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Reduction of Integral Dose with Posterior Limited-Angle VMAT Planning in the Treatment of CSI

S Lachowicz*, J Molloy , J Feddock , W St.Clair , D Pokhrel , University of Kentucky, Lexington, KY


(Monday, 7/30/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 3

Purpose: Conventional 3D-conformal craniospinal irradiation (CSI) techniques require field-matching junction-shifts, often resulting in prolonged treatment times which can be challenging for treatment of children. Sparing critical organs remains limited due to fixed beam angle; efforts to improve this process while minimizing long-term risks such as second cancers are warranted. This report aimed to quantify reduction in integral and organ-at-risk (OAR) doses to patients by utilizing posterior limited-angle VMAT avoiding field-matching and junction-shifts.

Methods: Five patients underwent conventional CSI to a total dose of 36Gy in 20 fractions, each requiring 3-junction changes were re-computed with AcurosXB 3-isocenter VMAT requiring only one-dimensional longitudinal shifts between isocenters. 3 sets (brain, upper spine, lower spine) of 2 posterior partial arcs (clockwise/counter-clockwise rotation) with ±5degree collimator rotation were optimized with Truebeam 6MV VMAT fields allowing 5-8cm field overlap at junction areas. Conventional technique and limited-angle VMAT technique were compared analyzing total MU, beam-on time, integral dose, and critical OAR dose.

Results: VMAT technique allows for increased homogeneity and conformity. Total MU and beam on time increased by 30%. Simple setup allows relatively faster treatment. Normal tissue volume under 5Gy, 10Gy and 18Gy decreased by factors up to 1.4, 2.1 and 3.2 respectfully. Most critical structures max and mean doses exhibited statistically significant decrease with this OAR sparing, including skin. For example, average decreases observed were lung V20 by 99.6% (p-value=0.02), esophagus mean dose by 58.9% (p-value=0.002), mean heart dose by 62.5% (p-value=0.004), mean parotid dose by 63.5% (p-value=0.001) and small-bowel max dose by 68.6% (p-value=0.001).

Conclusion: Posterior limited-angle VMAT allows us to obtain simple setup only requiring one-dimensional shifts leading to shorter overall treatment delivery, more conformal dose distribution, decrease in integral and OAR dose. Eliminating fields matching/junctions change on patient’s skin, we have begun implementing this technique into our clinic with supine patient setup.


Dose Uniformity, Treatment Techniques, Treatment Planning


TH- External beam- photons: VMAT dose optimization algorithms

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