Field-in-Field Technique With Intrafractionally Modulated Junction Shifts for Craniospinal Irradiation Planning With Three-Dimensional Conformal Radiation Therapy

Abstract

Background: Planning craniospinal irradiation (CSI) with ''field-in-field'' (FIF) homogenization technique in combination with daily, intrafractional modulation of the field junctions is needed to avoid spinal cord overdose. Photon-based techniques for CSI may result in dose inhomogeneity within the treatment volume and usually require a weekly manual shift of the field junctions to minimize the possibility of spinal cord overdose. Nowadays, FIF technique is used to feather out the dose inhomogeneity caused by multiple fields. We have started using this technique after acquiring advanced technology machines in recent years.

Methods and materials: Sixteen patients treated with three-dimensional conformal radiation therapy (3D-CRT) for CSI were retrospectively chosen for analysis. These patients were treated during 2019-2020. Contouring of planning target volume (PTV) and organs at risk (OAR) was done, and planning was done on Varian Eclipse^TM Treatment Planning System (TPS) (Varian Medical Systems, Palo Alto, CA). These patients were planned with lateral craniocervical fields and posterior spinal fields using a forward-planned FIF technique. Field junctions were automatically modulated and custom-weighted for maximal homogeneity within each treatment fraction. Dose-volume histogram (DVH) was used for analysis of results. A corresponding plan without FIF technique was planned; then maximum dose at the junction was noted for each patient with both plans, and the readings were evaluated. Paired sample t-test was used to compute the p-values for the inferential statistics.

Results: Without FIF technique, the volume receiving 110% of the prescribed dose ranged from 39% to 74% (mean: 62.12%) and volume receiving 120% dose ranged from 8% to 28% (mean: 17.68%), whereas with FIF technique, the thecal sac volume receiving 110% of dose ranged from 2% to 18% (mean: 11%) and volume receiving 120% ranged from 0% to 2%. Volume receiving 100% of the dose was also calculated in both techniques; mean values of this dose range was almost similar in both groups. Later p-value was calculated, and in both dose ranges of thecal sac volume receiving 110% and 120%, the difference in values was statistically significant. Therefore, it proved that plan inhomogeneity improved with FIF technique. This technique provided consistent dose delivery during each fraction of treatment across the junctions. The maximum doses calculated at the junction were higher in the CSI plans without FIF compared to those with FIF technique.

Conclusion: This study concludes that better dose homogeneity is achieved with FIF technique as compared to non-FIF technique, and the difference in values was statistically significant.

Keywords: 3d-crt; craniospinal irradiation (csi); field-in-field technique; medulloblastoma.

Figure 1. Comparison of two plans of a medulloblastoma child patient with and without field-in-field technique and modulated junctions

A: Sagittal DRR of a medulloblastoma child planned without field-in-field technique. Note the junction of the craniocervical field with the PA field showing deficient coverage of the anterior area of the PTV with 95% coverage (shown in green lines) of the prescribed radiation dose and the subsequent hot spot areas showed with purple-colored dose distribution areas. B: Sagittal CT images of the same patient planned with field-in-field technique and modulated junctions, and dose coverage is showed in color wash. The junction can now be seen covered with ≥95% coverage of the prescribed dose with minimum hot spot areas. DRR, Digitally reconstructed radiograph; PA, posteroanterior; PTV, planning target volume.

Figure 2. Comparison of DVH of the same patient planned with and without FIF technique

The graph shows two curves, one without FIF technique and modulated junction of which the curves show deficient 95% coverage and along with that more hot spot. In contrast, the second curve with the FIF technique shows good 95% coverage with lesser hot areas. DVH, Dose-volume histogram; FIF, field-in-field.