Physics contribution
Intensity-modulated radiation therapy (IMRT) for locally advanced paranasal sinus tumors: incorporating clinical decisions in the optimization process

Presented in part at the 42nd Annual Meeting of the American Society for Therapeutic Radiology and Oncology, San Francisco, CA, November 2001.
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Abstract

Purpose: Intensity-modulated radiotherapy (IMRT) plans require decisions about priorities and tradeoffs among competing goals. This study evaluates the incorporation of various clinical decisions into the optimization system, using locally advanced paranasal sinus tumors as a model.

Methods and Materials: Thirteen patients with locally advanced paranasal sinus tumors were retrospectively replanned using inverse planning. Two clinical decisions were assumed: (1) Spare both optic pathways (OP), or (2) Spare only the contralateral OP. In each case, adequate tumor coverage (treated to 70 Gy in 35 fractions) was required. Two beamlet IMRT plans were thus developed for each patient using a class solution cost function. By altering one key variable at a time, different levels of risk of OP toxicity and planning target volume (PTV) compromise were compared in a systematic manner. The resulting clinical tradeoffs were analyzed using dosimetric criteria, tumor control probability (TCP), equivalent uniform dose (EUD), and normal tissue complication probability.

Results: Plan comparisons representing the two clinical decisions (sparing both OP and sparing only the contralateral OP), with respect to minimum dose, TCP, V95, and EUD, demonstrated small, yet statistically significant, differences. However, when individual cases were analyzed further, significant PTV underdosage (>5%) was present in most cases for plans sparing both OP. In 6/13 cases (46%), PTV underdosage was between 5% and 15%, and in 3 cases (23%) was greater than 15%. By comparison, adequate PTV coverage was present in 8/13 cases (62%) for plans sparing only the contralateral OP. Mean target EUD comparisons between the two plans (including 9 cases where a clinical tradeoff between PTV coverage and OP sparing was required) were similar: 68.6 Gy and 69.1 Gy, respectively (p = 0.02). Mean TCP values for those 9 cases were 56.5 vs. 61.7, respectively (p = 0.006).

Conclusions: In IMRT plans for paranasal sinus tumors, tradeoff values between OP toxicity and PTV coverage can be compared for different clinical decisions. The information derived can then be used to individualize the parameters within the optimization system. This process of determining clinical tradeoffs associated with different clinical decisions may be a useful tool in other sites.

Introduction

Locally advanced paranasal sinus tumors are difficult to treat with radiotherapy, because of their close proximity to the optic pathway (OP). The prescription dose required for these tumors usually exceeds the normal-tissue tolerance doses of the adjacent critical structures. Radiotherapy treatment plans often require a compromise between target coverage and sparing of the visual pathway structures. A clinical decision accounting for the patient’s preference must often be made between limiting OP toxicity and maximizing target coverage.

Recent advances in radiotherapy planning using conformal therapy and inverse planning (IP) have sought to improve the therapeutic ratio by increasing the likelihood of tumor control while decreasing the risk of normal-tissue complication. Yet, a tradeoff between these two goals often remains. Inverse planning provides the clinician with a new set of tools to assess clinical tradeoffs in a systematic manner. In some instances, clinical decisions can be incorporated into the inverse plan by altering specific variables in the cost function. A continuum of tradeoff values between sparing the optic pathway and target coverage can therefore be obtained while keeping the dose to other normal structures as constant as possible.

The purpose of this study is to evaluate how IP can be used in the planning process for difficult treatment planning cases where clinical tradeoffs are required. Clinical decisions were incorporated into the IP process for direct comparison of their effects. Namely, two distinct clinical decisions were used in the IP process for advanced paranasal sinus cancer cases: (1) Spare both OP, or (2) Spare only the contralateral OP. The resulting tradeoffs between target coverage and OP toxicity for the different plans were then evaluated using multiple metrics.

Section snippets

Patient selection

Fifty patients with locally advanced paranasal sinus tumors were treated at the University of Michigan between 1990 and 1999. Fifteen patients were retrospectively reviewed based on the following selection criteria: (1) All patients had locally advanced unresectable paranasal sinus cancers, and (2) All patients were treated definitively with three-dimensional (3D) conformal radiotherapy with curative intent. In all cases, the original treatment plans had compromises in target coverage to limit

Results

For each case, geometric considerations such as the tumor volume, location, and proximity to the OP were analyzed to predict patients that may benefit from dose optimization. Although the study had a small number of patients, the proximity of the PTV to the expanded OP was an important factor in predicting for a greater benefit from dose-optimized plans. The average distance between OP and PTV for plans with large differences in V95% (i.e., greater than median of 16.8%) was 3.2 mm (range: 2–5

Discussion

Other studies have also demonstrated that, in comparison to 3D conformal radiotherapy, IMRT plans may potentially decrease OP toxicity while maintaining similar tumor coverage for paranasal sinus tumors. Claus et al. recently reported their findings in a prospective study of 11 patients regarding the ability to spare bilateral OPs using intensity-modulated radiotherapy for the treatment of ethmoid sinus tumors (17). Patients were treated principally in the postoperative setting with 1 patient

Conclusion

In locally advanced paranasal sinus tumors, automated dose-optimized plans for IMRT may allow us, in some cases, to spare bilateral optic pathways. However, this may not be possible in all cases. Our results demonstrate that automated dose optimization planning can provide useful tools to assess clinical tradeoffs. Clinical decisions derived from this information can then be incorporated into the plan optimization process. In locally advanced paranasal sinus tumors, quantitative data regarding

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Supported in part by NIH Grant P01-CA59827.

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