Imaging Features following Thermal Ablation of Lung Malignancies

Abstract

Percutaneous image-guided thermal ablation is gaining attraction as an effective alternative to surgical resection for patients with primary and secondary malignancies of the lung. Currently, no standard follow-up imaging protocol has been established or uniformly accepted. The early identification of residual or recurrent tumor would in theory enable the practitioner to offer expeditious retreatment or alternative treatment. This review elaborates on the imaging findings following thermal ablation, both heat- and cold-based, of nonresectable pulmonary malignancies.

Keywords: computed tomography; cryotherapy; imaging findings; lung cancer; microwave; positron emission tomography; radiofrequency; thermal ablation.

Figure 1

Combined positron emission tomography/computed tomography (PET/CT) and CT modalities used in the follow-up imaging protocol after ablation. Initial PET/CT is required for staging. Thereafter, chest CT with CT nodule densitometry through the ablation zone is performed at 1 to 2 months, PET/CT at 3 months, and thereafter alternating every 3 months with chest CT for up to 2 years.

Figure 2

(A, B) Computed tomography (CT) and positron emission tomography (PET) images of a 77-year-old man with adenocarcinoma of the lung in the superior segment of the right lower lobe obtained 1 week before ablation demonstrates a hypermetabolic nodule. (C, D) Microwave ablation (MWA) was performed with the patient in the prone position using a posterior approach and using 45 W for 7 minutes. Images are rotated to maintain continuity of lesion location. Immediately after the ablation there is central tissue desiccation and cavitation along the antennae tract. A second layer and rim of faint ground-glass opacity (GGO) surrounds the tumor and represents tissue necrosis. An outermost rim of denser GGO (white arrows) represents congested lung tissue and hemorrhage that retains viability. (E) Approximately 48 hours after MWA, there is an area of cavitation in the postablation zone, and the antennae tract remains desiccated. The ablation zone is larger than the pretreatment tumor and represents hemorrhage, edema, and necrotic tissue. With time the ablation zone should regress in size. (F, G) CT and PET at 6 weeks postablation. There is a decrease in size of the postablation zone and surrounding hemorrhage, with persistent central cavitation. The PET activity has returned to mediastinal blood pool activity with a central area of absent uptake corresponding to cavitation (arrow).

Figure 3

(A) A 50-year-old woman with metastatic uterine leiomyosarcoma. Microwave ablation of a pleural-based right lung metastasis at the inferior edge of a prior radiation treatment. (B, C) By the second postprocedure day, the patient developed fever and extensive subcutaneous emphysema, with computed tomography showing necrotizing postablation zone and air track to subcutaneous tissue from a bronchopleural fistula. A drainage tube was placed in the cavity. The patient was taken to the operating room for debridement and rib resection. Four weeks after the procedure and surgical debridement, there was near complete resolution of the bronchopleural fistula and subcutaneous emphysema (not shown). (Courtesy of Dr. Majid Maybody.)

Figure 4

(A, B) An 84-year-old man with a biopsy-proven cavitary left upper lobe squamous cell carcinoma. Based on significant cardiovascular risk factors precluding surgical resection, the patient was referred for radiofrequency ablation. A multi-array electrode was used to perform overlapping ablations at 3 cm and 4 cm, each for 15 minutes. (B) The intra-ablation imaging demonstrated a rim of ground glass (white arrows) completely surrounding the mass. (C) At 5 weeks following treatment, the ablation zone (white arrows) consists of eccentric cavitation, covering the extent of the target lesion. (D) By 6 months, a nodule (white arrow) remains along the margin of the otherwise fully scarred ablation zone. (E, F) At 13 months, this nodule has grown (white arrow) and displays intense fluorodeoxyglucose uptake on positron emission tomography, compatible with residual tumor.

Figure 5

(A) A 50-year-old woman with solitary metastatic colon carcinoma to left lower lobe. Multiple attempts were made to place the probe through the nodule; however, the nodule changed position, a common finding while placing larger cryoprobes in smaller subcentimeter nodules. Ultimately the cryoprobe was placed at the superior surface of the nodule. (B) Initial 3 minutes of freeze cycle followed by passive thaw was performed to create hemorrhage surrounding the nodule and to allow for the definite freeze cycle. Peripheral rim of hyperdensity represents the edge of the melting ice ball (arrows), and the inner hypodensity surrounding the probe represents the ice ball itself. (C) Following a second freeze cycle of 10 minutes, the edge of the ice ball is seen (arrow) extending beyond the initial hemorrhage. (D) Immediately postablation, there is development of hemorrhage at the ablation site representing necrosis and hemorrhage. (E) The patient was placed in a supine position to limit the transbronchial spread of hemorrhage. (F, G) At 1 week following cryoablation, computed tomography demonstrates an elliptical ablation zone with resolution of hemorrhage. The size of the ablation zone is larger than the ablated nodule and depends on the number and size of the original ablation probes.

Figure 6

(A, B) A 90-year-old woman with history of primary lung carcinoma status postradiation, with recurrence of the tumor. High central metabolic activity is noted on positron emission tomography (PET). (C) Three probes were placed in the mass. Simultaneous cryoablation was performed with the ice balls (arrow), merging to create a large ablation zone. (D) At 2 months postablation, PET scan demonstrated a central metabolic area with thin rim of metabolic activity not exceeding that of the mediastinum. Despite a presumed favorable response, PET should be repeated in 6 months to confirm absence of residual tumor.

Figure 7

(A) Metastatic renal cell carcinoma to the left upper lobe, with a larger vessel (4 mm) in close proximity of the metastasis. (B) Following two cycles of ablation, the ice ball and cryo-zone did not grow to an expected size, despite prolonged ablation. It is hypothesized that the vessel adjacent to the nodule and ablation zone may have contributed to a cold sink effect. (C) At 1-month follow-up computed tomography scan, the size of the nodule is smaller but remains nodular. (D) At 5 months the nodule had grown, suggestive of recurrence.

Figure 8

(A) Axial prone computed tomography (CT) image demonstrates a peripheral right lower lobe metastasis in this patient with colon cancer. (B, C) Axial prone and coronal CT images demonstrate two cryoprobes placed in the mass with multiple layering ice balls, indicating the multiple cycles of ablation. (D) 1 week following initial ablation, the patient presented for repeat ablation but was found to have a large pleural effusion. The effusion was drained, and repeat ablation was performed.

Figure 9

(A) Metastatic renal cell carcinoma to the right lower lobe. (B) A single cryoprobe was used to ablate the lesion. Alternating circumferential layers of different intensities within the ice ball are seen engulfing the tumor. (C) Immediately postablation, the patient was placed in a supine position to limit hemorrhage into surrounding structures. A postablation zone with associated hemorrhage engulfs the initial tumor. (D-F) Follow-up at 1 month with (D) computed tomography (CT), (E) positron emission tomography (PET), and (F) fused CT/PET scans demonstrate an oval postablation zone with thin rim of peripheral enhancement and mild internal metabolic activity. (G, H) CT scan obtained 6 months after the ablation demonstrates oblong postablation zone with thin rim of peripheral enhancement. (I-K) Follow-up at 11 months with (I) CT, (J) PET, and (K) fused CT/PET scans demonstrate significant decrease in size of the ablation zone to a slit-like scar with no metabolic activity.