Development and long‐term stability of a comprehensive daily <scp>QA</scp> program for a modern pencil beam scanning (<scp>PBS</scp>) proton therapy delivery system

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Purpose The purpose of this study was twofold: (a) report the long‐term monthly quality assurance (QA) dosimetry results of the uniform scanning beam delivery system, and (b) derive the machine‐specific tolerances based on the statistic process control (SPC) methodology and compare them against the AAPM TG224 recommended tolerances. Methods The Oklahoma Proton Center has four treatment rooms (TR1, TR2, TR3, and TR4) with a cyclotron and a universal nozzle. Monthly QA dosimetry results of four treatment rooms over a period of 6 yr (Feb 2014–Jan 2020) were retrieved from the QA database. The dosimetry parameters included dose output, range, flatness, and symmetry. The monthly QA results were analyzed using the SPC method, which included individuals and moving range (I‐MR) chart. The upper control limit (UCL) and lower control limit (LCL) were set at 3σ above and below the mean value, respectively. Results The mean difference in dose output was −0.3% (2σ = ±0.9% and 3σ = ±1.3%) in TR1, 0% (2σ = ±1.4% and 3σ = ±2.1%) in TR2, −0.2% (2σ = ±1.0% and 3σ = ±1.6%) in TR3, and −0.5% (2σ = ±0.9% and 3σ = ±1.3%) in TR4. The mean flatness and symmetry differences of all beams among the four treatment rooms were within ±1.0%. The 3σ for the flatness difference ranged from ±0.5% to ±1.2%. The 3σ for the symmetry difference ranged from ±0.4% to ±1.4%. The SPC analysis showed that the 3σ for range 10 cm (R10), R16, and R22 were within ±1 mm, whereas the 3σ for R28 exceeded ±1 mm in two rooms (3σ = ±1.9 mm in TR2 and 3σ = ±1.3 mm in TR3). Conclusion The 3σ of the dose output, flatness, and symmetry differences in all four rooms were comparable to the TG224 tolerance (±2%). For the uniform scanning system, if the measured range is compared against the requested range, it may not always be possible to achieve the range difference within ±1 mm (TG224) for all the ranges.

Section: C | Data Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Determination of machine‐specific tolerances using statistical process control analysis of long‐term uniform scanning proton machine QA results

Rana¹,

Eckert²,

Singh³

et al. 2020

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Radiobiological and dosimetric impact of RayStation pencil beam and Monte Carlo algorithms on intensity‐modulated proton therapy breast cancer plans

Rana

Greco

Samuel

et al. 2019

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RayStation treatment planning system employs pencil beam (PB) and Monte Carlo (MC) algorithms for proton dose calculations. The purpose of this study is to evaluate the radiobiological and dosimetric impact of RayStation PB and MC algorithms on the intensity-modulated proton therapy (IMPT) breast plans. Methods: The current study included ten breast cancer patients, and each patient was treated with 1-2 proton beams to the whole breast/chestwall (CW) and regional lymph nodes in 28 fractions for a total dose of 50.4 Gy relative biological effectiveness (RBE). A total clinical target volume (CTV_Total) was generated by combining individual CTVs: AxI, AxII, AxIII, CW, IMN, and SCVN. All beams in the study were treated with a range shifter (7.5 cm water equivalent thickness). For each patient, three sets of plans were generated: (a) PB optimization followed by PB dose calculation (PB-PB), (b) PB optimization followed by MC dose calculation (PB-MC), and (c) MC optimization followed by MC dose calculation (MC-MC). For a given patient, each plan was robustly optimized on the CTVs with same parameters and objectives. Treatment plans were evaluated using dosimetric and radiobiological indices (equivalent uniform dose (EUD), tumor control probability (TCP), and normal tissue complication probability (NTCP)). Results: The results are averaged over ten breast cancer patients. In comparison to PB-PB plans, PB-MC plans showed a reduction in CTV target dose by 5.3% for D 99% and 4.1% for D 95% , as well as a reduction in TCP by 1.5-2.1%. Similarly, PB overestimated the EUD of target volumes by 1.8─3.2 Gy(RBE). In contrast, MC-MC plans achieved similar dosimetric and radiobiological (EUD and TCP) results as the ones in PB-PB plans. A selection of one dose calculation algorithm over another did not produce any noticeable differences in the NTCP of the heart, lung, and skin. Conclusion: If MC is more accurate than PB as reported in the literature, dosimetric and radiobiological results from the current study suggest that PB overestimates the target dose, EUD, and TCP for IMPT breast cancer treatment. The overestimation of dosimetric and radiobiological results of the target volume by PB needs to be further interpreted in terms of clinical outcome.

Impact of magnetic field regulation in conjunction with the volumetric repainting technique on the spot positions and beam range in pencil beam scanning proton therapy

Rana

Bennouna

Gutiérrez

et al. 2020

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Purpose The objective of this study was to evaluate the impact of the magnetic field regulation in conjunction with the volumetric repainting technique on the spot positions and range in pencil beam scanning proton therapy. Methods “Field regulation” — a feature to reduce the switching time between layers by applying a magnetic field setpoint (instead of a current setpoint) has been implemented on the proton beam delivery system at the Miami Cancer Institute. To investigate the impact of field regulation for the volumetric repainting technique, several spot maps were generated with beam delivery sequence in both directions, that is, irradiating from the deepest layer to the most proximal layer (“down” direction) as well as irradiating from the most proximal layer to the deepest layer (“up” direction). Range measurements were performed using a multi‐layer ionization chamber array. Spot positions were measured using two‐dimensional and three‐dimensional scintillation detectors. For range and central‐axis spot position, spot maps were delivered for energies ranging from 70–225 MeV. For off‐axis spot positions, the maps were delivered for high‐, medium, and low‐energies at eight different gantry angles. The results were then compared between the “up” and “down” directions. Results The average difference in range for given energy between “up” and “down” directions was 0.0 ± 0.1 mm. The off‐axis spot position results showed that 846/864 of the spots were within ±1 mm, and all off‐axis spot positions were within ±1.2 mm. For spots (n = 126) at the isocenter, the evaluation between “up” and “down” directions for given energy showed the spot position difference within ±0.25 mm. At the nozzle entrance, the average differences in X and Y positions for given energy were 0.0 ± 0.2 mm and −0.0 ± 0.4 mm, respectively. At the nozzle exit, the average differences in X and Y positions for given energy were 0.0 ± 0.1 mm and −0.1 ± 0.1 mm, respectively. Conclusion The volumetric repainting technique in magnetic field regulation mode resulted in acceptable spot position and range differences for our beam delivery system. The range differences were found to be within ±1 mm (TG224). For the spot positions (TG224: ±1 mm), the central axis measurements were within ±1 mm, whereas for the off‐axis measurements, 97.9% of the spots were within ±1 mm, and all spots were within ±1.2 mm.