Parametric characterization of penumbra reduction for aperture-collimated pencil beam scanning (PBS) proton therapy

Abstract

Recently, a commercial treatment planning system (TPS) has implemented aperture collimators for PBS dose calculations which can serve to reduce lateral penumbra. This study characterized the variation in magnitude of lateral penumbra for collimated and un-collimated PBS fields versus the parameters of air gap, depth, and range shifter thickness. Comparisons were performed in a homogenous geometry between measured data and calculations made by a commercial TPS. Beam-specific target volumes were generated for collimated and un-collimated PBS fields and optimized for various range shifter thicknesses and air gaps. Lateral penumbra (80%-20% distance) was measured across each target volume to characterize penumbra variation with depth and air gap. An analytic equation was introduced to predict the reduction in lateral penumbra between un-collimated and collimated PBS treatments. Calculated penumbra values increased with depth across all combinations of range shifters for a constant air gap. At 2 cm depth, the reductions in penumbra due to the aperture were 2.7 mm, 3.7 mm and 4.2 mm when using range shifter thicknesses of 0 cm, 4.0 cm and 7.5 cm, respectively. At a depth of approximately 20 cm and air gap of 5 cm, differences between penumbras of collimated and un-collimated beams were less than 1 mm. Penumbra reductions for the collimated beams were largest at small air gaps. All TPScalculated penumbra values derived in this study were within 1 mm of film measurement values. Finally, the analytic equation was tested using a clinical CT scan, and we found good dosimetric agreement between the model predictions and the result calculated by the TPS. In conclusion, application of collimators to PBS fields can sharpen penumbra by several mm and are most beneficial for shallow targets. Furthermore, measurements indicate that the dose calculation accuracy in the penumbra region of PBS-collimated fields is adequate for clinical use. AbstractRecently, a commercial treatment planning system (TPS) has implemented aperture collimators for PBS dose calculations which can serve to reduce lateral penumbra. This study characterized the variation in magnitude of lateral penumbra for collimated and uncollimated PBS fields versus the parameters of air gap, depth, and range shifter thickness. Comparisons were performed in a homogenous geometry between measured data and calculations made by a commercial TPS. Beam-specific target volumes were generated for collimated and un-collimated PBS fields and optimized for various range shifter thicknesses and air gaps. Lateral penumbra (80%-20% distance) was measured across each target volume to characterize penumbra variation with depth and air gap. An analytic equation was introduced to predict the reduction in lateral penumbra between uncollimated and collimated PBS treatments. Calculated penumbra values increased with depth across all combinations of range shifters for a constant air gap. At 2 cm depth, the reductions in penumbra due to the aperture were 2.7 mm, 3.7 mm and 4.2 mm when using ran...

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

“…The average lateral penumbra was 10 mm at 15 cm depth when using AA for our system. Meas et al 16 also reported a similar result of lateral penumbra of 8.5 mm when using brass aperture with their measurement condition of range 23.5 g/cm 2 and modulation width 23 g/cm 2 with 7.5 cm WET range shifter. These values are equivalent to those for conventional linacs.…”

“…The maximum lateral penumbra reduction by AA was 10.4 mm at 5 cm WET depth for the 10 cm cube target. However, Meas et al 16 reported penumbra reduction of ~4 mm at 5 cm depth for 10 × 10 cm 2 field size, when they were using range shifter of 40 mm WET with brass aperture. The reason for this large discrepancy in penumbra reduction is that the spot sigma at isocenter varied from ~7.3 to 3.2 mm for the IBA system with long beam line, 28 compared to ~19 to 3.9 mm for our system.…”

“…For our Varian (Palo Alto, CA) TrueBeam system, the penumbrae are 8.0 mm for 6 MV and 8.4 mm for 15 MV at 10 cm depth for 10 × 10 cm 2 field size. One of the limitations reported by the Maes et al 16 was that the penumbra measurements were done with a single energy and one target size. The present work includes penumbra measurements by varying the spot and target sizes.…”

“…The quantification and sharpening of lateral penumbra with an aperture for different proton beam delivery techniques have been previously published. [14][15][16] Bues et al 14 did a Monte Carlo (MC) study regarding the use of multileaf collimator (MLC) for spot scanning proton therapy. They studied lateral penumbra, leaf transmission, scalloping effect and found that for energy less than 159 MeV, MLC achieved superior penumbra as compared to the open field.…”

“…They concluded that air gap, depth of measurements, proton beam range, and compensator thickness were the main factors upon which lateral penumbra is dependent. Maes et al 16 concluded that aperture‐collimated PBS fields sharpen the penumbra by several millimeters. This penumbra reduction was beneficial for shallow targets.…”