Esophageal cancer is the eighth most common cancer in Japan
. Despite modern multimodality therapy, esophageal cancer still has a poor prognosis
. Surgical therapy remains the mainstay of curative therapy for esophageal cancer. However, many patients with esophageal cancer are diagnosed at an advanced, inoperable stage. Furthermore, some patients may be deemed unsuitable for surgery because of co-morbidity or increasing age
. On the other hand, some studies have reported that definitive chemoradiotherapy (CRT) is an effective alternative to surgical therapy and can achieve long-term disease control, with the overall 5-year survival rate reaching 20-27% for locally advanced esophageal cancer
Conventional CRT for advanced esophageal cancer is generally challenging because of the surrounding radiosensitive organs such as the lung, and the proximity of critical structures such as the heart and spinal cord. In order to reduce the risk of morbidity in these organs, PBT has advantages based on the fundamental physical dose distribution
. As described above, however, the available field size of PBT is insufficient to treat locally advanced esophageal cancer existing as an extended esophageal lesion. Field sizes up to 25 × 25 cm2 can be achieved with the double scattering system
. This problem is common to many facilities.
On the other hand, the so-called ‘patch-field’ technique has been introduced in several studies
[4, 5, 15, 16]. This technique is used to optimize dose distribution within an irregular volume in close proximity to critical normal structures. Hug et al. reported excellent sparing of the lens and selected intraorbital and ocular normal structures, while maintaining conformal dose-target coverage in orbital rhabdomyosarcoma of children
. According to their report, the target volume was divided into two segments, each treated by a separate radiation field. Utilizing sharp dose distribution, the distal edge of one field was matched with the lateral field edge of the second field while taking care to avoid match lines in critical structures. In fact, this method provided long disease-free periods, without severe morbidity.
In the present study, the distal or cranial field edge of one field was matched with that of the other field. To date, several studies concerning PBT for esophageal cancer have been reported, and to our knowledge, this study is the first report on PBT using ‘patch-field’ technique for esophageal cancer. Traditionally, distal-cranial matching has been used for craniospinal irradiation with photons as well as other diseases
[17, 18]. However, hot or cold spots around the field junction cannot be avoided. On the other hand, in the present study, the average minimum dose and maximum dose was between 95.9% and 105.3% around the field junction along the CC axis by the treatment-planning system. This result is acceptable compared with the dose homogeneities of previous reports using the ‘patch-field’ technique
[4, 5], and satisfies the criteria of the International Commission on Radiation Units reports 50 and 62
In some cases in the present report, hot or cold spots were actually present in the target volume around the field junction. Passing through highly complex heterogeneities and some minor inherent lateral penumbra of protons could have caused the dose distribution at the field junction to be non-uniform. Therefore, shifting the field junction should be considered to avoid over- or under-dosage. In the present study, the average ± S.D. of discordant distance was 10.6 ± 4.2 mm. From these results, it seems necessary to slide the field junction by at least 19.0 mm (two-sided 95% confidence interval).
The present study has several limitations. In particular, the effects of respiratory and peristaltic motions on the esophagus were not well considered. Yaremko et al. reported esophageal motion using respiratory-gated four-dimensional CT
. In their study, the mean of peak-to-peak displacement of the esophagus was 7.1 mm in the CC axis in 31 consecutive patients treated for esophageal cancer. Although the present report used CT images obtained during the expiratory phase under a respiratory gating system, it is insufficient to evaluate motion of the esophagus within the expiratory phase and to evaluate peristaltic motion during treatment. Shifting the field junction several times is one possible way to avoid the uncertainty due to these motions in clinical practice. Further physical and clinical studies are needed.