Interplay effects during enhanced dynamic wedge deliveries

[Impact of Respiratory Motion on Point Doses for Three Whole-breast Irradiation Techniques after Breast-conserving Surgery]

PURPOSE

For whole-breast irradiation after breast-conserving surgery, computed tomography simulation (CTS) and irradiation are generally performed during free breathing. In treatment planning, there are three techniques: field-in-field (FIF), physical wedge (PW), and enhanced dynamic wedge (EDW). The aim of this study was to investigate the impact of respiratory motion on doses for these three irradiation techniques.

METHODS

All doses were measured using an ionization chamber in a cylindrical phantom on a respiratory motion platform. Doses for each technique were measured with and without phantom motion. The dose without phantom motion was defined as the reference. The reference was compared to the dose with the phantom motion. The positions of the isocenter with respect to the ranges of phantom motion were set as exhale and intermediate. The phantom motion amplitude was set to 5 mm or 10 mm. The respiratory phase to initiate irradiation was varied as inhale, intermediate-inhale, exhale and intermediate-exhale.

RESULTS

When the motion amplitude was 10 mm, the dose differences for the FIF, PW, and EDW techniques were 4.2%, 0.5%, and 0.8%, respectively, at the maximum. However, the dose difference for the FIF technique was -0.5% when the isocenter position was set to the intermediate phase of phantom motion.

CONCLUSION

We found that the dose difference per fraction was reduced when the respiratory phase during CTS image acquisition was set to the intermediate phase. Meanwhile, the dose differences per fraction for the PW and EDW techniques were less affected by the respiratory motion.

Basic evaluation of a novel 4D target and human body phantom

Stereotactic body radiation therapy (SBRT) is usually verified with a dynamic phantom or solid phantom, but there is a demand for phantoms that can accurately simulate tumor dynamics within an individual that would allow customized validation in every patient. We developed a new 4D dynamic target phantom (multi-cell 4D phantom) that allows simulation of tumor movement in patients. The basic quality and dynamic reproducibility of this new phantom was verified in this investigation. The newly developed multi-cell 4D phantom comprises four main components: soft tissue, bones, lungs, and tumor (target). The phantom structure was based on computed tomography (CT) data of a male. In this study, we investigated the basic performance of a multi-cell 4D phantom. All the CT numbers of the phantom were very close to those of human data. The geometric maximum amplitudes were 4.57 mm in the lateral direction, 4.59 mm in the ventrodorsal direction, and 3.68 mm in the cranio-caudal direction. Geometric errors were 0.84, 0.58, and 0.40 mm, respectively. Movements of the abdominal surface were stable for 60 s. Repeated measurements show no actual differences in target movements between multiple measurements and indicated high reproducibility (r  >  0.97). End-to-end tests using Gafchromic film revealed a gamma pass rate of 98% or above (2 mm/3%). Although our phantom performed limited reproducibility in the movement of the patient tumor at present, a satisfactory level of precision was confirmed in general. This is a very promising device for use in the verification of radiation therapy for moving targets.

[Impact of Respiratory Motion on Dose Variation in Radiation Therapy for Glottic Cancer]

In radiation therapy for glottic cancer, respiratory motion of larynx may change the dose variation in the target. The purpose of this study is to measure the respiratory motion of the larynx, and quantify the impact of the motion on the dose variation. This study included 10 patients treated by opposing portal irradiation for glottic cancer. We acquired fluoroscopy and respiratory waveform of the patients simultaneously and formulated the relationship between the displacement of larynx and the respiratory phase. We divided one field into 39 sub-fields on the basis of control points. Dose distributions accounting for the displacement were calculated by shifting isocenter calculated using the formula in every sub-fields. Dose variations of clinical target volume (CTV) were evaluated by subtracting dose distributions with displacement consideration and dose distributions without it. Average amplitude and the maximum amplitude of respiratory motion were 2.5 and 8.7 mm, respectively. Average of mean dose variation in CTV was 0.1% of the prescribed dose, and maximum of local dose variation was 2.0% of the prescribed dose. Hence, it is realized that dose variation in CTV by respiratory motion was slight.