Impact of setup errors on multi‐isocenter volumetric modulated arc therapy for craniospinal irradiation

Abstract Multi‐isocenter volumetric modulated arc therapy (VMAT) is recommended for craniospinal irradiation (CSI) to smooth the dose distribution in the junction regions relying solely on inverse optimization. However, few studies have measured the dosimetric impact of setup errors on this multi‐isocenter VMAT in the junction areas. The purpose of this study is to evaluate the impact of positional errors during VMAT CSI with two‐dimension (2D) and three‐dimension (3D) dosimetric measurements. A total of 20 patients treated by three‐isocenter VMAT CSI were retrospectively reviewed and analyzed. A 3D diode array ArcCHECK and radiochromic film EBT3 were applied to measure the percentage gamma passing rates (%GPs) and dose distributions in the junction areas between the cranial/upper‐spinal and the upper/lower‐spinal fields with intentionally introduced setup errors of ± 1 mm, ±2 mm, ±3 mm, ±5 mm, and ± 8 mm, respectively. The length and volume of planning target volume (PTV) for these CSI patients ranged from 50.14 to 80.8 cm, and 1572.3 to 2114.5 cm3, respectively. The %GPs for ±3 mm, ±5 mm, and ±8 mm positional errors were around 95%, 90%, and 85%, respectively, in the junction areas. The dosimetric verification results with EBT3 films indicated that cold and hot areas were observed with the increase of introduced setup errors. In conclusion, the dosimetric verification with intentionally introduced setup errors demonstrated that positional errors within 3 mm have a little impact for VMAT CSI, although setup errors should be minimized. Relying on the inverse optimization of VMAT to smooth the dose distribution in the junction areas is feasible for CSI.


| INTRODUCTION
Craniospinal irradiation (CSI) is a complex radiotherapy technique indicated for patients with high risk of whole central nervous system involvement. 1,2 The challenge of CSI is the exceeding length of planning target volume (PTV) over the maximum treatment field of a common linear accelerator (linac), which requires combining treatment fields to cover the whole targets and causes over-and/or underdose problems in the junction areas between fields. 3,4 Traditionally, 3D conformal radiotherapy (CRT) technique is applied for CSI by using two lateral opposed photon beams for the brain and matching to one or more posterior photon beams for the spine. 5 Weekly junction displacements, known as feathering by moving the treatment field junction weekly, have been adopted to reduce the larger over-or underdose in the junction areas. 5 However, junction issue remains challenging for 3DCRT due to the intrinsic limitations of the dose calculation accuracy, collimator positioning accuracy, patient positioning accuracy, etc. 3,6 In order to deliver more conformal doses to the target volumes and to better spare the surrounding health tissues outside of the target, in particular the thyroid, heart, and intestines, 7 modern techniques, such as intensity-modulated radiation therapy (IMRT), 8 volumetric modulated arc therapy (VMAT), 9 tomotherapy, 10 and proton therapy have been intensively investigated in the dose delivery of CSI. 11 Studies demonstrated superior dosimetric results from these modern techniques in comparison with conventional 3DCRT for CSI; however, there is still no consensus on the recommendation of CSI radiotherapy technique due to a larger number of organs at risk involved and overlap among different techniques and studies. [8][9][10][11][12] Dosimetric hot and cold spots in the field junction regions are still a major concern. A variety of techniques, such as "jagged-junction", "overlap" technique, and "gradient optimization" technique, have been employed for these modern techniques to address the dosimetric problem in the field junctions. 5,8,10,13 Due to its advantage of less treatment time and potential decrease of movement uncertainty, especially for pediatric patients, linac-based VMAT CSI has been investigated and applied in clinical practice. Controlling linear dose gradient across the junction, 14 silico ideal-based universal field matching solution, 15 low gradient junction technique, 16 etc., have been investigated specifically for VMAT CSI to optimize the junction doses. However, as an inversely optimized technique, careful planning of field junctions may complicate the VMAT planning process. 17 Reports demonstrated that it is feasible to optimize a set of overlapping fields concurrently without explicitly controlling the junction dose with VMAT. 18,19 A smooth dose across the junction could be achieved with this simple approach by relying on the optimization algorithm. One limitation of this simple approach is that it renders the dosimetric distribution sensitive to positional errors.

2.B | Dosimetric verification with ArcCHECK
Three-dimensional dosimetric verification of VMAT CSI plans was carried out by using a 3D diode array ArcCHECK (Model 1220) and   Detailed characteristics of these patients are presented in Table 1.
Dosimetric verification results with ArcCHECK are presented in   The idea of irradiating the whole central nervous system for patients with cerebellar medulloblastoma was firstly advanced by Dr.
Edith Paterson. 22 The challenge along with this CSI technique for whole central nervous system is the extreme long target volume. As shown in Table 1, the PTV length for the enrolled patients in this study is around 50-80 cm. The maximum treatment field of linacbased MLC is usually around 40 cm. Two to three isocenters were commonly required during CSI with modern IMRT and VMAT techniques. 7,19 Three-isocenter VMAT plans were generated in this study to cover the brain and spinal as one PTV.
The purpose of treating the whole nerve system as one PTV is to decrease the dose inhomogeneity at the field junctions and to smooth the dose distribution with the intrinsic characteristics of the inverse optimization of VMAT. 4 because of the existence of low dose gradients at the junction between fields. 16 When the setup error was greater than 5 mm, lower %GPs and cold or hot junctions were observed in this study.
However, even when the positional error reached 8 mm, the %GPs only had a minor degradation of around 85%. Similar results were demonstrated by Meyer et al., in which they simulated the isocenter shifting by 1, 2, 5, and 10 mm and concluded that isocenter shifting should be minimized, but the treatment plan accuracy will not be deteriorated even when larger errors of 5-10 mm were simulated. 20 In general clinical practice, the setup errors are usually around 1-3 mm, especially for the application of modern IMRT and VMAT techniques, and high accuracy of setup error is a precondition. 23

| CONCLUSION S
The dosimetric verification with intentionally introduced setup errors demonstrated that positional errors less than 3 mm have little impact on VMAT CSI, although setup errors should be minimized during practice. Relying on the inverse optimization of VMAT to smooth the dose distribution in the junction areas is feasible for CSI.