TU-G-BRB-01: Continuous Path Optimization for Non-Coplanar Variant SAD IMRT Delivery Using C-Arm Machines

PURPOSE

To develop and investigate a continuous path optimization methodology to traverse prescribed non-coplanar IMRT beams with variant SADs, by orchestrating the couch and gantry movement with zero-collision, minimal patient motion consequence and machine travel time.

METHODS

We convert the given collision zone definition and the prescribed beam location/angles to a tumor-centric coordinate, and represent the traversing path as a continuous open curve. We proceed to optimize a composite objective function consisting of (1) a strong attraction energy to ensure all prescribed beams are en-route, (2) a penalty for patient-motion inducing couch motion, and (3) a penalty for travel-time inducing overall path-length. Feasibility manifold is defined as complement to collision zone and the optimization is performed with a level set representation evolved with variational flows. The proposed method has been implemented and tested on clinically derived data. In the absence of any existing solutions for the same problem, we validate by: (1) visual inspecting the generated path rendered in the 3D tumor-centric coordinates, and (2) comparing with a traveling-salesman (TSP) solution obtained from relaxing the variant SADs and continuous collision-avoidance requirement.

RESULTS

The proposed method has generated delivery paths that are smooth and intuitively appealing. Under relaxed settings, our results outperform the generic TSP solutions and agree with specially tuned versions.

CONCLUSIONS

We have proposed a novel systematic approach that automatically determines the continuous path to cover non-coplanar, varying SAD IMRT beams. The proposed approach accommodates patient-specific collision zone definition and ensures its avoidance continuously. The differential penalty to couch and gantry motions allows customizable tradeoff between patient geometry stability and delivery efficiency. This development paves the path to achieve safe, accurate and efficient non-coplanar IMRT delivery with the advanced robotic controls in new-generation C-arm systems, enabling practical harvesting of the dose benefit offered by non-coplanar, variant SAD IMRT treatment.

SU-E-T-421: 6MV Radiation Small Field Dose: Off Axis and Penumbra Effects, a Study with Radiochromic Film

PURPOSE

To accurately determine relative dose factor scaling, tissue depth relation, and penumbra details of small 6MV radiation fields with an emphasis on far-off-central-axis fields for various MLC designs -subject matter important in the context of SRS, FSRT, SBRT, and IMRT.

METHODS

Radiochromic film designed for quantitative dosimetry, EBT3™ film (Ashland Inc.) can be reliably used as a relative dosimeter to within 1% accuracy if one controls for both film and flatbed scanner variation. This enabled a study of the fine detail of small radiation fields and the impact upon these small dose profiles by different MLC designs both on central axis and off. In particular, the impact of leaf-end penumbra from bifocal MLC Optifocus (Siemens) and Synergy Beam Modulator (Elekta) is examined.EBT3 films were placed at 100cm SAD in solid water phantom and irradiated to 200 MU at various depths 1.5cm - 20cm and for various off-axis locations. The films were analysed to give the detailed dosimetric profile of each small field and relate them to both a standard reference field (10 × 10)cm² and to a broadfield flatness/symmetry irradiation (20 × 20)cm² . Films were scanned at ∼20 hours post irradiation on an Epson 2000 flatbed scanner and analysed using FilmQAPro and in-house methods.

RESULTS

As small field size decreases, the field center falls within the penumbra of each MLC edge for megavoltage photon energies. The penumbral effects cause the conventional dosimetric quantity SP to sharply depart from the smooth fieldsize dependence for larger conventional fields.

CONCLUSIONS

Careful characterization of small field dose and leaf end modeling within a TPS is crucial in both predicting accurate small field dosimetry and off-axis dosimetry.

SU-E-T-239: In Vivo Dosimetry with Surface Diodes during Total Body Irradiation: A Patient Thickness Factor to Correct Midline Dose

PURPOSE

In vivo dosimetry (IVD) assessment of treatment dose is important when delivering total body irradiation (TBI). One method is to average AP and PA surface diode measurements and compare them to prescribed midline doses. We designed phantom studies to examine the impact of patient thickness on surface IVD measurements under TBI conditions.

METHODS

Phantom studies were designed to assess the effects of patient thickness on diode IVD. Sun Nuclear QED diodes with inherent buildup were placed on anterior and posterior surfaces of a solid water phantom. Phantom thickness was varied between 20 and 40 cm. A PTW farmer chamber was inserted in the center of the phantom at 425 SSD to reflect prescribed midline dose, and 50 cGy was delivered to midline with 18 MV photons. Averaged entrance and exit diode doses were then compared to farmer chamber measurements of phantom midline dose.

RESULTS

A trend of increased deviation with increasing umbilicus thickness was observed between averaged surface diodes and midline farmer chamber measurements. Averaged surface diode dose ranged from 49.6 cGy (20 cm thickness) to 52.1 cGy (40 cm thickness). Interpolation of diode measurements to midline resulted in linear overestimation of delivered dose relative to farmer chamber measurements at midline, up to 6.8% at 40 cm umbilicus thickness.

CONCLUSION

Accurate in vivo dosimetry at time of patient TBI is important to allow individual correction of MU exposure and tissue compensation. Without patient thickness correction, overresponse of surface diodes may lead to unnecessary clinical intervention to treatment MU or compensation and insufficient midline dose. Additionally, SAD setup is preferable to SSD setup to minimize thickness non-linearity. In conclusion, thickness correction factors should be used to generate expected diode readings for patients with thickness greater than 30 cm.