Clinical outcomes of stereotactic single high dose irradiation of lung tumors under respiratory gating

Abstract

Purpose/Objective: To investigate the clinical response and the morbidities of a stereotactic single high-dose irradiation for lung tumors. Materials/Methods: The Microtron (Hitachi Medical Co., Tokyo, Japan) was modified to deliver stereotactic irradiation of 6MV X-rays under respiratory gating. The treatment couch as well as the gantry move automatically around the isocenter according to the treatment planning. The circular collimators up to 50 mmf can be attached to the gantry. Diameter of the collimator is defined by the 50% isodose line at 100 cm. The collimator moves by computer control to reduce the geometrical errors evoked by gantry and couch movement into within 0.5mm. Respiratory movement of the chest wall is measured continuously by laser displacement monitor placed over the patient’s chest. The obtained respiratory movements of the chest wall are sent to the respiratory gating system, where on-signals for irradiation are delivered to Microtron at an arbitrary respiratory phase. The Microtron can begin irradiation with a lag time of 20 msec after the on-signals from the respiratory gating system. As for the time of radiation delivery, end-expiratory phase with +25% of the amplitude of respiratory cycles are used. For the actual treatment procedures, the patient is fixed non-invasively in the custom-made bed extending from the head to the thighs, and CT images about 20cm from the tumor caudally and rostrally are obtained in the end-expiratory phase with the patient lying in the custom-made bed. The areas around the tumor neighborhood are studies with thin slices of 2-mm thickness and 2 mm intervals. CT images of the tumor at the inspiratory phase are obtained as well. The isocenter is calculated and marked onto the patient skin as well as onto the custom-made bed. The patient lying on the bed is transferred to the treatment couch of the Microtron and after reproducing the isocenter, the range of gantry and treatment couch angles are measured which permits treatment delivery without collision. Multiportal 3-dimensional converging planning is performed. Multi-isocentric radiation was not employed in this series. In this study, clinical target volume (CTV) is considered to be equal to the GTV displayed by CT images of lung field. For planning target volume (PTV), at least 5-mm margins are taken into account in all directions. In the day of the treatment, the patient is laid in the custom-made bed and verification CT is performed to correct patient positioning and to calculate the deviation of isocenter coordinates. Immediately afterwards, the patient lying in the bed is transferred to the treatment couch and the isocenter is reproduced after appropriate corrections. The treatment ensues by delivering X-rays only at the predetermined respiratory phase. Between August 1998 and October 2002, 35 malignant peripheral lung tumors less than 40 mm in a diameter were treated by a single fractional irradiation after informed consent was obtained. Nine tumors were treated by minimal dose of GTV less than 30 Gy and 26 tumors were treated by 30 Gy. The GTV ranged from 1 cc to 19 cc with a mean of 5 cc. The morbidity and response were evaluated. The follow up length ranged from 3 to 24 months with a median of 13 months. Results: As for maximal response, 12 tumors were CR, 17 were PR and 6 was NC. Regrowth was seen in 2 of 9 tumors irradiated less than 30 Gy and 2 (the maximal response NC case) of 26 tumors treated by 30 Gy with follow up length of 2 to 24 months. As for morbidities, RTOG/EORTC grade 2 radiation pneumonitis was seen in a patient. One patient with active tuberculosis and idiopathic interstitial pneumonia had grade 3 respiratory symptoms that necessitated oxygen supplement. In 2 patients with peripheral tumors skin erythema was noted which healed without any scarring. All other tumors could be treated without any acute as well as chronic morbidities. Conclusions: Our results clearly demonstrated that the single high dose irradiation is quite tolerable and can sterilize small lung tumors up to 40 mm in a diameter. Two regrowth was seen in 9 tumors irradiated with minimal dose 30 Gy. Now, we increased minimal dose to 35 Gy.