Aberrant mediastinal basal pulmonary artery encountered at anatomical lung resection: A case report and review of the literature

Introduction Abnormal branching of the pulmonary artery is often encountered in anatomical lung resection, which can potentially result in accidental vessel injury with life-threatening bleeding or extra lung resection. The mediastinal basal pulmonary artery (Arteria Praebronchialis, AP) is a very rare but potentially critical variant. Presentation of case We present the case of a patient with lung cancer accompanied by the left basal segmental pulmonary artery, independent A8a+9, which was liable to be injured during lower lobectomy with poor interlobar fissure development. This variation was preoperatively recognized using three-dimensional contrast-enhanced computed tomography (3D-CECT) angiography, and vessel injury was avoided. Discussion and literature review 3D-CECT angiography was effective in identifying this rare but potentially critical variation, and it is desirable to perform it routinely before anatomical lung resection. A review of 31 AP cases revealed that the branching pattern of AP was independent (15 patients, 48 %) and common trunk type (16 patients, 52 %), one half for each. Mediastinal branching of the lingular artery was more frequent among the reported AP cases (71 %) than in general reports. Conclusion When mediastinal branches of left pulmonary artery are encountered, the possibility that it is AP should be always taken into account.


Introduction
Abnormal branching of the pulmonary artery is often encountered during anatomical lung resections.A mediastinal basal pulmonary artery on the left side, also known as Arteria Praebronchialis (AP), is a very rare anatomical variation (0.05 %) [1], but can be potentially critical by mistaking it for the mediastinal lingular artery to result in accidental extra-lung resection or severe bleeding.Herein, we present a case of AP detected on preoperative 3D-CECT angiography that underwent safe left lower lobectomy for lung cancer.In addition, literature on 31 AP cases was reviewed.

Presentation of case
A 69-year-old woman with a history of bilateral breast cancer surgery was diagnosed with primary lung adenocarcinoma in the left lower lobe S 9 a, 18 × 12 × 16 mm in size, which was asymptomatic, identified by follow-up CT scan, and histologically diagnosed by CT-guided needle biopsy.The disease was classified as c-T1bN0M0 stage IA 2, according to the Union for International Cancer Control (UICC) classification (seventh edition), following fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT (Fig. 1) and brain magnetic resonance imaging.Preoperative 3D-CECT angiography clearly detected an anatomical variant of the left basal segmental pulmonary artery A 8a+9 , which branches independently from the proximal portion of the left main pulmonary trunk medio-distally adjacent to the independent mediastinal A 4+5 , descends behind both the upper pulmonary vein and the upper lobe bronchus, and flows into the basal segments S 8 a and S 9 just lateral to the inferior pulmonary vein on the anteromedial aspect of the lower lobe.3D-CECT angiography also demonstrated an independently branching mediastinal lingular artery (A 4+5 ) without interlobar branches (Fig. 2).Extremely poor development of the interlobar fissure of the left lung was also observed on CT (Fig. 1c).There were no anomalies of the bronchi or pulmonary veins.Video-assisted thoracoscopic left lower lobectomy and lymph node dissection were performed for lung cancer without injuring the AP, A 8a+9 .After identifying the aberrant A 8a+9 branch entering the lower lobe from the anteromedial aspect (Fig. 3a), the pulmonary artery trunk was posteriorly dissected, and the A 6 branch was dissected and divided first.A poorly developed interlobar fissure was plicated by dividing the lung parenchyma step-by-step along the lateral surface of the pulmonary artery trunk in the direction from the posterior to anterior using a stapling device.Just before separation of the most anterior portion of the interlobar fissure, the aberrant A 8a+9 segmental pulmonary artery branch was ligated and divided to avoid injury (Fig. 3b, c; Video 1).Following division of the main trunk of the pulmonary artery (A 8a+10 ) to the lower lobe in the interlobar fissure by stapling, the lower lobe bronchus beneath was divided by stapling, and lobectomy was completed.The postoperative course was uneventful and the patient was discharged on postoperative day 8.The present case is consistent with the SCARE criteria [2].

Discussion
We performed thoracoscopic left lower lobectomy in a lung cancer patient with AP.The number of case reports on this variation has been increasing in recent years, owing to the wider use of 3D-CECT angiography (Table 1).Pulmonary vascular variation is reported in 16.4 % of patients, of whom 47.8 % exist in the left lung [3].An abnormal pulmonary vessel branch can potentially induce unexpected vessel injury, with life-threatening bleeding or accidental extra-lung resection.Careful surgery is required to dissect the branch that enters the preserved lobe.This is especially emphasized in video-assisted thoracoscopic lung resection, in which it is difficult to obtain a comprehensive picture of vessel anatomy.Preoperative 3D-CECT angiography reconstruction is helpful for identifying vessel anomalies and preventing such accidents.Hence, it is desirable to confirm vascular branching patterns routinely using this noninvasive investigation before anatomical lung resection, enabling surgeons to perform lung surgery more safely with fewer complications [4].In patients with mediastinal branches of the pulmonary artery on the left side, vascular treatment should be managed by considering the possibility that the branches may enter either the lingular or basal segments [4].
In the literature, we identified 30 patients with AP in the left lung other than the present case (Table 1).Among the 31 patients, the median age at diagnosis was 68.5 years (range: 29-84 years).Twenty-three cases (74 %) occurred in males, seven (23 %) in females, and one (3 %) was unknown, with definite male predilection [1].Reports of this variation are all from Asian countries: 23 patients from Japan [5,6,[8][9][10][11][12][13][14][16][17][18][19], five from Korea [1,15], and one each from China [3], Malaysia, and Singapore [7].The racial predilection remains unclear.Twenty-six patients (84 %) underwent anatomical lung resection for left lung cancer, comprising 18 left upper lobectomies, seven left lower lobectomies, and one left S 8+9 segmentectomy.The remaining five patients (16 %) had no surgery, one had pneumonia, one had lung cancer, one had colon cancer, and one had an unknown diagnosis.Among the 26 surgical patients, a preoperative diagnosis of AP was made in 17 patients (65 %) using the following modalities: 3D-CECT angiography in 12, two-dimensional (2D) CECT in 2, 2D-plain CT in 1, and angiography in 2. The remaining nine patients without preoperative diagnosis (35 %) underwent seven left upper lobectomies and two left lower lobectomies, and anomalous AP branches were recognized intraoperatively for the first time.Particularly in left upper lobectomies, incorrect division of the AP branch can occur by mistaking it for the mediastinal lingular artery, which could result in left pneumonectomy in the worst-case scenario.This occurred in two patients (Patients 4 and 30 in Table 1) during left upper lobectomy in the literature, and reconstruction of divided branches was performed to avoid pneumonectomy in both patients [6,7].The diagnosis of AP among five patients without surgery was obtained by 2D-CECT in three patients and 3D-CECT angiography in 2. Furthermore, size dominance of the AP compared to the main pulmonary artery trunk was observed in 8 (26 %), and poor interlobar fissure development of the left lung was reported in six (21 %) of the studies.Consistent with the description by Hong et al. [1], no A 1+2 or A 6 The branching pattern of AP is diverse [1] and is anatomically classified into the following four types (Table 2a): independent (15 patients, 48 %), common trunk with the lingular artery (13 patients, 42 %), common trunk with arteries other than the lingular artery (0 patients, 0 %), and common trunk with both the lingular artery and arteries other than the lingular artery (3 patients, 10 %).In the independent type, the AP branches independently from the main trunk of the pulmonary artery.In the common trunk type, the AP forms a common trunk with lingular artery branches, other than the lingular artery, or both.These results can be simplified as independent (15 patients, 48 %) and common trunk types (16 patients, 52 %), with one-half each.
In contrast, among the 31 patients with AP, the branching pattern of the lingular pulmonary artery was categorized into the following four groups (Table 2b): purely interlobar (6 patients, 19 %), purely mediastinal (14 patients, 45 %), a combination of both (8 patients, 26 %), and uncertain (3 patients, 10 %).The mediastinal branching patterns of the lingular artery among AP cases accounted for 71 %, which was remarkably larger than that generally reported [5].A summary of the lingular artery branching patterns among several literatures reported by He et al. [20] showed that mediastinal origin accounts for approximately 30 % and purely interlobar origin accounts for approximately 70 % in general.Based on the fact that AP branches that form the commontrunk with mediastinal lingular artery account for as many as 52 % of all reported AP cases (Table 2a and b), it is critical to understand the relationship between AP and the mediastinal lingular artery and discriminate them in surgery.
Table 2a shows the branching patterns of the lingular artery for the four groups of AP branching patterns.The independent type of AP (15 patients, 48 %) mainly consisted of a purely interlobar lingular artery in six patients and a purely mediastinal lingular artery in seven patients, equally divided in half.The common trunk with lingular artery type of AP (13 patients, 42 %) consisted of a purely mediastinal lingular artery in seven patients and a combination type of lingular artery (mediastinal and interlobar branches) in six patients, equally divided in half.No patients had a common trunk with other than lingular artery alone as a branching type of AP, and only three patients showed a common trunk with both lingular artery and other than lingular artery.
In contrast, Table 2b shows the branching patterns of AP in the four groups of lingular artery branching patterns.In the purely interlobar type of lingular artery (6 patients, 19 %), all 6 patients showed an independent branching pattern of AP.In the purely mediastinal type of lingular artery (14 patients, 45 %), half (7 patients) showed an independent branching pattern of AP, and the remaining half (7 patients) showed a common trunk with lingular artery pattern of AP, one half for each.In the combination type of the lingular artery (eight patients, 26 %), the common trunk with lingular artery pattern of AP accounted for almost all (six out of eight patients).These findings suggest that the embryonic origin of the AP intimately correlates with the mediastinal branching of the lingular artery.

Conclusion
A lung cancer patient with AP (A 8a+9 ) underwent a thoracoscopic left lower lobectomy.Preoperative 3D-CECT angiography is effective for identifying this rare but potentially critical variation.Thirty-one AP cases in the literature demonstrated a branching pattern of AP as an independent or common trunk type, half of each.AP was also more frequently accompanied by a mediastinal lingular artery (71 %) than generally reported.

Fig. 1 .
Fig. 1.Preoperative chest computed tomography (CT) and FDG-PET/CT images.(a) CT, axial section image; (b) CT, coronal section image: primary lung adenocarcinoma in the left lower lobe S 9 a, 18 × 12 × 16 mm (red arrow); (c) CT, sagittal section image showing a poorly developed interlobar fissure of the left lung.(d) FDG-PET/CT: FDG uptake into the cancer is significant (SUVmax: 7.44) (red arrow).(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 .
Fig. 2. 3D-CECT angiography images.(a) Combination 3D image of the pulmonary vessels and bronchial tree: green dotted line: A 4+5 , blue dotted line: aberrant mediastinal basal artery A 8a+9 .Green mass: S 9 a lung cancer.(b) 3D image of the bronchial tree; the left bronchial tree is depicted with a blue dotted line.(c) Combination 3D image of the pulmonary artery and bronchial tree: green dotted line: A 4+5 , blue dotted line: aberrant mediastinal basal artery A 8a+9 .By utilizing a combination of 3D images of the accompanying pulmonary artery and bronchial tree, accurate identification of AP, A 8a+9, becomes easier.(d) 3D image of the pulmonary artery: green dotted line: independent A 4+5 ; blue dotted line: independent AP, A 8a+9 .(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)