Dose evaluation of conventional radiotherapy using facial-cervical fields in nasopharyngeal carcinoma / 中华放射肿瘤学杂志

ABSTRACT

Objective To evaluate the dose distribution of the target volume and the cranial base in nasopharyngeal carcinoma ( NPC ) treated with facial-cervical fields, and to analyze the differences of dose distribution using different isoeenters with the CT-simulator and treatment planning system (TPS). Methods Eleven patients with nasopharyngeal carcinoma were treated by conventional radiotherapy as their primary treatment. All patients were simulated by the conventional simulator and the field borders were marked with thin lead wires on the mask. Then the patients were scanned by the CT-sim with the same immobilization. The planning CT images were transferred to the TPS and the field borders were copied on the DRR, and then GTV and the cranial base were contoured on the coronal CT slices. Two isoeenters were chosen, including one in front of the 1 st cervical vertebra to measeure the depth of the nasopharynx and the other in front of the 3rd cervical vertebra to measure the depth of the upper neck. The prescription dose of 36 Gy was given in 18 fractions. Dose distributions of GTV and the cranial base were calculated with TPS. Results The actual dose of 95% volume of GTV was 33.31 -35.54 Gy (median 34.83 Gy) and 31.43 -33.36 Gy (median 32.44 Gy) when the isoeenters were set in the nasopharynx and the superior neck, respectively. The corre-sponding actual dose of 95% volume of the cranial base was 17.76 - 34.60 Gy ( median 30.28 Gy ) and 16.52 -32.60 Gy (median 28.52 Gy), respectively. Conclusions For NPC patients treated with conven-tional radiotherapy using facial-cervical fields, the actual dose of GTV and the cranial base is lower than the prescribed dose whenever the isocenter is set in the nasopharynx or the upper neck,which is more significant in the latter. The isocenter should be set in the nasopharynx when the conventional radiotherapy is applied and a boost of 4- 8 Gy should be given when the cranial base is involved.