Volumetric-modulated arc therapy for the treatment of a large planning target volume in thoracic esophageal cancer

Abstract

Recently, volumetric-modulated arc therapy (VMAT) has demonstrated the ability to deliver radiation dose precisely and accurately with a shorter delivery time compared to conventional intensity-modulated fixed-field treatment (IMRT). We applied the hypothesis of VMAT technique for the treatment of thoracic esophageal carcinoma to determine superior or equivalent conformal dose coverage for a large thoracic esophageal planning target volume (PTV) with superior or equivalent sparing of organs-at-risk (OARs) doses, and reduce delivery time and monitor units (MUs), in comparison with conventional fixed-field IMRT plans. We also analyzed and compared some other important metrics of treatment planning and treatment delivery for both IMRT and VMAT techniques. These metrics include: 1) the integral dose and the volume receiving intermediate dose levels between IMRT and VMATI plans; 2) the use of 4D CT to determine the internal motion margin; and 3) evaluating the dosimetry of every plan through patient-specific QA. These factors may impact the overall treatment plan quality and outcomes from the individual planning technique used. In this study, we also examined the significance of using two arcs vs. a single-arc VMAT technique for PTV coverage, OARs doses, monitor units and delivery time. Thirteen patients, stage T2-T3 N0-N1 (TNM AJCC 7th edn.), PTV volume median 395 cc (range 281-601 cc), median age 69 years (range 53 to 85), were treated from July 2010 to June 2011 with a four-field (n = 4) or five-field (n = 9) step-and-shoot IMRT technique using a 6 MV beam to a prescribed dose of 50 Gy in 20 to 25 F. These patients were retrospectively replanned using single arc (VMATI, 91 control points) and two arcs (VMATII, 182 control points). All treatment plans of the 13 study cases were evaluated using various dose-volume metrics. These included PTV D99, PTV D95, PTV V9547.5Gy(95%), PTV mean dose, Dmax, PTV dose conformity (Van't Riet conformation number (CN)), mean lung dose, lung V20 and V5, liver V30, and Dmax to the spinal canal prv3mm. Also examined were the total plan monitor units (MUs) and the beam delivery time. Equivalent target coverage was observed with both VMAT single and two-arc plans. The comparison of VMATI with fixed-field IMRT demonstrated equivalent target coverage; statistically no significant difference were found in PTV D99 (p = 0.47), PTV mean (p = 0.12), PTV D95 and PTV V9547.5Gy (95%) (p = 0.38). However, Dmax in VMATI plans was significantly lower compared to IMRT (p = 0.02). The Van't Riet dose conformation number (CN) was also statistically in favor of VMATI plans (p = 0.04). VMATI achieved lower lung V20 (p = 0.05), whereas lung V5 (p = 0.35) and mean lung dose (p = 0.62) were not significantly different. The other OARs, including spinal canal, liver, heart, and kidneys showed no statistically significant differences between the two techniques. Treatment time delivery for VMATI plans was reduced by up to 55% (p = 5.8E-10) and MUs reduced by up to 16% (p = 0.001). Integral dose was not statistically different between the two planning techniques (p = 0.99). There were no statistically significant differences found in dose distribution of the two VMAT techniques (VMATI vs. VMATII) Dose statistics for both VMAT techniques were: PTV D99 (p = 0.76), PTV D95 (p = 0.95), mean PTV dose (p = 0.78), conformation number (CN) (p = 0.26), and MUs (p = 0.1). However, the treatment delivery time for VMATII increased significantly by two-fold (p = 3.0E-11) compared to VMATI. VMAT-based treatment planning is safe and deliverable for patients with thoracic esophageal cancer with similar planning goals, when compared to standard IMRT. The key benefit for VMATI was the reduction in treatment delivery time and MUs, and improvement in dose conformality. In our study, we found no significant difference in VMATII over single-arc VMATI for PTV coverage or OARs doses. However, we observed significant increase in delivery time for VMATII compared to VMATI.

Figure 1

Box‐and‐whisker plot of DVH metrics representation showing the median, max, min, and outliers of IMRT and ${\mathrm{V}\mathrm{M}\mathrm{A}\mathrm{T}}_{\mathrm{I}}$: (a) PTV (D99, D95, D mean, and ${\mathrm{D}}_{\text{max}}$) dose comparison between IMRT and ${\text{VMAT}}_{\mathrm{I}}$ plans; (b) PTV $\mathrm{V}{95}_{47.5\mathrm{G}\mathrm{y}(95\mathrm{\%})}$ coverage for IMRT and ${\text{VMAT}}_{\mathrm{I}}$ plans; (c) organ at risk volume of lungs, heart, liver, kidneys, and spinal canal 3 mm prv comparison; (d) organ at risk mean doses (lungs, right and left kidney).

Figure 2

Integral‐dose comparison [J] of ${\text{VMAT}}_{\mathrm{I}}$ and IMRT plans: (a) linear plot representing a typical patient dose score; (b) box plot represent the minimal and maximum ranges of integral dose.

Figure 3

Irradiated volume difference: (a) box plot of ${\text{VMAT}}_{\mathrm{I}}$ and IMRT representing percentage of volume differences $({\mathrm{V}\mathrm{M}\mathrm{A}\mathrm{T}}_{\mathrm{I}}-\mathrm{I}\mathrm{M}\mathrm{R}\mathrm{T})$ receiving the doses in (Gy); (b) linear plot representing the doses in (Gy) received by the volume in (cc) in ${\text{VMAT}}_{\mathrm{I}}$ and IMRT plans.

Figure 4

Dose distribution comparison between the IMRT (left), ${\text{VMAT}}_{\mathrm{I}}$ single arc (middle), and ${\text{VMAT}}_{\text{II}}$ two full arcs (right) plans for a selected patient, shown in the axial, sagittal, and coronal plane.

Figure 5

Box plot of: (a) Delta4 patient specific QA results $(\mathrm{n}=13)$, representing the median, maximum, minimum, and outliers for in‐house stringent criteria of DTA $\le 2\hspace{0.17em}\mathrm{m}\mathrm{m}$ and gamma index $\le 1(\pm 2\mathrm{\%}/\pm 2\hspace{0.17em}\mathrm{m}\mathrm{m})$ with pass rate of 95% for ${\text{VMAT}}_{\mathrm{I}}$ and IMRT plans; (b) delivery time (sec), ${\text{VMAT}}_{\mathrm{I}}$ (single arc) vs. IMRT treatment plans for all 13 study cases.