A quantitative investigation on lung tumor site on its motion tracking in radiotherapy with external surrogates

Introduction: In external beam radiotherapy each effort is done to deliver 3D dose distribution onto the tumor volume uniformly, while minimizing the dose to healthy organs at the same time. Radiation treatment of tumors located at thorax region such as lung and liver has a challenging issue during target localization since these tumors move mainly due to respiration. There are several methods to compensate the effect or tumor motion error, clinically. Recently, stereotactic body radiation therapy strategy has been proposed clinically that can lead to better local control using external surrogates. In this method, the exact information of tumor position in real-time is obtained using consistent correlation models between tumor motion and external surrogates motion. In this work, the influence of motion variability and the site of lung tumor has been taken into account over total treatment time.   Materials and Methods: In this study, motion information of lung tumor of real patients treated with the CyberKnife Synchrony system, have been utilized. Real time tumor motion tracking is performed by developing a fuzzy logic based correlation model to predict tumor position using external surrogate’s motion. In this work the variabilities of lung tumor motion in the Superior-Inferior (SI), Latero-Lateral (LL), and Anterior-Posterior (AP) directions and their geometrical location in lung region have been investigated on the performance accuracy of correlation model, as metric. With divided lung tumors location into four upper, lower, left and right sites.   Results: Targeting errors between of 20 patients with lung tumors located at various sites have been recorded using Root Mean Square Error (RMSE). Intra-fraction variation of lung tumors ranges from 4.0 to 70.5 mm at SI, 1.7 to 75.5 mm at LL, 1.8 to 40.7 mm at AP directions. Total treatment time ranges between 27.0 to 118.8 minutes over total patients. Moreover, the average of RMSEs during tumor tracking by means of correlation model are 15.6 mm, 6.7 mm, 7.8 mm and 9.5 mm for upper, lower, left and right tumors, respectively in one division.   Conclusion: There are many influencing factors and patterns on lung tumor motion management at external radiotherapy. By understanding these factors and considering to them during real time tumor tracking, the treatment quality may have enhanced significantly at stereotactic body radiation therapy. In this work we assess tumors motion variability and their geometrical sites on the accuracy of our developed fuzzy correlation model, quantitatively.