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Investigation of Clinically Relevant Dose of Small Field Brain IMRT Using Planned Dose Perturbation

H Jin*, I Ali, V Keeling, S Ahmad, University of Oklahoma Health Science Center, OKLAHOMA CITY, AA

SU-E-T-351 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Clinically relevant dose errors of small field brain IMRT were evaluated using planar per-beam dosimetry and planned dose perturbation (PDP; used in the Sun Nuclear 3DVH+MapCHEKC2) algorithm.

Methods: Ten BrainLAB IMRT plans were retrospectively selected. PTVs ranged from 0.5 to 29.0 cc and most of the field sizes were smaller than 4x4 cm² (beamlets were much smaller). The Varian TrueBEAM STx with high-definition MLC (2.5 mm width at isocenter) was employed to deliver treatment plans. The conventional per-beam IMRT QA was performed using the 2D diode array (diode detector size of 0.7x0.7 mm²). Two shifted QA measurements were merged for each plan to achieve a grid spacing of 5 mm. The 3DVH reconstructed a 'true' DVH of plans using the measured errors back-projected on to 3D patient dose calculation. Dose coverage in PTVs and critical structures was compared between treatment planning and actual delivery. 2D (conventional planar QA) and 3D (PDP-reconstructed dose) gamma analyses were also performed.

Results: In DVH-based analysis using 3DVH, the percent differences of maximum dose between planning and delivery for PTV were -0.4 to 20.2% (p-value: 0.02) and for normal tissue -5.67 to 1.32% (brainstem), -2.50 to 2.51% (eye), and -4.9 to 4.9% (optic nerve). The D95 percent differences for PTV were -3.2 to -0.56% (p-value: 0.0001).
Average pass rates (1 standard deviation) of the gamma test with 3%-3 mm criteria were 95.7 (4.9)% (2D) and 97.6 (3.4)% (3D), respectively. The correlation coefficients between 2D/3D gamma tests and the change in PTV coverage were 0.76/0.68 (Dmax of PTV) and -0.86/-0.96 (D95 of PTV), respectively.

Conclusions: There was a strong correlation between the gamma analysis and the predicted PTV coverage in small field dosimetry. Using 3D reconstructed dose maps, statistically significant dose difference between planning and delivery was observed in PTVs for the small field IMRT.

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