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. 2022 Feb;23(2):e13512.
doi: 10.1002/acm2.13512. Epub 2022 Jan 6.

Small spot size versus large spot size: Effect on plan quality for lung cancer in pencil beam scanning proton therapy

Suresh Rana  1   2   3   4 , Anatoly B Rosenfeld  4
Affiliations

Small spot size versus large spot size: Effect on plan quality for lung cancer in pencil beam scanning proton therapy

Suresh Rana et al. J Appl Clin Med Phys. 2022 Feb.

Abstract

Purpose: The purpose of the current study was to evaluate the impact of spot size on the interplay effect, plan robustness, and dose to the organs at risk for lung cancer plans in pencil beam scanning (PBS) proton therapy METHODS: The current retrospective study included 13 lung cancer patients. For each patient, small spot (∼3 mm) plans and large spot (∼8 mm) plans were generated. The Monte Carlo algorithm was used for both robust plan optimization and final dose calculations. Each plan was normalized, such that 99% of the clinical target volume (CTV) received 99% of the prescription dose. Interplay effect was evaluated for treatment delivery starting in two different breathing phases (T0 and T50). Plan robustness was investigated for 12 perturbed scenarios, which combined the isocenter shift and range uncertainty. The nominal and worst-case scenario (WCS) results were recorded for each treatment plan. Equivalent uniform dose (EUD) and normal tissue complication probability (NTCP) were evaluated for the total lung, heart, and esophagus.

Results: In comparison to large spot plans, the WCS values of small spot plans at CTV D95% , D96% , D97% , D98% , and D99% were higher with the average differences of 2.2% (range, 0.3%-3.7%), 2.3% (range, 0.5%-4.0%), 2.6% (range, 0.6%-4.4%), 2.7% (range, 0.9%-5.2%), and 2.7% (range, 0.3%-6.0%), respectively. The nominal and WCS mean dose and EUD for the esophagus, heart, and total lung were higher in large spot plans. The difference in NTCP between large spot and small spot plans was up to 1.9% for the total lung, up to 0.3% for the heart, and up to 32.8% for the esophagus. For robustness acceptance criteria of CTV D95% ≥ 98% of the prescription dose, seven small spot plans had all 12 perturbed scenarios meeting the criteria, whereas, for 13 large spot plans, there were ≥2 scenarios failing to meet the criteria. Interplay results showed that, on average, the target coverage in large spot plans was higher by 1.5% and 0.4% in non-volumetric and volumetric repainting plans, respectively.

Conclusion: For robustly optimized PBS lung cancer plans in our study, a small spot machine resulted in a more robust CTV against the setup and range errors when compared to a large spot machine. In the absence of volumetric repainting, large spot PBS lung plans were more robust against the interplay effect. The use of a volumetric repainting technique in both small and large spot PBS lung plans led to comparable interplay target coverage.

Keywords: NTCP; interplay; lung cancer; pencil beam scanning; plan robustness; proton therapy.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

FIGURE 1
FIGURE 1
Example of spot profiles from the small spot and large spot models at the isocenter for 100, 150, and 225 MeV
FIGURE 2
FIGURE 2
Beam delivery directions in a volumetric repainting plan with an alternating order; Note: beam delivery starts from the distal energy layer to the proximal energy layer, and then follows an alternating order. All plans in the current study included five repaintings
FIGURE 3
FIGURE 3
(a) The worst‐case scenario (WCS) values in the large spot plans with five volumetric repaintings (LSVR) and small spot plans with five volumetric repaintings (SSVR) at different dosimetric metrics; (b) Number of perturbed scenarios passing meeting the robustness criteria in LSVR and SSVR plans; (c) Nominal homogeneity index (HI) in LSVR and SSVR plans; and (d) WCS HI in LSVR and SSVR plans
FIGURE 4
FIGURE 4
(a,b) Nominal dose distributions in large spot and small spot plans with and without volumetric repainting technique in an example patient; (c,d) Interplay dose distributions without volumetric repainting in large spot and small spot plans in the same patient; (e,f) Interplay dose distributions with volumetric repainting in large spot and small spot plans in the same patient. Note: Red contour = CTV, Blue = 6930 cGy(RBE), yellow = 6650 cGy(RBE); green = 3000 cGy(RBE); LSNR = large spot plan with no volumetric repainting, SSNR = small spot plan with no volumetric repainting, LSVR = large spot plan with five volumetric repaintings, SSVR = small spot plan with five volumetric repaintings
FIGURE 5
FIGURE 5
Difference in clinical target volume (CTV) D95%, D99%, and homogeneity index (HI) from interplay dose‐volume histograms (DVHs) between small spot and large spot plans
FIGURE 6
FIGURE 6
(a) Difference in D0.03cc of nominal results for the spinal cord between large spot plans with five volumetric repaintings (LSVR) and small spot plans with five volumetric repaintings (SSVR); (b) Difference in D0.03cc of worst‐case scenario (WCS) results for the spinal cord between LSVR and SSVR plans; (c) Difference in Dmean of nominal results for the organs at risk (OARs; esophagus, heart, and total lung) between LSVR and SSVR plans; (d) Difference in Dmean of WCS results for the OARs (esophagus, heart, and total lung) between LSVR and SSVR plans; (e) Difference in V20 and V5 of nominal results for the total lung between LSVR ad SSVR plans; (f) Difference in V20 and V5 of WCS results for the total lung between LSVR ad SSVR plans
FIGURE 7
FIGURE 7
Equivalent uniform dose (EUD) of the total lung for 12 perturbed scenarios (S1–S12) in large spot plans with five volumetric repaintings (LSVR) and small spot plans with five volumetric repaintings (SSVR) of all 13 patients
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