxel size, calculation dose grid, type-A uncertainty for MC-based systems)

• Prescription

• Patient-specific QA

• Delivery (i.e., treatment unit and energy, image

verification device, and data set)

• Dose reporting

(1) Dose in PTV and, if applicable in CTV and/or GTV

(2) Dose in OAR and PRV

These metrics should always be reported for each SRS/SBRT plan:

1. PTV median absorbed dose, D_50%

2. SRT near-maximum dose, D_near-max (D_2%):

   • For PTV larger than or equal to 2 cm³, the volume near-max represents 2% of the PTV.

   • For PTV V of less than 2 cm³, near-max is an absolute volume of 35 mm³, in which case D_35mm³ is reported.

3. SRT D_near-min (D_98%):

   • For PTV V larger or equal than 2 cm³, the volume near-min represents 98 % of the PTV

   • For PTV V of less than 2 cm³, near-min is an absolute volume of 35 mm³, in which case D_35mm³ is reported.

With Level 3 reporting one should consider:

• integral dose

• clinical and biological evaluation metrics (i.e., TCP & NTCP, and dose coverage histogram)

• equivalent uniform dose-based optimization

• reporting of probability coverage

What can a 2 mm leaf position error cause in small fields?

Dose errors easily exceeding 10% for leaf gaps ≤2 cm

Why is a TPS commissioned only with large fields inaccurate for small field calculations?

Sensitivity of endpoints such as output factor to source size causes errors; small fields require separate verification.

Why must collimator settings be independently verified in small field shaping?

Because small errors in collimation significantly alter penumbra and dose distribution

What grid size is recommended for high-dose gradient areas (e.g., SRS/SBRT)?

2 mm or finer

What key factors should be verified to optimize TPS performance for small field applications?

• Source size & output factors

• Jaw and MLC leaf-end positioning

• MLC transmission at off-axis positions

• Backup jaw & MLC interaction

• Interleaf & intraleaf leakage

• Penumbra behavior in small fields

• Effects of low-density media on dose (widened penumbra, builddown, rebuild-up)

What must be evaluated and recorded in SRT treatments?

Pre-treatment target shifts and changes in target volume or internal anatomy during delivery.