Comparing proton treatment plans of pediatric brain tumors in two pencil beam scanning nozzles with different spot sizes.

John C Kralik,Timothy D Solberg,Charles B Simone,Liyong Lin,Liwen Xi

doi:10.1120/jacmp.v16i6.5389

John C Kralik, Timothy D Solberg + Show 3 more

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https://doi.org/10.1120/jacmp.v16i6.5389

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Abstract

Target coverage and organ‐at‐risk sparing were compared for 22 pediatric patients with primary brain tumors treated using two distinct nozzles in pencil beam scanning (PBS) proton therapy. Consecutive patients treated at our institution using a PBS‐dedicated nozzle (DN) were replanned using a universal nozzle (UN) beam model and the original DN plan objectives. Various cranial sites were treated among the patients to prescription doses ranging from 45 to 54 Gy. Organs at risk (OARs) evaluated were patient‐dependent; 15 unique OARs were analyzed, all of which were assessed in at least 10 patients. Clinical target volume (CTV) coverage and organ sparing were compared for the two nozzles using dose‐volume histogram data. Statistical analysis using a confidence‐interval approach demonstrates that CTV coverage is equivalent for UN and DN plans within ±5% equivalence bounds. In contrast, average mean and maximum doses are significantly higher for nearly all 15 OARs in the UN plans. The average median increase over all OARs and patients is approximately 1.7 Gy, with an increase in the 25%–75% of 1.0–2.3 Gy; the median increase to the pituitary gland, temporal lobes, eyes and cochleas are 1.8, 1.7, 0.7, and 2.7 Gy, respectively. The CTV dose distributions fall off slower for UN than for the DN plans; hence, normal tissue structures in close proximity to CTVs receive higher doses in UN plans than in DN plans. The higher OAR doses in the UN plans are likely due to the larger spot profile in plans created with UN beams. In light of the high rates of toxicities in pediatric patients receiving cranial irradiation and in light of selected brain tumor types having high cure rates, this study suggests the smaller DN beam profile is preferable for the advantage of reducing dose to OARs.PACS number: 87.55.D‐

Highlights

42 Kralik et al.: Pediatric brain plan comparison with different PBS nozzles dosimetric evaluation of patients with pediatric brain tumors using pencil beam scanning beam lines that utilize one of two distinct treatment nozzles
The 90% confidence interval (CI) for all of the metrics lie within the equivalence bounds, indicating that in terms of Clinical target volume (CTV) coverage, the universal nozzle (UN) plans are equivalent to dedicated nozzle (DN) plans
The CTV coverage quality was quantified using the homogeneity index (HI) and the inhomogeneity coefficient (IC).(5) These metrics, which are derived from the quantities described above, are given by HI = (Dmax - Dmin)/DRx

Summary

Introduction

42 Kralik et al.: Pediatric brain plan comparison with different PBS nozzles dosimetric evaluation of patients with pediatric brain tumors using pencil beam scanning beam lines that utilize one of two distinct treatment nozzles. Our center delivers pencil beam scanning (PBS) proton therapy using two different nozzles: a dedicated nozzle (DN) commissioned in 2012, and a universal nozzle (UN) commissioned in 2013. As the DN and the UN share the same energy selection system, and beamline components within the treatment nozzles consist of only MU chambers and thin vacuum windows, the shape of the two Bragg peaks is indistinguishable. Both DN and UN are commissioned using the same measurement technique and for the same proton dose algorithm (convolution superposition version 10.0.28). Spot profiles are measured using a 0.5 mm resolution scintillation detector and approximated by two-dimensional Gaussian models; the Bragg peaks are collected using an 80 mm diameter parallel chamber.[8,9,11,12]

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