Noninvasive cardiac radioablation for ventricular tachycardia: dosimetric comparison between linear accelerator- and robotic CyberKnife-based radiosurgery systems

Noninvasive cardiac radio-ablation by stereotactic ablative radiotherapy represents an alternative treatment modality for ventricular tachycardia (VT). It effectively reduced the VT burden and improved the quality of life for patients who are refractory to medical therapy or catheter ablation or who cannot tolerate it [1, 2]. Currently, the linear accelerator (LA)-based system and the CyberKnife (CK) robotic radiosurgery system are used for noninvasive cardiac radio-ablation. Notably, several studies on the LA-based system from the USA, Asia, and Europe applied coplanar and non-coplanar volumetric-modulated arc radiotherapy (VMAT) to generate a highly conformal ablative radiation dose to the VT substrate [3‐9]. Meanwhile, studies on the CK robotic radiosurgery system from the USA and Europe used a six-dimensional robotic arm, multiple non-coplanar small beams, and a continuous image-tracking system to deliver ablative radiation doses accurately [10‐13].

Noninvasive cardiac radio-ablation for VT is a new and unique challenge for clinicians and radiation physicists. One case report presented the dosimetric difference between the VT substrate’s LA- and CK-based systems at the anterior basal heart [14]. The report concluded that CK was superior to the LA-based planning system for a steeper dose gradient and better real-time target tracking. The RAVENTA benchmark study included three patients’ plans using LA- and CK-based systems from five academic centers [15]. They demonstrated that VMAT plans had steeper dose gradients in the high-dose region, while the CK plans had smaller low-dose regions. They concluded that plans from both systems were considered deliverable based on the internal guidelines and protocols for SBRT and noninvasive cardiac radio-ablation [16].

Few dosimetric comparisons have been published between the LA-based and CK-based robotic radiosurgery systems for cardiac radio-ablation. Therefore, this study aimed to compare the dosimetry of noninvasive cardiac radio-ablation deliverables on CK with LA systems and discuss related clinical issues.

The clinical goal of 95% PTV coverage was optimally achieved, except in patient No. 12, who had a large PTV of 93.7 cm³ located at the apical lateral wall of the LV adjacent to the stomach. The PTV coverage was traded off in both the LA and CK systems to reach the dose constraint of the stomach. The plan quality parameters are listed in Table 1 and Fig. 3. The HI ratio was significantly higher in the CK plans than in the LA plans (1.34 ± 0.06 vs. 1.24 ± 0.03, p < 0.001), reflecting that a higher maximal dose within target volumes was usually tolerable for CK-based radiosurgery plans. The CK and LA plans achieved acceptable CI ratios, with no significant difference (CK vs. LA: 0.84 ± 0.08 vs. 0.87 ± 0.07, respectively, p = 0.093).

The CK plans achieved significantly lower GI ratios (3.12 ± 0.71 vs. 3.48 ± 0.55, p = 0.031) and GM values (1.00 ± 0.29 cm vs. 1.17 ± 0.29 cm, p < 0.001) than the LA plans, indicating that the CK plans provided a steeper dose gradient benefit. The D_2cm was comparable between the LA and CK plans (CK vs. LA: 50.8 ± 9.9% vs. 53.1 ± 5.3%, respectively, p = 0.423), indicating that in both systems, the radiation dose could be decreased to approximately half values at 2 cm from the PTV.

The dosimetric parameters of the OARs are listed in Table 2. The maximal doses to the esophagus, stomach, spinal cord, trachea, large bronchus, and coronary artery were similar between the CK and LA plans. The volumes of the normal heart receiving ≥ 16 Gy, the normal heart’s mean dose, and the normal lung receiving ≥ 7 Gy were similar between the CK and LA plans. These results suggested that the OARs were well protected with the two systems.

The treatment delivery parameters are listed in Table 3. An average of 232.2 non-coplanar beams in the CK system and a median of 10 arcs (six coplanar arcs and four non-coplanar arcs) in the LA system were designed for one fully qualified plan. Compared with the LA plans, the CK plans had significantly longer treatment times (22.4 ± 1.6 min vs. 119.2 ± 43.3 min, p = 0.007).

The dosimetric comparison between large (PTV > 50 cm³, n = 6) and small (PTV ≤ 50 cm³, n = 7) target volumes for subgroup analyses is shown in Table 4. The dosimetric advantages of CK providing steeper dose gradients were prominent in patients with small target volumes, as evidenced by the lower GI ratios (2.78 ± 0.36 vs. 3.38 ± 0.46, p = 0.028), GM values (0.76 ± 0.13 cm vs. 0.93 ± 0.15 cm, p = 0.007), and D_2cm values (42.9 ± 4.1% vs. 51.0 ± 5.8%, p = 0.027). Meanwhile, the dosimetric advantages of CK were less evident in patients with large target volumes, as evidenced by the lower GM values (1.28 ± 0.10 cm vs. 1.45 ± 0.07 cm, p < 0.001) but similar GI ratios (CK vs. LA: 3.52 ± 0.83 vs. 3.60 ± 0.66, respectively, p = 0.619) and D_2cm values (CK vs. LA: 59.9 ± 5.4% vs. 55.5 ± 3.8%, respectively, p = 0.273). There was no difference in the estimated treatment time between large and small target volumes in both the LA system (small vs. large: 21.7 ± 1.8 vs. 23.2 ± 0.8 min, respectively, p = 0.072) and the CK system (small vs. large: 120.2 ± 18.1 vs. 118.2 ± 37.4 min, p = 0.907).

Additional analysis was conducted for dosimetric parameters by stratifying the PTV shape into spherical (n = 6) or non-spherical (n = 7). The benefits of CK plans for treating spherical targets were prominent, as manifested by lower GI ratios (2.63 ± 0.23 vs. 3.35 ± 0.50, p = 0.029), GM ratios (0.73 ± 0.12 vs. 0.93 ± 0.17, p = 0.006), and D_2cm values (42.9 ± 4.4% vs. 51.5 ± 6.2%, p = 0.046). When treating non-spherical PTVs, the LA plans achieved equivalent dosimetric advantages, as supported by similar GI ratios (CK vs. LA: 3.51 ± 0.76 vs. 3.59 ± 0.60, respectively, p = 0.531) and D_2cm values (CK vs. LA: 57.5 ± 8.0% vs. 54.4 ± 4.4%, respectively, p = 0.395).

There have been few dosimetric comparisons between the LA-based and CK-based robotic radiosurgery systems for cardiac radio-ablation. This study found that the LA and CK systems are suitable for noninvasive cardiac radio-ablation, with adequate dose coverage and appropriate normal tissue sparing. CK plans provided dosimetric advantages of steeper dose gradients in patients with small target volumes (PTV ≤ 50 cm³) or spherical PTVs. Meanwhile, CK plans required longer treatment times.

The present study demonstrated complex-shaped target volumes at the LV basal, middle, or apical areas surrounded by non-identical critical structures, indicating noninvasive cardiac radio-ablation heterogeneity and dosimetric challenges. The dose distribution of homogeneity and gradient measures were significantly more favorable in the CK plans than in the LA plans, especially in patients with small target volumes and spherical PTVs. This suggested a much sharper and rapid dose fall-off of the margin from target volumes and a more centralized uniform radiation dose within target volumes in CK plans. These results align with the published case report establishing the dosimetric benefit of using the CK system to treat non-ischemic basal VT with a small and spherical PTV (31.8 cm³) [14].

The RAVENTA trial demonstrated three benchmark cases with PTVs of 36.5 cc, 62.4 cc, and 52.8 cc [15]. The present study parallels their study regarding dose distribution and conformity values. The RAVENTA trial showed that the dose distributions were more axial in VMAT plans and more spherical in CK plans, and a trend toward higher CI ratios in the LA-based VMAT plans (LA 0.87 vs. CK 0.79), which following our distribution shown in Fig. 2 and the CI ratio results (LA 0.87 vs. CK 0.84). The GI advantage of CK plans in the present study was merely seen in the RAVENTA trial. In their series, the LA-based VMAT plans had steeper dose gradients in the high-dose region, while the CK plans had smaller low-dose regions. This may partly be related to their series’ target volumes and cardiac substructures constraint. The dosimetric advantages of CK were less evident in our series in patients with PTV > 50 cm³ showing similar GI ratios (CK 3.52 vs. LA 3.60). Moreover, the RAVENTA trial adopted dose constraints to cardiac substructures, including the aorta, the left coronary arteries, the superior vena cava, and the left atrium, which are not implemented in our study. The trade-offs between the gradient dose fall-off and OAR sparing varied from plan to plan, depending on individual planners, different institutions, and varied clinical situations [26, 27]. In our series, we observed that LA plans exhibited dosimetric superiority in thin and flat targets in the apical LV lateral wall, as shown in patients No. 6 and No. 13. This implies that treatment systems should be considered based on PTV volumes, locations, and shapes.

Literature has shown that for stereotactic radiotherapy, different dose calculation approaches like AAA, ray-tracing, or pencil beam algorithms may have differences regarding dose distributions, with the largest differences in the lung encompassing high tissue heterogeneities[28]. AAA algorithm accounts for changes in electron transport and volume scatter with improved calculation results, especially in regions of high heterogeneities [29], thus may be more reliable in circumstances where VT targets overlapping lung regions, while both the AAA algorithm and the pencil beam algorithm are accurate enough for cases where VT targets reside within the heart, where almost no heterogeneities are present. In the RAVENTA benchmark study, the dosimetric accuracy was tested during patient-specific quality assurance using an electronic portal imaging device (EPID) for VMAT plans and PTW PinPoint or SRS array for CK plans; all fulfilled the criteria without major discrepancy. Therefore, plans were considered deliverable by both CK and LA-based systems [15].

In the present study, significantly longer treatment times were shown in the CK system. This finding aligns with previous dosimetric comparisons between CK-based robotic radiosurgery and LA-based systems under variable circumstances [30]. Importantly, this finding has pertinent implications for patients whose medical conditions do not allow long irradiation periods, especially those experiencing recurrent VT that leads to life-threatening electrical storms.

Dose limits to heart substructures are largely unknown and have only been recently reported, though not yet correlated to toxicity; however, our institution did not set dosimetry constraints for the coronary arteries. In the present study, as VT substrate could be very close to or overlapping the coronary arteries, the maximal dose of the coronary arteries was as high as 105%-114% of the prescribed dose. The RAVENTA trial has set the major violation for left coronary arteries to be 20 Gy[15]. So far, in our institution, there has been no evidence of short-term toxicities in the coronary arteries. However, patients are followed up after study completion to obtain further long-term safety data. Long-term data will define the full safety profile of cardiac radio-ablation for toxicities on healthy intra-cardiac structures.

Motion management varies between the LA and CK treatment strategies. The internal target volume (ITV) was delineated using four-dimensional CT in both systems to cover respiratory and cardiac movements. Abdominal compression was employed to minimize respiratory motion during the LA-based treatments, which usually required short treatment time with acceptable patient compliance. The advantage of time efficiency in LA-based systems may have biological benefits to treatment outcomes [31] and is also more suitable to life-threatening VT patients experiencing electrical storms. For CK plans with longer treatment times, the CK-based system adopted the synchrony fiducial tracking system (Accuray, Sunnyvale, CA, USA) in which the distal dipole of the right ventricular lead of the implanted cardiac defibrillator is used as a fiducial, allowing real-time tracking of both respiratory and cardiac movements during the long treatment session [12, 13]. Furthermore, the CK-based InCise MLC mode has been used to treat large and irregularly shaped targets with the advantage of shorter delivery time [32], was adopted for patient 12’s treatment plan, and resulted in a relatively acceptable treatment time of 46 min.

The present study had some limitations. First, it only included 13 patients with heterogeneous PTV sizes, shapes, and locations. The small target volumes (PTV ≤ 50 cm³) and spherical PTVs were mainly located at the basal segments. However, the large target volumes (PTV > 50 cm³) and non-spherical PTVs were mostly located at the apical segments. The small number of patients and heterogeneous PTV volumes, shapes, and locations may limit the comparison of dosimetric advantages between the two systems. Second, the study did not investigate the correlation between dosimetry and clinical outcomes. Further prospective studies with a larger patient size may be needed to validate whether the dosimetric difference affects the clinical efficacy of reducing the VT burden.

In conclusion, the LA and CK systems are ideal modalities for noninvasive cardiac radio-ablation, with adequate dose coverage and appropriate normal tissue sparing. Moreover, the CK plans provide dosimetric advantages of steeper dose gradients in patients with small target volumes (PTV ≤ 50 cm³) and spherical PTVs, although they also require longer treatment times. LA plans may be beneficial in treating large PTVs (> 50 cm³) or non-spherical PTVs located at the apical LV lateral wall, with the advantage of time efficiency. Therefore, upfront treatment should be considered based on the clinical intent.