Impact of Gantry Rotation Time on Plan Quality and Dosimetric Verification – Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiotherapy (IMRT)

Der Einfluss der Gantry-Rotationszeit auf Planqualität und dosimetrische Verifikation – Volumetrisch modulierte Rotationsbestrahlung (VMAT) vs. intensitätsmodulierte Strahlentherapie (IMRT)


Background and Purpose:

To compare plan quality criteria and dosimetric accuracy of step-and-shoot intensity-modulated radiotherapy (ss-IMRT) and volumetric modulated arc radiotherapy (VMAT) using two different gantry rotation times.

Patients and Methods:

This retrospective planning study based on 20 patients was comprised of 10 prostate cancer (PC) and 10 head and neck (HN) cancer cases. Each plan contained two target volumes: a primary planning target volume (PTV) and a boost volume. For each patient, one ss-IMRT plan and two VMAT plans at 90 s (VMAT90) and 120 s (VMAT120) per arc were generated with the Pinnacle© planning system. Two arcs were provided for the PTV plans and a single arc for boost volumes. Dosimetric verification of the plans was performed using a 2D ionization chamber array placed in a full scatter phantom.


VMAT reduced delivery time and monitor units for both treatment sites compared to IMRT. VMAT120 vs. VMAT90 increased delivery time and monitor units in PC plans without improving plan quality. For HN cases, VMAT120 provided comparable organs at risk sparing and better target coverage and conformity than VMAT90. In the VMAT plan verification, an average of 97.1% of the detector points passed the 3 mm, 3% γ criterion, while in IMRT verification it was 98.8%.


VMAT90, VMAT120, and IMRT achieved comparable treatment plans. Slower gantry movement in VMAT120 plans only improves dosimetric quality for highly complex targets.



Vergleich von Planqualität und dosimetrischer Genauigkeit von „step-and-shoot“ intensitätsmodulierter Strahlentherapie (ss-IMRT) und volumetrisch modulierter Rotationstherapie (VMAT) bei zwei unterschiedlichen Gantry-Rotationszeiten.

Patienten und Methoden:

Diese retrospektive Planungsstudie basierte auf 20 Patienten: 10 mit Prostatatumor (PC) und 10 mit Kopf-Hals-Tumor (HN). Jeder Plan enthielt 2 Zielvolumina: das primäre Planungszielvolumen (PTV) und ein Boostvolumen. Pro Patient wurden je ein ss-IMRT- und zwei VMAT-Pläne mit je 90 (VMAT90) und 120 (VMAT120) Sekunden pro Rotation im Pinnacle©-Planungssystem generiert. VMAT-Pläne für primäre Zielvolumina wurden mit 2 Vollrotationen erstellt, Pläne für Boostvolumina mit 1 Rotation. Für die dosimetrische Verifikation der Bestrahlungspläne wurden eine 2D Ionisationskammer-Matrix und ein oktagonales Festkörperphantom verwendet.


Im Vergleich zur IMRT wurden durch VMAT-Pläne die Bestrahlungszeit und die Anzahl der Monitoreinheiten für beide Behandlungslokalisationen reduziert (Tabellen 2 und 3). VMAT120 erhöhte gegenüber VMAT90 sowohl die Bestrahlungszeit als auch die Monitoreinheiten für PC-Pläne, ohne eine Verbesserung der Planqualität zu erzielen. Für HN-Patienten konnte mit VMAT120 eine vergleichbare Risikoorganschonung sowie eine bessere Zielvolumenabdeckung als mit VMAT90 erzielt werden (siehe Tabellen 1 und 3, sowie Abbildung 2b). VMAT-Pläne erreichten einen Gamma-Index < 1 (3 mm Abstand und 3% Dosis) für 97,1% der Detektorpunkte, IMRT-Pläne erreichten 98,8% der Detektorpunkte.


VMAT90, VMAT120 und IMRT erzielten vergleichbare Bestrahlungspläne. Eine langsamere Bewegung der Gantry in VMAT120-Plänen konnte die Planqualität nur für sehr komplexe Zielvolumina verbessern.

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Correspondence to Marlies Pasler.

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Pasler, M., Wirtz, H. & Lutterbach, J. Impact of Gantry Rotation Time on Plan Quality and Dosimetric Verification – Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiotherapy (IMRT). Strahlenther Onkol 187, 812–819 (2011).

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Key Words

  • VMAT
  • IMRT
  • Head and neck cancer
  • Prostate cancer


  • VMAT
  • IMRT
  • Kopf-Hals-Tumor
  • Prostatatumor