Physics Contribution
Restricted Field IMRT Dramatically Enhances IMRT Planning for Mesothelioma

https://doi.org/10.1016/j.ijrobp.2007.06.075Get rights and content

Purpose

To improve the target coverage and normal tissue sparing of intensity-modulated radiotherapy (IMRT) for mesothelioma after extrapleural pneumonectomy.

Methods and Materials

Thirteen plans from patients previously treated with IMRT for mesothelioma were replanned using a restricted field technique. This technique was novel in two ways. It limited the entrance beams to 200° around the target and three to four beams per case had their field apertures restricted down to the level of the heart or liver to further limit the contralateral lung dose. New constraints were added that included a mean lung dose of <9.5 Gy and volume receiving ≥5 Gy of <55%.

Results

In all cases, the planning target volume coverage was excellent, with an average of 97% coverage of the planning target volume by the target dose. No change was seen in the target coverage with the new technique. The heart, kidneys, and esophagus were all kept under tolerance in all cases. The average mean lung dose, volume receiving ≥20 Gy, and volume receiving ≥5 Gy with the new technique was 6.6 Gy, 3.0%, and 50.8%, respectively, compared with 13.8 Gy, 15%, and 90% with the previous technique (p < 0.0001 for all three comparisons). The maximal value for any case in the cohort was 8.0 Gy, 7.3%, and 57.5% for the mean lung dose, volume receiving ≥20 Gy, and volume receiving ≥5 Gy, respectively.

Conclusion

Restricted field IMRT provides an improved method to deliver IMRT to a complex target after extrapleural pneumonectomy. An upcoming Phase I trial will provide validation of these results.

Introduction

Malignant pleural mesothelioma is a deadly disease with an extremely invasive local growth pattern. Extrapleural pneumonectomy (1) has emerged as a safe and effective strategy in some patients with disease limited to the thorax (2). However, the local recurrence rate with surgery alone is exceedingly high (3), and, therefore, adjuvant therapies have been investigated to reduce the local recurrence rate. Significant among these is the use of thoracic radiotherapy (RT).

Hemithoracic RT has been used after extrapleural pneumonectomy (EPP) to reduce the local recurrence rate 4, 5. The radiation dose used is limited because of radiosensitive normal tissue structures, such as the heart, liver, and contralateral lung. Consequently, the dose of full hemithoracic RT has been limited to 30–40 Gy, with focal boosts to 54 Gy 4, 5. We have recently shown these doses to be helpful at reducing the local recurrence rate from 50% to 25% with newer techniques (6).

A novel method of radiation delivery, intensity-modulated RT (IMRT) has been developed that allows for significant improvements in the dose delivery of RT and can allow for improved local control 7, 8, 9. However, in the initial experience with this technique, a high rate of death from treatment was seen 10, 11. This was hypothesized to have resulted from the increased radiation dose to the contralateral lung with IMRT. Therefore, we have developed a modification of the IMRT approach designed to maintain the improved coverage of the target volumes achieved with the previous IMRT protocols but with a substantially reduced contralateral lung dose.

Section snippets

Case selection

Thirteen plans for patients who were treated with IMRT after EPP for mesothelioma and previously reported (10) were used for this study. All the contours and structures were maintained without modifications.

Dose–volume constraints

The new and old dose–volume histogram constraints are described in Table 1. The significant changes included constraints on the remaining lung: mean lung dose (MLD) of <9.5 Gy, volume receiving ≥20 Gy (V20) of <10%, and volume receiving ≥5 Gy (V5) of <60%.

Planning technique

In addition to these stricter

Results

In all 13 replanned cases, we were able to achieve a much-improved plan with both adequate target coverage and improved normal tissue sparing. On average, 97% of the planning target volume receiving 54 Gy (PTV54) was covered by the prescription dose. The average boost PTV coverage was >98%. In all but 2 cases, the targeted PTV54 coverage was achieved. In both of these cases, >92% of the PTV54 was covered by the prescription dose, and the regions with undercoverage were directly adjacent to

Discussion

Local control remains an important endpoint in mesothelioma patients after EPP. Multiple investigators have demonstrated that IMRT provides superior dose distribution to conventional planning in the post-EPP setting 7, 10, 11, 12. In particular, IMRT is helpful in the mediastinum and upper abdomen, where conventional techniques fail (6). However, one must balance these target goals against the normal tissue tolerances.

Data from our own institution, as well as from others, have shown that IMRT

Conclusion

We believe that IMRT is the best way to treat the complex target volume of mesothelioma after EPP. The restricted field technique we have presented provides excellent results for target volume coverage while sparing normal tissues and resulting in much lower contralateral lung doses than the traditional IMRT approach. We plan to validate this approach in an upcoming Phase I trial.

References (21)

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Presented in part at the 48th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO), Philadelphia, PA, November 5–9, 2006.

Conflict of interest: none.

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