2376 - Investigating the Feasibility of Using Spot-Scanning Proton Arc Therapy to Mitigate the Dosimetric Perturbations in the Presence of Large Metal Implants

D. Mumaw¹, W. Zheng¹, X. Cong², P. M. Liu², X. Li², T. J. Quinn², X. Ding², and R. L. Deraniyagala Jr²; ¹William Beaumont University Hospital - Corewell Health, Royal Oak, MI, ²Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI

Purpose/Objective(s): Conventional intensity modulated proton therapy (IMPT) techniques are exquisitely sensitive to setup and range uncertainties in the presence of high-density metal implants: even small setup errors can generate large magnitude under- and overdosing of targets. Herein we introduce a spot-scanning proton arc (SPArc) approach to overcome these dosimetric challenges in close proximity to large metal implants. Materials/

Methods: Single dose-level, curative-intent plans were generated for 5 patients with metal implants circumscribing or within the target volume. Both 4-field IMPT and single-arc SPArc plans were generated for each patient using the same optimization functions. Direct interaction of the spot with metal implants was prohibited with use of structure avoidance. Robust optimization parameters of 3.5% range and 3/5 mm setup uncertainties were used, generating a total of 26 dose perturbation scenarios per technique. For each perturbation, dose volume histograms (DVH) relative to prescription dose were extracted and used to calculate a mean dose distribution. The nominal plan quality was judged on the basis of target coverage at V_90%, V_95%, V_99%, D_90%, D_95%, and D_98% and hotspots were assessed via V_105%, V_110%, D_2% and D_5% with significance determined via Wilcoxon. Target volume homogeneity was assessed via standard deviation (SD), interquartile range (IQR), range width, Levenes variance test, and homogeneity index (HI) (D_2% - D_98%) as per Wu 2003 (PMID: 12738335). Robustness was quantified via Levene and mean DVH 95% confidence interval (CI) width. DVH-difference curves were constructed, allowing for direct dose comparison via paired t-test. a=0.05 for all analyses.
Results: Table 1 arrays mean DVH parameters and levels of significance. In comparison to IMPT, SPArc significantly improved target coverage at all levels and there were strong trends towards hotspot reduction. Examining the mean DVH-difference curve reveals significantly greater volumes at dose levels from 84.7% to 100.2% with an 8.5% volume advantage over this range. Volume advantages were also seen above 102.3%, though these were not significant. SPArc generates more homogenous plans than IMPT as measured by SD (2.3 v. 6.2%), IQR (1.9 v. 4.1%), range (22.7 v. 56.2%), HI (mean: 0.07 v. 0.21, p=0.03), and by Levene (statistic: 65, p<1x10^-14). SPArc also showed greater robustness than IMPT as measured by perturbation-to-perturbation Levene (mean statistic: 1.1 v. 1.6) and a strong trend towards a reduced mean DVH 95% CI width (mean: 0.91 v. 1.26%, p=0.052).
Conclusion: In comparison to IMPT, SPArc therapy achieved substantially greater homogeneity, robustness, and target coverage in the immediate presence of large metal artifacts. This provides a novel solution for a challenging clinical scenario.

%	V90%	V95%	V99%	V105%	V110%		D90%	D95%	D99%	D2%	D5%
SPArc	99.6	99.0	93.6	1.0	0.0		99.4	98.7	97.0	104.4	103.8
IMPT	95.5	89.9	70.6	8.0	0.5		95.0	90.8	84.3	107.7	105.9
p-value	0.03	0.03	0.03	0.03	0.05		0.03	0.03	0.03	0.06	0.06