Fixed‐jaw technique to improve IMRT plan quality for the treatment of cervical and upper thoracic esophageal cancer

Abstract The purpose of this study was to investigate the potential advantages of the fixed‐jaw technique (FJT) over the conventional split‐field technique (SFT) for cervical and upper thoracic esophageal cancer (EC) patients treated with intensity‐modulated radiotherapy. The SFT and FJT plans were generated for 15 patients with cervical and upper thoracic EC. Dosimetric parameters and delivery efficiency were compared. An area ratio (AR) of the jaw opening to multileaf collimator (MLC) aperture weighted by the number of monitor units (MUs) was defined to evaluate the impact of the transmission through the MLC on the dose gradient outside the PTV50.4, and the correlation between the gradient index (GI) and AR was analyzed. The FJT plans achieved a better GI and AR (P < 0.001). There was a positive correlation between the GI and AR in the FJT (r = 0.883, P < 0.001) and SFT plans (r = 0.836, P < 0.001), respectively. Moreover, the mean dose (Dmean), V5Gy–V40Gy for the lungs and the Dmean, V5Gy–V50Gy for the body‐PTV50.4 in the FJT plans were lower than those in the SFT plans (P < 0.05). The FJT plans demonstrated a reduction trend in the doses to the spinal cord PRV and heart, but only the difference in the heart Dmean reached statistical significance (P < 0.05). The FJT plans reduced the number of MUs and subfields by 5.5% and 17.9% and slightly shortened the delivery time by 0.23 min (P < 0.05). The gamma‐index passing rates were above 95% for both plans. The FJT combined with target splitting can provide superior organs at risk sparing and similar target coverage without compromising delivery efficiency and should be a preferred intensity‐modulated radiotherapy planning method for cervical and upper thoracic EC patients.


2.A | Patient characteristics
Fifteen medically inoperable patients with cervical and upper thoracic EC previously treated with definitive IMRT at our department were included in this study. The patients consisted of 13 men and 2 women. Their median age was 63 yr old (range: 46 to 76 yr). All patients were histologically or cytologically confirmed to have squamous cell carcinoma. According to the clinical staging of esophageal carcinoma receiving nonsurgical treatment, 16 three patients had stage Ⅰ disease, eight had stage Ⅱ disease, and four had stage Ⅲ disease. Among the 15 patients, there were six cases with the primary tumor located in the cervical esophagus and nine in the upper thoracic esophagus.

2.B | CT simulation, target, and OAR delineation
All patients were immobilized with a head, neck, and shoulders thermoplastic mask in a supine position with both arms along the body.
The CT simulation was performed with a 5-mm slice thickness from the cranial base to the lower edge of the liver using a Philips Brilliance Big Bore CT scanner (Philips Medical Systems, Inc., Cleveland, OH, USA). The CT images were then transferred to the Eclipse treat- were contoured as OARs.

2.C | Treatment planning
Treatment planning was performed on the Eclipse TPS using the configured 6-MV photon beam data for a Varian Clinac iX linear accelerator. The accelerator is equipped with a Millennium 120 MLC, which has 40 central leaf pairs with a projected width of 5 mm and 20 outer leaf pairs with a projected width of 10 mm at the isocenter. Two static gantry IMRT plans, that is, the SFT plan and the FJT plan, were generated for each patient using a simultaneously integrated boost scheme. The prescribed dose was 50.4 Gy in 28 fractions to the PTV50.4 and 60 Gy in 28 fractions to the PTV60.
In the SFT plan, the Eclipse TPS automatically determined the collimator jaw positions to cover the entire PTV50.4 in each beam direction. Treatment fields wider than 14.5 cm were split into two or more subfields by the TPS because the maximum travel distance of MLC leaves in one carriage group were limited to 14.5 cm. To minimize the transmitted radiation to the surrounding OARs in the FJT plan, we split the PTV50.4 into a superior part (PTV50.4-sup) and an inferior part (PTV50.4-inf) at the level of the sternal notch, and the collimator jaw positions were manually adjusted and fixed to modify SONG ET AL. | 25 the jaw opening of each field for these two parts. Figure 1 illustrates the jaw positions in the FJT and SFT plan for a representative patient. Several strategies were employed to determine the jaw positions as follows: (a) a minimal margin of 7 mm from the PTV50.4 to each jaw edge was kept to ensure the dose coverage at the edge of PTV50.4; 17 (b) to minimize the volume of normal tissues surrounding the PTV50.4 shielded by MLC alone at the gantry angle of 0°, the large field was manually split into two subfields with a collimator rotation of 90°to maintain the number of subfields. One of the subfields covered the PTV50.4-sup [ Fig. 1(a)], and the other one covered the PTV50.4-inf [ Fig. 1(b)]. The two jaw openings were allowed to overlap 2 cm in the superior and inferior direction to feather their junction; (c) for other beam directions such as the gantry angle of 145°, 180°, and 285°, some parts of the PTV50.4 further away from the radiation source were manually shielded with the collimator jaws to further reduce the volume of surrounding normal tissues exposed to the transmission through the MLC. For example, the contralateral The optimization parameters were kept the same for both plans.
The planning goal for the PTV50.4 and PTV60 was 95% of the target volume should be covered by 100% of the prescribed dose. The dose constraints for the OARs were as follows: lung, the mean dose (D mean ) < 13 Gy, V 5Gy ≤ 55% (i.e., the percentage volume of the   The CI was calculated as follows: 18

2.D | Plan evaluation
where TV is the target volume, PIV is the prescription isodose volume, and TV PIV is the target volume enclosed by PIV. A CI closer to 1 indicates a better target conformity.
The HI was calculated as follows: 18 An HI close to zero indicates an ideal target dose homogeneity.
The GI was defined as follows: where R 100% is the equivalent sphere radius of the prescription dose volume, and R 50% is the equivalent sphere radius of the half prescription dose volume. 19 A smaller GI indicates a sharper dose fall-off outside the target volume.
To compare the volume of surrounding normal tissues shielded by the MLC alone in both plans, an area ratio (AR) of the jaw opening to MLC aperture weighted by the number of MUs was defined as follows: where To evaluate OAR sparing, DVH parameters for the lung (D mean , V 5Gy , V 10Gy , V 13Gy , V 20Gy , V 30Gy , and V 40Gy ), spinal cord PRV (D 1% ), heart (D mean , V 30Gy , and V 40Gy ), body-PTV50.4 (D mean , V 5Gy , V 10Gy , V 20Gy , V 30Gy , V 40Gy , and V 50Gy ) were calculated and compared.
In addition, both plans were delivered to a phantom in clinical mode to evaluate the treatment delivery efficiency. The number of MUs, number of subfields, and delivery time were recorded according to the ARIA 10 record and verify system (Varian Medical Systems, Palo Alto, CA, USA).

2.F | Statistical analysis
The statistical analysis was performed using SPSS Statistics19.0 software (IBM Corp., Armonk, NY, USA). The Shapiro-Wilk test showed that all the parameters were normally distributed. Differences between the two plans were analyzed using the paired t-test. All the parameters were reported as the mean ± standard deviation (SD).
The correlation between the GI and AR was evaluated using the Pearson correlation test. A P <0.05 (two-tailed) was considered statistically significant.

| RESULTS
Both the FJT and SFT plans were clinically acceptable. Figure 2 shows   Fig. 4, the GI and AR of the FJT plan were evidently improved for each patient. Interestingly, Fig. 5   PRV, and the D mean , V 30Gy , and V 40Gy of the heart in the FJT plans were lower than those in the SFT plans, but only the difference in the heart D mean reached statistical significance (P < 0.05). Since most heart volumes were located outside of the treatment field, the doses to the heart were considerably lower in both plans and should be less of a concern. The dose to the spinal cord PRV was also within the tolerance level.

| DISCUSSION
Owing to the complexity of the target and surrounding anatomy, radiotherapy planning for cervical and upper thoracic EC is very challenging. 4 IMRT has a much greater potential for dose sculpting and normal tissue sparing than 3D-CRT and has been widely adopted for EC treatment in clinical practice. 2   IMRT. [23][24][25] In combination with our data, it can be reasonably speculated that SG-IMRT with FJT will further reduce the lung V 5Gy , V 10Gy , and V 13Gy in comparison to VMAT. Meanwhile, the difference in the lung V 20Gy and V 30Gy could be even smaller. Jaw tracking techniques (JTT), which continuously adjust the jaw positions to and acceptable. The treatment delivery efficiency is another major concern in IMRT. Decreasing the delivery time may not only improve patient throughput but also reduce the patients' discomfort and the probability of patient movement during treatment. 15,25 Moreover, it has been found that the biological effect of radiotherapy decreased with elongation of delivery time. 27 Similar to the results from the study by Lee et al., 15

| CONCLUSIONS
The FJT combined with target splitting can provide superior OAR sparing and similar target coverage without compromising delivery efficiency compared with the SFT and should be a preferred IMRT planning method for cervical and upper thoracic EC patients.

ACKNOWLEDG MENTS
Financial support for this work was provided by the National Natural Science Foundation of China (81703758, 81301971).

CONFLI CT OF INTEREST
The authors declare that they have no competing interests.