Physics contribution
Benefit of respiration-gated stereotactic radiotherapy for stage I lung cancer: An analysis of 4DCT datasets

https://doi.org/10.1016/j.ijrobp.2005.01.032Get rights and content

Purpose: High local control rates have been reported with stereotactic radiotherapy (SRT) for Stage I non–small-cell lung cancer. Because high-dose fractions are used, reduction in treatment portals will reduce the risk of toxicity to adjacent structures. Respiratory gating can allow reduced field sizes and planning four-dimensional computed tomography scans were retrospectively analyzed to study the benefits for gated SRT and identify patients who derive significant benefit from this approach.

Methods and Materials: A total of 31 consecutive patients underwent a four-dimensional computed tomography scan, in which three-dimensional computed tomography datasets for 10 phase bins of the respiratory cycle were acquired during free breathing. For a total of 34 tumors, the three planning target volumes (PTVs) were analyzed, namely (1) PTV10bins, derived from an internal target volume (ITV) that incorporated all observed mobility (ITV10bins), with the addition of a 3-mm isotropic setup margin; (2) PTVgating, derived from an ITV generated from mobility observed in three consecutive phases (“bins”) during tidal-expiration, plus addition of a 3-mm isotropic margin; and (3) PTV10 mm, derived from the addition of a 10-mm isotropic margin to the most central gross tumor volumes in the three bins selected for gating.

Results: The PTV10bins and PTVgating were, on average, 48.2% and 33.3% of the PTV10 mm, and respective mean volumes of normal tissue (outside the PTV) receiving the prescribed doses were 57.1% and 39.1%, respectively, of that of PTV10 mm. A significant correlation was seen between the extent of tumor mobility (i.e., a three-dimensional mobility vector of at least 1 cm) and reduction in normal tissue irradiation achieved with gating. The ratio of the intersecting and the encompassing volumes of GTVs at extreme phases of tidal respiration predicted for the benefits of gated respiration.

Conclusion: The use of “standard population-based” margins for SRT leads to unnecessary normal tissue irradiation. The risk of toxicity is further reduced if respiration-gated radiotherapy is used to treat mobile tumors. These findings suggest that gated SRT will be of clinical relevance in selected patients with mobile tumors.

Introduction

In patients presenting with Stage I non–small-cell lung cancer (NSCLC), far higher local control rates have been reported using stereotactic radiotherapy (SRT) (1, 2, 3, 4, 5, 6, 7, 8) than have been reported with conventional fractionated radiotherapy (9). SRT generally combines high-precision irradiation with high fraction doses, and total biologic doses well in excess of 100 Gy. Although the reported clinical toxicity after SRT for lung cancer is low, this may reflect both patient selection (e.g., small peripheral lung cancers) and a relatively short follow-up in most series. The high dose per fraction used can result in toxicity to normal lung tissue and adjacent structures such as the esophagus, spinal cord, and thoracic wall (10); therefore, it is important that treatment portals for SRT should remain as small as possible. Because most SRT approaches use online setup correction protocols, measures to reduce the margins used to account for tumor mobility offer a possible solution for minimizing toxicity.

Recent studies show that individualized, and not standard population-based margins, are necessary for high-precision radiotherapy of Stage I NSCLC (11, 12). Approaches such as breath-hold techniques and respiration-gated radiotherapy have been used to reduce target volumes for lung cancer (13, 14). However, breath-hold techniques are often poorly tolerated by patients with lung cancer (13). Respiratory gating permits a reduction in field sizes, because irradiation can be limited to phases in which the mobile target volume is in a predetermined position. Commercially available gating systems (e.g., the Varian Real-Time Position Management [RPM], Varian Medical Systems, Palo Alto, CA) are already in clinical use (15).

For SRT planning in Stage I NSCLC, use of a single four-dimensional (4D) or respiration-correlated computed tomography (CT) scan was superior to the use of multiple conventional CT scans (16). Because our 4DCT scan technique also uses RPM gating equipment for coregistering respiratory signals, we analyzed 4D scans of 31 patients to establish the benefit with use of respiratory gating in SRT and establish the criteria for identifying patients who could derive significant benefit from this approach.

Section snippets

Methods and materials

A total of 34 tumors in 31 consecutive patients with Stage I NSCLC, who were treated with fractionated SRT using 4DCT scans at the VU Medical Center, Amsterdam were included in this study.

Results

A total of 34 tumors were analyzed. Tumor characteristics are shown in Table 1. There were 24 T1N0 tumors (70.6%) and 10 T2N0 tumors (29.4%). The mean GTVs of T1 and T2 tumors were 4.8 mL and 27.3 mL, respectively. Included were 11 lower lobe tumors, 22 upper lobe tumors, and 1 middle lobe tumor, and 8 tumors were adjacent to the pleura.

Discussion

The reduction of the planning margins required to account for tumor mobility is the subject of active, ongoing research. Most of the patients with Stage I NSCLC who are referred for radiotherapy are ineligible for surgery because of poor pulmonary function (18), which explains an inability to tolerate procedures such as deep-inspiration breath-hold (DIBH) in up to 50% of patients (13). As such, passive respiration-gated radiotherapy would appear to be an ideal approach for reducing the risk of

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