Verification of dose distribution in carbon ion radiotherapy for stage I lung cancer

Publication date: Available online 14 September 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Daisuke Irie, Jun-ichi Saitoh, Katsuyuki Shirai, Takanori Abe, Yoshiki Kubota, Makoto Sakai, Shin-ei Noda, Tatsuya Ohno, Takashi Nakano
PurposeTo evaluate robustness of dose distribution of carbon-ion radiotherapy (C-ion RT) in non-small-cell lung cancer (NSCLC) and to identify factors affecting the dose distribution by simulated dose distribution (SimDD).Methods and MaterialsEighty irradiation fields for delivery of C-ion RT were analyzed in 20 patients with stage I NSCLC. Computed tomography (CT) images were obtained twice before treatment initiation. SimDD was reconstructed on CT for confirmation under the same settings as actual treatment with respiratory gating and bony structure matching. Dose–volume histogram (DVH) parameters, such as %D95 (percentage of D95 relative to the prescribed dose) were calculated. Patients with any field for which the %D95 of gross tumor volume (GTV) was below 90% were classified as unacceptable for treatment, and the optimal target margin for such cases was examined.ResultsFive patients with a total of eight fields (10% of total number of fields analyzed) were classified as unacceptable according to %D95 of GTV, although most patients showed no remarkable change in the DVH parameters. Receiver operating characteristic (ROC) curve analysis showed that tumor displacement and change in water-equivalent pathlength (WEL) were significant predictive factors of unacceptable cases (p < 0.001 and p = 0.002, respectively). The main cause of degradation of the dose distribution was tumor displacement in seven of the eight unacceptable fields. A 6-mm planning target volume margin ensured a GTV %D95 of >90% except in one extremely unacceptable field.ConclusionsAccording to this simulation analysis of C-ion RT for stage I NSCLC, a few fields were reported as unacceptable and required resetting of body position and reconfirmation. In addition, tumor displacement and change in WEL (bone shift and/or chest wall thickness) were identified as factors influencing the robustness of dose distribution. Such uncertainties should be regarded in planning.

Teaser

Dose distribution of carbon ion radiotherapy for stage I lung cancer under the bony structure-matching method was simulated, and its robustness for interfractional changes was evaluated. Eight of 80 field irradiations showed unacceptable degradation of D95 of gross tumor volume, although most field irradiations were robust. The degradation resulted from tumor displacement without correlation with respiration and change in water-equivalent pathlength on the beam pathway. Such uncertainty factors should be regarded in planning.


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