[Dosimetric analysis of radiotherapy with middle shielding blocks of different widths at the lower cervical supraclavicular field for stage N2-3 nasopharyngeal carcinoma].

Background & Objective: Anterior tangential field irradiation with middle shielding block at the lower cervical and supraclavicular region is needed in the conventional radiotherapy for stage N2-3 nasopharyngeal carcinoma (NPC), but there are still some disagreements on block width. This study was to explore a reasonable block width by dosimetric analysis of anterior tangential field irradiation with middle shielding blocks of different widths designed by the 3-dimensional treatment planning system (3D-TPS) at the lower cervical supraclavicular region for stage N2-3 NPC.

Methods: Ten untreated patients with stage N2-3 NPC received 3D-TPS-designed irradiation plan. For every patient, a gradual shrinking field technique was adopted. Single anterior tangential fields were set at the lower cervical and supraclavicular region and irradiated with middle shielding blocks of different widths: 0 cm (Plan A), 2.1 cm (Plan B), 2.5 cm (Plan C), and 3 cm (Plan D) for the first 40 Gy, then 3 cm for residual dose for all 4 plans. The prescribed doses were the same for 4 plans for every patient. The irradiated volumes and doses of target volumes and organs at risk among the 4 plans were compared.

Results: The high dose coverage (V95 and V90) of plan target volume (PTV) for the subclinical lesion region at the lower cervical supraclavicular region (PTV50a) was significantly higher in Plan A than in Plans B, C, and D (82.44% vs. 78.21%, 77.10% and 73.80% for V95, 87.89% vs. 84.03%, 82.68% and 77.50% for V90, P<0.05), and significantly higher in Plans B and C than in Plan D (P<0.05), but there was no difference between Plans B and C (P>0.05). There was no significant difference in high dose coverage (V95 and V90) of PTV for the primary gross tumor region (PTVnx), PTV for the cervical metastatic nodes (PTVnd), PTV for the high risk region around primary gross tumor (PTVnx60), PTV for the high risk region around metastatic nodes (PTVnd60), and subclinical lesion region above cricoid cartilage (PTV50b) among the 4 plans. There was no difference in the doses for the spinal cord and larynx among the 4 plans. The maximal doses for 50% volumes of target organs (D50) were significantly higher in Plan A than in Plans B, C, and D (49.47 Gy vs. 41.95 Gy, 38.73 Gy, and 26.82 Gy for the thyroid gland, 44.52 Gy vs. 8.41 Gy, 7.03 Gy, and 5.63 Gy for the esophagus, 44.18 Gy vs. 10.16 Gy, 8.55 Gy, and 7.60 Gy for the trachea, P<0.05), and higher in Plans B and C than in Plan D (P<0.05), but there was no difference between Plans B and C (P>0.05). The normal tissue complication probability (NTCP) of the thyroid gland was significantly higher in Plan A than in Plans B, C, and D (7.9% vs. 4.8%, 4.3%, and 3.0%, P<0.05), and higher in Plans B and C than in Plan D, but there was no difference between Plans B and C (P>0.05). There were no difference in the doses for the spinal cord and larynx among the 4 plans. The maximal doses for 50% volumes of target organs (D50) were significantly higher in Plan A than in Plans B, C, and D (49.47 Gy vs. 41.95 Gy, 38.73 Gy, and 26.82 Gy for the thyroid gland, 44.52 Gy vs. 8.41 Gy, 7.03 Gy, and 5.63 Gy for the esophagus, 44.18 Gy vs. 10.16 Gy, 8.55 Gy, and 7.60 Gy for the trachea, P<0.05), and higher in Plans B and C than in Plan D (P<0.05), but there was no difference between Plans B and C (P>0.05). The normal tissue complication probability (NTCP) of the thyroid gland was significantly higher in Plan A than in Plans B, C, and D (7.9% vs. 4.8%, 4.3%, and 3.0%, P<0.05), and higher in Plans B and C than in Plan D, but there was no difference between Plans B and C (P>0.05). There were no difference in the NTCP of other organs at risk among the 4 plans (P>0.05).

Conclusions: Not obviously increasing the irradiation doses for critical organs at risk, Plan A has the best high dose coverage at the subclinical lesion region of the lower cervical and supraclavicular region, while Plan D the worst. We recommend to use no middle shielding block in the anterior tangential field for the first 40 Gy, then use individual middle shielding blocks of 2.1-2.5 cm in the institutes at where set up error is small.