Helical versus static approaches to delivering tomotherapy to the junctional target for patients taller than 135 cm undergoing total body irradiation

This analysis was conducted for routine quality assurance in line with requirements of the German radiation protection law. Therefore, ethical approval was not required.

Patients

Data from all patients taller than 135 cm at the time of TBI delivery between 2012 and 2020 who underwent planning with a direct field connection between upper and lower CT scans were analyzed. Patients who underwent planning using a dose gradient in the JT volume were excluded.

Treatment planning

In preparation for TBI, two planning CT scans were required, because the TomoTherapy® Hi-ART II is limited to a maximal couch movement of 135 cm. These CT scans were performed using a fixation mask for the head and a vacuum cushion for the body to help stabilize the patient and prevent significant alterations in the patient’s position during CT, between CT and treatment, and during treatment. The first CT scan was performed in the cranio-to-caudal direction, and the second CT scan was performed in the caudo-to-cranial direction, both with a slice thickness of 5 mm. To correctly match these two scans and assist in treatment planning, a radio-opaque marker was placed on the patient’s upper thigh. The exact position of the marker depended on the patient’s height to ensure that neither CT scan (and thus treatment plan) exceeded the maximum length of 135 cm.

The matching of CT scans and all delineations and planning were performed using an Eclipse Treatment Planning unit (Varian Medical System, Palo Alto, CA, USA). To calculate the optimal dose for irradiating the junctional area, two treatment plans were fused and matched, with JT5 being the lowest part of the upper treatment plan and JT6 being the highest part of the lower treatment plan. The connecting area (JT4–7) was planned with 50% of the prescribed dose. Contouring of the whole body and organs at risk as well as generation of the planning target volume (PTV), sparing the lungs, were performed according to current institutional and international standards [18]. Nine patients underwent treatment planning and delivery using a helical approach (Fig. 1), and 10 patients underwent treatment planning and delivery using a static approach (Fig. 2). Both approaches used fixed jaws, a field width of 5 cm, a pitch of 0.4, and a constant feed rate fitting the pitch and prescribed dose. The modulation factors were 1.6 for the static approach and 2 for the helical approach. With the static approach, the dose was delivered from four angles, all covering the entire PTV. Treatment and planning times were equivalent between the two approaches.

Fig. 1figure 1

Simulation of the helical approach

Fig. 2figure 2

Simulation of the static four-field box approach

Considering the inverse square law, which states that the dose is inversely related to the square of the distance from the radiation source, lateral movement of the patient can be compensated due to an increase in field width, which can be addressed by opening five additional leaves of the multileaf collimator. As these five additionally opened leaves do not target the PTV directly, their dose is calculated using the mean opening time of the three outermost leaves that target the PTV directly and using that calculated amount as the dose for the additionally opened leaves. This is not possible when using a helical treatment plan. Because of these technical limitations when planning TBI with the TomoTherapy Hi-ART II, the static approach offers 30 mm safety before possible subtherapeutic doses, when the position of the patient changes laterally. Because a patient must be precisely positioned during irradiation to ensure an optimal treatment outcome, a simulation demonstrates the impact of lateral movement of the patients’ legs in the static approach as compared with the helical approach (Fig. 3). In this stimulation, we virtually misplaced the patient laterally at different distances and compared the resulting changes in the DVH between the helical and static approaches.

Fig. 3figure 3

Comparison of 5-, 10-, and 15-mm lateral movement between helical and static approaches

To evaluate the performance of the static approach using a four-field box method compared with the usual helical approach and in consideration of TBI guidelines, we divided the JT volume between the upper and lower CT scans into ten 1-cm-thick volumes (JT1-JT10) covering the entire PTV, spanning from 5 cm above to 5 cm below the marker on the patient’s thigh (Fig. 4). This additional contouring was performed after the completion of treatment for all patients. The dose-volume histogram and the D5, D50, D95, D98, and Dmean as well as the homogeneity index (HI) of each JT and all ten JT volumes combined (JTtotal) were calculated. The HI was calculated using the formula proposed by Kataria et al. (HI = D5/D95) [19].

Fig. 4figure 4

Contouring of 1 cm JT volumes (J1–J10) for patient treatment planning

Two-sided t tests were used to compare groups using SPSS v26.0 (IBM, Armonk, New York, USA).

A total of 19 patients were included; 10 patients underwent a static approach to planning and treatment, and 9 patients underwent a helical approach to planning and treatment. All patients underwent TBI in preparation for BMT. The most common disease for patients in the helical group was AML, and the most common disease for patients in the static group was ALL (Table 1). In addition, two patients in the static group were treated for diffuse large B cell lymphoma and mixed phenotype acute leukaemia, respectively. The delivered dose ranged from a single 2 Gy fraction to 2 × 2 Gy, 4 × 2 Gy, or 6 × 2 Gy, resulting in a total dose of 2, 4, 8, or 12 Gy, respectively. Most patients in the helical group received 4 Gy, and most patients in the static group received 8 Gy (Table 2).

Table 1 Diagnosis of patientsTable 2 Prescribed total dose

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