Abstract 6504: Field-in-field technique with intra-fractionally modulated junction shifts for craniospinal irradiation (CSI) planning with 3D-CRT
Authors: Ali Shabbir Hussain A. , Gohar Rahim , Nazim Hira , Mallick Jawaid 
Published: September 2020

Abstract
Purpose: To plan craniospinal irradiation with ‘‘field-in-field'' (FIF) homogenization technique in combination with daily, intra-fractional modulation of the field junctions, to minimize the possibility of spinal cord overdose. Photon-based techniques for craniospinal irradiation (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 field-in-field 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: 16 patients (2 adults, 14 children) treated with 3D-CRT for craniospinal irradiation were retrospectively chosen for this analysis. These patients were planned and treated during 2016-2017. Contouring of Brain and Spine Cord and organ at risk were already done and planning done on Eclipse TM Treatment Planning System (Varian). All of these patients were planned Lateral cranio-cervical fields and posterior spinal fields were planned 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 and maximum dose (in terms of volume of the PTV covering &gt;110 % and &gt;120 %) at the junction was noted for each patient with both plans and the readings were evaluated.
Results: Plan inhomogeneity improved with FIF technique. Planning with daily-modulated junction shifts provided consistent dose delivery during each fraction of treatment across the junctions. Without FIF or modulated junctions, the thecal sac volume receiving 110% of the prescribed dose ranged from 12% to 25.8% (mean: 15%) and volume receiving 120% dose ranged from 0% to 8% (mean: 4%). Whereas with FIF and modulated junctions, the thecal sac volume receiving 110% of dose ranged from 0.59% to 3% and volume receiving 120% was 0%.
Conclusion: Without FIF or modulated junctions, the thecal sac volume receiving 110% of the prescribed dose ranged from 12% to 25.8% (mean: 15%) and volume receiving 120% dose ranged from 0% to 8% (mean: 4%). Whereas with FIF and modulated junctions, the thecal sac volume receiving 110% of dose ranged from 0.59% to 3% and volume receiving 120% was 0%. Therefore it was concluded that Field-in-field technique improved dose conformality and reduced inhomogeneity.
Citation Format: Shabbir Hussain A. Ali, Rahim Gohar, Hira Nazim, Jawaid Mallick. Field-in-field technique with intra-fractionally modulated junction shifts for craniospinal irradiation (CSI) planning with 3D-CRT [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6504.