Validation of Pinnacle treatment planning system for use with Novalis delivery unit

Abstract

For an institution that already owns the licenses, it is economically advantageous and technically feasible to use Pinnacle TPS (Philips Radiation Oncology Systems, Fitchburg, WI) with the BrainLab Novalis delivery system (BrainLAB A.G., Heimstetten, Germany). This takes advantage of the improved accuracy of the convolution algorithm in the presence of heterogeneities compared with the pencil beam calculation, which is particularly significant for lung SBRT treatments. The reference patient positioning DRRs still have to be generated by the BrainLab software from the CT images and isocenter coordinates transferred from Pinnacle. We validated this process with the end-to-end hidden target test, which showed an isocenter positioning error within one standard deviation from the previously established mean value. The Novalis treatment table attenuation is substantial (up to 6.2% for a beam directed straight up and up to 8.4% for oblique incidence) and has to be accounted for in calculations. A simple single-contour treatment table model was developed, resulting in mean differences between the measured and calculated attenuation factors of 0.0%-0.2%, depending on the field size. The maximum difference for a single incidence angle is 1.1%. The BrainLab micro-MLC (mMLC) leaf tip, although not geometrically round, can be represented in Pinnacle by an arch with satisfactory dosimetric accuracy. Subsequently, step-and-shoot (direct machine parameter optimization) IMRT dosimetric agreement is excellent. VMAT (called "SmartArc" in Pinnacle) treatments with constant gantry speed and dose rate are feasible without any modifications to the accelerator. Due to the 3 mm-wide mMLC leaves, the use of a 2 mm calculation grid is recommended. When dual arcs are used for the more complex cases, the overall dosimetric agreement for the SmartArc plans compares favorably with the previously reported results for other implementations of VMAT: gamma(3%,3mm) for absolute dose obtained with the biplanar diode array passing rates above 97% with the mean of 98.6%. However, a larger than expected dose error with the single-arc plans, confined predominantly to the isocenter region, requires further investigation.

Figure 1

The mMLC leaf sketch with the circle tangent to all three edges. The X‐axis scale is expanded for clarity.

Figure 2

Dose profiles through abutting MLC fields. The dose profile consists of the sum of two $3\times 3\hspace{0.17em}{\text{cm}}^2$ MLC fields, each offset by 1.5 cm from the central axis, and measured at a depth of 10 cm and with an offset of 1.5 mm in the X direction. Also plotted are the corresponding profiles calculated by the TPS for three selected values of the leaf‐radius, $\mathrm{R}=15$, 20, and 28 cm.

Figure 3

Representative DVHs: a) Mock Prostate; b) Multi‐Target; c) Mock H&N (SmartArc planned with two arcs); d) C‐shape (SmartArc with two arcs). Thick solid $\text{line}=\text{DMPO}$, thin $\text{solid}=\text{SmartArc}$ with MLC constrained to 0.22 cm/deg, $\text{dashed}=0.33\hspace{0.17em}\text{cm}/\text{deg}$.

Figure 4

Mock H&N SmartArc plan comparison between single and double‐arcs: a) DVH comparison; b) isodoses for the single‐arc plan; c) isodoses for the double‐arc plan. $\text{Solid}=\text{single}‐\text{arc}$, $\text{dashed}=\text{double}‐\text{arc}$ plan.

Figure 5

Delta4 dose profiles taken in the transverse plane, through isocenter, at a 50° angle, corresponding to the Delta4 main detector board orientation. H&N plans constrained to 0.33 cm/deg MLC movement: (a) single‐arc; (b) double‐arc with individual beams ALPO 4.8 and 1.7 cm; (c) double‐arc with individual beams ALPO 3.8 and 2.3 cm.

Figure 6

C‐Shape single‐arc plans with MLC apertures for control points separated by 4°: (a) variable dose rate (DR) plan on a hypothetical linac; (b) constant dose rate plan, real Novalis linac; Insert (c) unobscured posterior view of the structure set.