7. Beyond metal implant artifacts

Metallic surgical implants are commonly used in patients who undergo RT for bone metastasis. In computed tomography, metallic hardware can dramatically attenuate the X-ray beam and severe beam hardening effect and lead to faulty or inconsistent projection data [57, 58]. Consequently, so-called metallic artifacts or bright and dark streak artifacts can dramatically degrade the image quality. Figure 4 illustrates a typical case of a patient who underwent spine-stabilization before adjacent RT. Strong artifacts induced by the titaniumbased pedicle screws make it difficult to distinguish target lesions from surrounding normal tissues.

80% of ambulant patients before RT will be expected to maintain the walking ability, whereas less than 20% of not ambulant patients will recover the function [49, 50]. SBRT is well suited for reirradiation of the spine due to its superiority of dose distribution compared with conventional techniques. SBRT has a major potential to provide superior local control without increasing toxicity (Table 8) [51–54]. A care must be taken when SBRT is applied to patients with MSCC, because the existence of tumor too close to the spinal cord is a risk factor for local recurrence due to underdose [51]. Relatively little is known regarding the long-term toxicities of reirradiation. Because reirradiation has the potential to exceed normal tissue tolerance, it might be appropriate to sum the biologically effective doses (BEDs) from the initial and repeat treatment regimens to estimate the safety of treatment. The BED is calculated according to the liner-quadratic model [BED = D × (1+ d/α/β), D: total dose, d: fractional dose] with generally using α/β value of 2 (Gy2) for the late effects. For example, the BED for 30 Gy in 10 fractions is 75 Gy2 and 8 Gy in single fraction is 40 Gy2. Regarding the spinal cord, higher cumulative RT doses (BED > 135.5 Gy2), higher doses for each RT course (BED > 98 Gy2) and a short interval between the courses (<6 months) could be associated with a higher probability of developing radiation-induced myelopathy [55]. These dose constraints for the spinal cord seem to be

Metallic surgical implants are commonly used in patients who undergo RT for bone metastasis. In computed tomography, metallic hardware can dramatically attenuate the X-ray beam and severe beam hardening effect and lead to faulty or inconsistent projection data [57, 58]. Consequently, so-called metallic artifacts or bright and dark streak artifacts can dramatically degrade the image quality. Figure 4 illustrates a typical case of a patient who underwent spine-stabilization before adjacent RT. Strong artifacts induced by the titaniumbased pedicle screws make it difficult to distinguish target lesions from surrounding nor-

24 Gy in 3 fx, 25–30 Gy in

8-22 Gy in 1 fx, 14–50 Gy

SBRT, Stereotactic body radiotherapy; EBRT, external beam radiotherapy; Fx, fraction; NA, not available; G3, grade3

Median dose/Fx (range) Local control Overall

survival

11 month (median)

64% at 6 months

year

76% at 1 year 76% at 1

93% at last follow-up

93% at 6 months

Neural toxicity

2 of G3 Radiculopathy

None

None

reproducible in SBRT [56].

Patients/ lesions (n) Previous EBRT dose/Fx

Garg [52] 59/63 NA 30 Gy in 5 fx, 27 Gy in 3

60/81 30 Gy in 10 fx (median)

215/247 30 Gy in 10 fx (median)

fx

5 fx

in 3(2–20) fx

Author (year)

16 Radiotherapy

Mahadevan [53]

Hashmi [54]

7. Beyond metal implant artifacts

Table 8. Outcomes of reirradiation by spinal SBRT.

mal tissues.

Figure 4. Artifacts of metallic surgical implants. (A) 2D radiography image shows a patient with implanted titanium pedicle screws. (B) Computed tomography image of a patient with titanium pedicle screws. Streak artifacts are present around the metallic implants.

For modern-era RT protocols, target delineation and dose calculation are performed on CT images using treatment-planning systems. Therefore, metallic artifacts, that are commonly located directly adjacent to the target volume and organs-at-risk and that degrade the delineation accuracy and dose calculation might lead to poor local control and a high risk of complications. Several studies have investigated the metallic-implant-related dosimetric impact using Monte Carlo simulations. Spadea et al. [59] reported that low-Z materials such as titanium might not cause relevant dose discrepancies, while high-Z materials including gold and platinum might lead to underestimation of the delivered dose during photon beam irradiation. Verburg et al. [60] investigated the effect of titanium implants on dosimetric errors in photon therapy treatment planning. They revealed dose discrepancies of up to 10% with range differences of up to 10 mm in artifact-contaminated areas. Figure 5 illustrates examples of dose differences caused by titanium-based artifacts introduced by Verburg [58]. Factors including the beam-implant interaction, radiation beam type and the physical characteristics of the metals differ and eventually lead to dose uncertainties. For bone metastasis, especially in cases of infield recurrence of metastatic spinal lesions, the high dosimetric accuracy for organs-at-risk becomes clinically significant because of the limited spinal cord radiation tolerance. Recently, several promising approaches to reduce metallic artifacts have been proposed, such as metal artifact reduction (MAR) algorithms [61–63] and monoenergetic extrapolations from dual-energy computed tomography (DECT) [64, 65]. Figure 6 briefly illustrated reduction of metal artifacts using a frequency split MAR method introduced by Meyer et al. Antiartifact approaches have proven useful for improving target delineation and dose calculation in RT, but to date, they have not been widely implicated for routine clinical use.

Figure 5. Differences in dose calculation of photon beams passing through the metal artifact region. (A) Dose calculation on the artifact-affected computed tomography image. The arrow indicates the titanium insert. (B) Dose calculation on the ground truth computed tomography image without the artifact.

Figure 6. Reduction of metal artifacts using a frequency split MAR method. (A) Patient with implanted pedicle screws. (B) Patient with implanted unilateral hip endoprosthesis, Left: original computed tomography image; right: MAR corrected computed tomography image.

In conclusion, in cases of bone metastases, the impact of dose uncertainties due to metallic implants is critical in modern RT, especially in patients undergoing reirradiation. Promising antiartifact approaches might be useful options to achieve the anticipated magnitude of clinical benefit.
