Necessity of daily IGRT? Dosimetric study of beam position offsets at central double-beam junction in pediatric craniospinal irradiation

Craniospinal irradiation (CSI) is an essential treatment modality for pediatric patients with tumors at high risk of leptomeningeal spread. However, accurate reproduction of the original simulation posture is difficult in subsequent CSI sessions as expansion of the target volume cannot fully compensate for variations, potentially resulting in dose uncertainties caused by the beam junction. This study aimed to evaluate the impact of offsets in beam positions on dose uncertainty at the central double-beam junction during CSI radiotherapy. To compute the dose distribution in different positional offsets, the relative positions of the beams and CT images of the brain and spine beam centers were adjusted according to the original treatment plan of individual patients. Four parameters were calculated to assess the quality of the radiotherapy plan: the conformity index (CI), homogeneity index (HI), percentage of 1-V95%, and percentage of V110% at the planning target volume (PTV) at the beam junction. The quality parameters (CI, HI, 1-V95%, V110%) were observed to deteriorate with increasing positional offsets. Overall, higher absolute values of beam center offset were associated with poorer dose distributions at the beam junction. Movement along the craniospinal axis toward the brain was defined as positive, while movement toward the spine was defined as negative. Shifting of the brain beams by + 3 mm and the spine beams by -3 mm resulted in an average CI value of 0.50, compared to 0.76 in the non-offset scenario, while the average HI was 0.33 versus 0.11 and the average 1-V95% value was 41.05% compared to 0.52% in the reference plan. When the brain beams were shifted by -3 mm and the spine beams by + 3 mm, the average V110% was 62.75%, compared to 2.01% without offset. Considering all beam offset scenarios, the worst-case doses to organs at risk were a maximum dose 7.78 Gy to the lens and average doses of 10.01, 8.58, 8.46, and 5.57 Gy to the parotid gland, thyroid gland, heart, and kidneys, respectively, while the lung V5 was 47%. Greater discrepancies in beam positioning during CSI radiotherapy result in poorer dose distributions at the beam junction. Therefore, opposing shifts of the brain and spinal fields can lead to significant underdosing or overdosing at the junction. Therefore, daily image-guided radiation therapy is recommended for CSI.