Clinical Investigation
Possible Misinterpretation of Demarcated Solid Patterns of Radiation Fibrosis on CT Scans as Tumor Recurrence in Patients Receiving Hypofractionated Stereotactic Radiotherapy for Lung Cancer

https://doi.org/10.1016/j.ijrobp.2007.07.2383Get rights and content

Purpose

To retrospectively analyze opacity changes near primary lung cancer tumors irradiated by using hypofractionated stereotactic radiotherapy (HSRT) to determine the presence or absence of tumor recurrence.

Methods and Materials

After review-board approval for a retrospective study, we examined data from 50 patients treated with curative intent for proven or highly suspected localized peripheral-lung cancer and followed up for at least 12 months. All patients had received 50 Gy in five fractions (80% isodose) and were followed up monthly with chest X-ray until clinical and X-ray findings stabilized. Follow-up computed tomography scans were performed 1 and 3 months after HSRT and thereafter at 3-month intervals during the first 2 years.

Results

Median follow-up was 30.4 months (range, 12.0–73.8 months). Abnormal opacities that were suspicious for recurrent tumor appeared in 20 patients at a median of 20.7 months (range, 5.9–61.4 months). Only 3 patients were finally found to have recurrence; 14 were recurrence free but were suspected to have fibrosis, and findings for the other 3 patients were considered equivocal because of a short follow-up period (≤6 months).

Conclusion

Radiation fibrosis, which may occur 1 year or longer after completion of HSRT, is difficult to distinguish from tumor recurrence. Even when opacities increase on follow-up radiologic scans, recurrence cannot be diagnosed conclusively based on image findings; biopsy occasionally is warranted.

Introduction

Delivery of a high dose of radiation by using hypofractionated stereotactic radiotherapy (HSRT) often results in radiopacity within the boundaries of a tumor, which would be considered tumor progression. To our knowledge, no study to date evaluated morphologic changes in tumors after HSRT, and only a few reports focused on HSRT-induced lung injuries 1, 2, 3.

Complete disappearance of the tumor is rarely seen during follow-up of patients with lung cancer with HSRT. As in many cases, after conventional radiotherapy, the primary tumor may be obscured by postradiation change. A degree of tumor shrinkage usually is observed, and a variety of radiologic patterns that may reflect modifications caused by irradiation also is seen 1, 3. In patients with a good response, tumor opacities usually shrink and thereafter appear as scar-like or linear patterns that persist on radiologic follow-up images. We generally consider these radiologic patterns to reflect tumor control. However, there are cases in which opacities are only minimally reduced in size. Moreover, diagnostic difficulty is often encountered when trying to determine whether the patterns reflect tumor recurrence or mass-like fibrosis after radiotherapy 4, 5. In patients who are candidates for such salvage treatments as surgery, cryoablation (6), and radiofrequency ablation (7), it is essential to diagnose recurrence as early as possible because the smaller the tumor, the easier it is to treat. For example, an adequate size for cryoablation of lung tumors is less than 3 cm in diameter (6). Needless to say, it is important to establish standards for determining tumor recurrence after HSRT to appropriately assess its treatment outcome.

In this study, we retrospectively reviewed follow-up computed tomography (CT) studies of 50 patients obtained during a period of at least 1 year after HSRT for localized peripheral primary lung cancer. Our goal was to analyze opacity changes in the vicinity of irradiated tumors to determine whether there was tumor recurrence.

Section snippets

Patients

Our institutional review boards approved this study and waived the need for informed consent based on the retrospective design. The study group was composed of 50 patients who underwent HSRT with curative intent for proven or highly suspected localized primary lung cancer located in the periphery of the lung. One case was located in segment 1 + 2 of the left lung and invasion to the aortic arch was noted; however, no patient with thoracic wall invasion was included. All were treated at

Results

Median follow-up was 30.4 months (range, 12.0–73.8 months). Excluding radiologic patterns typical of radiation pneumonitis that occurred 2 to 7 months after radiotherapy, abnormal opacities suspected of being recurrent tumors appeared in 20 patients (Fig. 1). Median time between completion of HSRT and a radiologically recognizable increase in tumor opacity was 20.7 months (range, 5.9–61.4 months). The remaining 30 patients were judged to have achieved tumor control because tumor opacities

Discussion

Careful follow-up reading of radiologic images from patients treated with HSRT showed that radiologic patterns seen after HSRT are likely to differ from those appearing after conventional radiotherapy. An earlier report found that radiation-induced lung injury occurred during the first year after HSRT (2). The conduction of longer term follow-up of radiologic patterns after HSRT is showing many cases in which it is difficult to determine whether residual opacities reflect viable malignant

Conclusion

In conclusion, it should be noted that radiation fibrosis, which may occur 1 year or more after completion of HSRT, is difficult to distinguish from tumor recurrence. Even when opacities increase on follow-up radiologic scans and high uptake is noted on FDG-PET scans, recurrence cannot be conclusively diagnosed based on image findings of increased opacities and high metabolism on PET scans. Biopsy occasionally is still warranted.

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