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Comparative Study
. 2013;8(4):e59729.
doi: 10.1371/journal.pone.0059729. Epub 2013 Apr 5.

Critical structure sparing in stereotactic ablative radiotherapy for central lung lesions: helical tomotherapy vs. volumetric modulated arc therapy

Alexander Chi  1 , Pan Ma, Guishan Fu, Gerry Hobbs, James S Welsh, Nam P Nguyen, Si Young Jang, Jinrong Dai, Jing Jin, Ritsuko Komaki
Affiliations
Comparative Study

Critical structure sparing in stereotactic ablative radiotherapy for central lung lesions: helical tomotherapy vs. volumetric modulated arc therapy

Alexander Chi et al. PLoS One. 2013.

Abstract

Background: Helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) are both advanced techniques of delivering intensity-modulated radiotherapy (IMRT). Here, we conduct a study to compare HT and partial-arc VMAT in their ability to spare organs at risk (OARs) when stereotactic ablative radiotherapy (SABR) is delivered to treat centrally located early stage non-small-cell lung cancer or lung metastases.

Methods: 12 patients with centrally located lung lesions were randomly chosen. HT, 2 & 8 arc (Smart Arc, Pinnacle v9.0) plans were generated to deliver 70 Gy in 10 fractions to the planning target volume (PTV). Target and OAR dose parameters were compared. Each technique's ability to meet dose constraints was further investigated.

Results: HT and VMAT plans generated essentially equivalent PTV coverage and dose conformality indices, while a trend for improved dose homogeneity by increasing from 2 to 8 arcs was observed with VMAT. Increasing the number of arcs with VMAT also led to some improvement in OAR sparing. After normalizing to OAR dose constraints, HT was found to be superior to 2 or 8-arc VMAT for optimal OAR sparing (meeting all the dose constraints) (p = 0.0004). All dose constraints were met in HT plans. Increasing from 2 to 8 arcs could not help achieve optimal OAR sparing for 4 patients. 2/4 of them had 3 immediately adjacent structures.

Conclusion: HT appears to be superior to VMAT in OAR sparing mainly in cases which require conformal dose avoidance of multiple immediately adjacent OARs. For such cases, increasing the number of arcs in VMAT cannot significantly improve OAR sparing.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Illustration of the possible scenarios of dose conformity described by the conformation number (CN).
The shade represents the target volume, the dotted line represents the desired isodose, the small solid in c) represent a critical structure that is immediately adjacent to the target. a). the ideal dose conformation with CN = 1. b). Less than optimal coverage of the target volume. c). In situations where the target is next to a critical structure, both adequate dose coverage of the target and the sparing of the critical structure are desired. As a result, more healthy tissue is irradiated in the context of the healthy tissue dose constraint as shown. The CN will be <1 is both b) and c).
Figure 2
Figure 2. Comparison of dose parameters to the organs at risk.
Organs at risk: a) Esophagus, b) heart, c) major airway, d) major vessels, e) spinal cord, f) and g) mean lung dose (MLD) and V20 for the total lung, after normalized to the absolute dose constraints between helical tomotherapy (Tomo), 2-arc, and 8-arc VMAT plans.
Figure 3
Figure 3. Illustration of a comparison of the 2 and 8 Arc plans demonstrating that the shape of the isodose covering the PTV is largely dependent on the immediately adjacent critical structures (yellow and blue) that need to be spared in one patient.
As a result, slightly increased volume of the normal lung tissue is included in the high dose volume lateral to the PTV (blue shade) away from the central structures. Also shown here is that when comparing to the 2 Arc plan, the high dose region included by the 77 Gy isodose in the 8 Arc plan is greatly diminished, demonstrating increased homogeneity.
See this image and copyright information in PMC

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