Comparison of setup errors between two immobilization techniques for image-guided pelvic radiotherapy: Can an ankle fixator be used as an alternative to a thermoplastic mask for immobilization without compromising treatment?

INTRODUCTION

Radiotherapy of solid tumors has moved from conventional two-dimensional (2D) techniques to conformal and intensity-modulated radiotherapy (IMRT) with image guidance.^[1] IMRT leads to highly conformal dose delivery to target volume with rapid dose fall off beyond the target, which reduces dose to surrounding organs at risk (OAR).^[2,3] However, it carries a higher chance of target miss, leading to treatment failure.^[4] Target miss can occur due to errors during treatment preparation (positioning, immobilization, CT simulation), treatment execution (patient setup during radiation delivery), and internal organ motion during treatment.^[5-7] To compensate for the above, a planning target volume (PTV) margin is added over the clinical target volume (CTV), and image verification is done during treatment delivery for highly conformal radiotherapy techniques.^[8,9]

A large PTV margin leads to increased irradiation of surrounding OARs. Various methods are implemented to reduce the PTV margin without compromising target failure, like rigid immobilization with a thermoplastic mask, respiratory motion management, and image verification of patient setup during treatment delivery.^[10] Image verification is usually done by onboard imaging (OBI) attachments on linear accelerator (LINAC) like kilovoltage/megavoltage (kV/MV) X-rays or Cone-beam Computed Tomography (CBCT).^[11]

Pelvic malignancies have reported large setup variations, thus requiring good treatment preparations, including positioning and immobilization.^[12,13] Various pelvic immobilization techniques have been reported in literature like thermoplastic mask, vacuum cushion, knee wedge, ankle fixator^[13–16]. Our institution uses multiple techniques based on physician preference. Use of ankle fixator or knee wedge rather than thermoplastic mask reduces treatment cost and may decrease setup time, and thus, overall treatment time. In this study, we have compared two immobilization techniques for translational setup errors during radiation delivery- thermoplastic mask and ankle fix immobilization.

MATERIAL AND METHODS

Protocol of Positioning and Immobilization

All patients were simulated in the supine position using one of the immobilization techniques based on physician preference. Carbon fiber base plate with extension, appropriate headrest, armrest, and hand grip pole was common to all patients, who were divided into two arms:

1. Thermoplastic mask immobilization (TPI) - 4 clamp thermoplastic mask was used for immobilization.

2. Ankle fix immobilization (AFI) - No thermoplastic mask used. An ankle fix was used, which would immobilize the legs and ankle, maintaining separation between lower limbs [Figures 1 and 2].

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Figure 1:

Ankle fixation device

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Figure 2:

Lateral view showing patient setup with ankle fixation device for pelvic immobilization

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Results

The basic characteristics of patients, immobilization, and the treatment technique has been described in Table 1. Most patients were treated with 3DCRT (54.9%) followed by IMRT (42.7%). Carcinoma cervix was the most commonly treated malignancy in the study population (45.1%). A total of 82 patients were included, 35 in TPI and 47 in AFI arm.

855 CBCT scans were analyzed. Average setup error for an individual patient is shown in Figure 3. Mean setup errors were 0.031 ± 0.23 cm, 0.079 ± 0.26 cm, 0.021 ± 0.20 cm in lateral (X), longitudinal (Y), vertical (Z) directions respectively [Table 2]. The corresponding population systematic errors (Σ) and random errors (σ) were 0.226, 0.257, 0.201 cm and 0.401, 0.419, 0.407 cm respectively. Overall calculated CTV-PTV margins: 0.78 cm in antero-posterior direction, 0.93 cm in supero- inferior direction, and 0.85 cm in lateral direction. To create a uniform CTV-PTV margin, at least 0.93 cm of margin is required all around the CTV.

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Figure 3:

Distribution of average translational errors for individual patients.

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Mean setup errors were not significantly different between the arms in all three translational axes, as compared by Mann Whitney U test (p = 0.729, 0.097, 0.548 for X, Y, Z shifts respectively) [Table 3]. Systematic, random errors and CTV-PTV margins for individual translation shifts between two arms have been tabulated in Table 3. The margins for TPI arm in antero-posterior, supero-inferior, lateral directions were 0.59, 0.94, 0.78 cm respectively. Corresponding margins for AFI arm were 0.84, 0.92, 0.87 cm respectively. To have a uniform CTV-PTV margin in all directions, minimum expansion required is: 0.94 cm and 0.92cm in TPI and AFI arm respectively.

TPI arm had significantly more longitudinal setup errors greater than 0.5 cm (38.5% vs. 27.9%, p 0.003), while, AFI arm had significantly more vertical setup errors, with values greater than 0.5 cm [Figure 4].

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Figure 4:

Clustered column chart showing the distribution of patients in arms with a shift of greater than 0.5 cm. TPI: Thermoplastic mask immobilization, AFI: Ankle fix immobilization

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The trend of overall CTV-PTV margins have decreased from 2018–2021 with symmetrical CTV-PTV margins; and were calculated to be 1.07, 0.93, 0.76 cm in 2018, 2019, and 2021 respectively (2020 not considered as very few patients had been treated here during early part of the COVID pandemic) [Table 4].

DISCUSSION

Immobilization for irradiation of pelvic malignancies has been a matter of debate, and many trials have tried to find an optimal immobilization that would improve reproducibility.^[12-15] Cost effectiveness is also important, especially in centers depending on state funds for patient treatment. Major problems with pelvic immobilization and setup include: significant mobility at hip joint, regular realignment if skin marks are placed, variable rectal and bladder filling, and significant pelvic rotation.^[14,16]

Although the use of thermoplastic mask negates the problem of skin mark alignments, the device is difficult to reuse. Thus, a new mask is required for every patient. In contrast, using simpler devices like ankle fixator requires placement of skin tattoos, and can be reused. Therefore, such a device per treatment room, can be used to treat all patients requiring pelvic irradiation, eventually reducing cost by reducing the wastage of thermoplastic masks.

In this study, 4-clamp thermoplastic mask and ankle fix immobilization, have been compared for setup errors and CTV-PTV margins. No significant difference has been reported between mean setup errors using the above mentioned techniques, with similar symmetrical CTV-PTV expansion. Setup errors of pelvic radiotherapy for various immobilization techniques [Table 5] have been reported in the literature.^{[12,14,17,18]} Two studies have compared thermoplastic mask immobilization with non-rigid versus no immobilization like ankle fixator and vacuum bag.^[12,14] Anand et al. reported no difference in mean errors between the thermoplastic mask and knee wedge, with similar CTV-PTV expansion.^[14] Udayshankar et al.^[12] reported no immobilization (use of leg separator) to be most reproducible, with a symmetrical CTV-PTV margin of 0.47 cm. Systematic and random errors were lowest with no immobilization, while random error was highest using thermoplastic masks. Patni et al. reported a CTV-PTV expansion of 1.03 cm for thermoplastic mask/skin marking, which was comparable to this study.^[17] However, a study comparing three immobilization devices for prostate radiotherapy, reported thermoplastic mask to have the most reproducible immobilization with minimum setup errors compared to alpha cradle or leg cushion.^[19]

Our study has shown a significantly higher individual setup error of >0.5 cm in supero-inferior (Y) direction in TPI arm (38.5%) and antero-posterior direction in AFI arm (28.5%). This probably has influenced the CTV-PTV expansion in TPI arm in supero-inferior direction, which is largest among 6 reported CTV-PTV expansion in this study (0.94 cm). Saini et al. and White et al. have reported maximum individual setup errors in supero-inferior direction of 23.38% and 23.6% respectively, with the use of 4-clamp pelvic thermoplastic mask, similar to our study.^[15,20] Maximum CTV-PTV expansion in this study was in supero-inferior direction in both arms, similar to many studies, that have reported a maximum CTV-PTV expansion in the superoinferior direction, ranging between 0.83–1.62 cm.^{[14,15,17,20]}

This study has several limitations: small sample size, small number of CBCT scans ,and only inter-fractional deviations being evaluated. Separate reports comparing immobilization techniques for individual sites of pelvic malignancy, are required, to validate the above findings. However, from the above results, it may be concluded that an ankle fixator or similar lower leg immobilization technique can be used as an alternative to thermoplastic mask immobilization, without compromising setup accuracy.

CONCLUSION

An ankle fixator or similar lower leg immobilization technique can be used as an alternative to thermoplastic mask immobilization without compromising setup accuracy. This has the potential to reduce treatment costs per patient by excluding the cost of thermoplastic masks.