Effect of interfractional shoulder motion on low neck nodal targets for patients treated using volumetric-modulated arc therapy (VMAT).

Kevin E Casey,Samuel S Tung,Pei‐Fong Wong

doi:10.1120/jacmp.v16i4.5206

Abstract

VMAT is an important tool in the treatment of head and neck cancers, many of which also require treatment to the supraclavicular lymph nodes. However, full VMAT arcs treating this nodal region necessarily cause entrance beam to pass through patients' shoulders. Thus, interfractional variations in shoulder position may cause unwanted dose perturbations. To assess this possibility, six patients undergoing treatment at our institution for head and neck cancers with associated supraclavicular lymph node treatment were imaged with in‐room CT‐on‐rails during the course of their treatments. This allowed for the establishment of a true record of the actual shoulder position during selected treatment fractions. Then, a full VMAT plan and a plan with VMAT arcs superior to the shoulder and a static anteroposterior field inferiorly were copied onto the patients' weekly image sets. The average one‐dimensional shoulder motion was generally within 10 mm of the simulated position, with some notable exceptions. The standard deviation in week‐to‐week shoulder position relative to simulation was 4.3 mm and 4.2 mm in the SI and AP dimensions, respectively. The average nodal target mean dose across all fractions sampled was within 5% of planned for all patients and both plans. Similarly, the average D95 for the nodal target was within 5% of planned across all fractions sampled, with the single exception of the full VMAT plan for one patient. In most cases, the standard deviation in both target mean dose and D95 was smaller with the VMAT+static AP field plan than it was with the full VMAT plan.PACS number: 87.55.‐x

Highlights

Volumetric-modulated arc therapy (VMAT)(1) has become an important technique in radiation oncology, including in the treatment of head and neck cancers
We have found interfractional shoulder motion for six patients to be largely consistent with previous literature.[6]. Using our immobilization device, most weekly shoulder positions were within 1 cm of the position at the time of simulation in the superior–inferior and anterior–posterior dimensions, with exceptional cases of motion up to approximately 2.5 cm in a single dimension
We found no clear evidence of superiority in the average supraclavicular target mean dose or average target D95 for either the Full VMAT or the Half-Beam VMAT technique in six patients

Summary

Introduction

Volumetric-modulated arc therapy (VMAT)(1) has become an important technique in radiation oncology, including in the treatment of head and neck cancers. Interfractional shoulder position changes could potentially cause clinically significant dose perturbations when using conventional arcs. Neubauer et al[6] investigated the effect of interfractional shoulder motion on supraclavicular targets using planning structures and found potential for loss of target coverage or increased OAR dose when treating with full 360° VMAT arcs. In-room CT-on-rails technology in our institution allows us to obtain weekly or daily diagnostic-quality CT scans of patients in the actual treatment position immediately prior to treatment delivery. By obtaining CT scans using this system periodically throughout the course of treatment, we may obtain a true record of the patient’s interfractional shoulder motion and use our treatment planning system (Pinnacle 9.4, Philips, Fitchburg, WI) to investigate the actual effect as VMAT arcs pass through the shoulder

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Discussion

Conclusion