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Modeling PTV Dose Fall-Off for Cervical Cancer SBRT Treatment Planning Using VMAT and Step-And-Shoot IMRT

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A Brito Delgado

A Brito Delgado*, D Cohen , T Eng , A Gutierrez , University of Texas Health Science Center San Antonio, San Antonio, TX


SU-F-T-590 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Due to the high dose per fraction in SBRT, dose conformity and dose fall-off are critical. In patients with cervical cancer, rapid dose fall-off is particularly important to limit dose to the nearby rectum, small bowel, and bladder. This study compares the target volume dose fall-off for two radiation delivery techniques, fixed-field IMRT & VMAT, using non-coplanar beam geometries. Further comparisons are made between 6 and 10MV photon beam energies.

Methods: Eleven (n=11) patients were planned in Pinnacle³ v9.10 with a NovalisTx (HD120 MLC) machine model using 6 and 10 MV photons. The following three techniques were used: (1) IMRT (10 non-coplanar beams) (2) Dual, coplanar 360° VMAT arcs (4° spacing), and (3) Triple, non-coplanar VMAT arcs (1 full arc and dual partial arcs). All plans were normalized such that 98% of the PTV received at least 28Gy/4Fx. Dose was calculated using a 2.0mm isotropic dose grid. To assess dose fall-off, twenty concentric 2mm thick rings were created around the PTV. The maximum dose in each ring was recorded and the data was fitted to model dose fall-off. A separate analysis was performed by separating target volumes into small (0-50cc), medium (51-80cc), and large (81-110cc).

Results: Triple, non-coplanar VMAT arcs showed the best dose fall-off for all patients evaluated. All fitted regressions had an R²≥0.99. At 10mm from the PTV edge, 10 MV VMAT3-arc had an absolute improvement in dose fall-off of 3.8% and 6.9% over IMRT and VMAT2-arc, respectively. At 30mm, 10 MV VMAT3-arc had an absolute improvement of 12.0% and 7.0% over IMRT and VMAT2-arc, respectively. Faster dose fall-off was observed for small volumes as opposed to medium and large ones—9.6% at 20mm.

Conclusion: Triple, non-coplanar VMAT arcs offer the sharpest dose fall-off for cervical SBRT plans. This improvement is most pronounced when treating smaller target volumes.

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