Session Item

The aim of radiotherapy for brain tumours is to maximize dose to tumour and at the same time preserving normal brain parenchyma as much as possible. In Image Guided Radiotherapy (IGRT), image guidance on the treatment couch enables better precision in patient setup and target localization which in turn maximizes therapeutic index of brain irradiation by reducing setup errors during treatment. The aim of study was to analyse setup errors in the radiation treatment of Glioblastoma Multiforme (GBM) and if decrease in Planning Target Volume (PTV) margin is feasible using daily Cone Beam CT (CBCT) and 6D couch correction.

Radiotherapy contouring and planning was done using 2 volume approach in which Gross Tumour Volume (GTV 1 and GTV 2) were delineated on T2W and T1C images of post-operative MRI. These were then expanded by 1.5cm to form Clinical Target Volume (CTV 1 and CTV 2). The standard practice at our institute is to give a PTV margin of 0.5cm. A total of 21 patients (630 fractions of radiotherapy) were studied in which setup corrections at the time of treatment were recorded in 6 dimensions i.e. vertical, lateral, and longitudinal along with pitch, roll and rotation. We determined the average setup errors in all directions, whether setup errors during the first three fractions would predict the setup uncertainties for the rest of the treatment with daily CBCT, mean difference in setup errors with or without application of 6D couch and volumetric benefit of reduction of PTV margin from 0.5cm to 0.3cm.

The mean shift in vertical, longitudinal and lateral directions were 0.17cm, 0.19cm and 0.11cm respectively. The mean values of pitch, roll and rotation were 1.1, 1.04 and 0.78 degrees respectively. The average 3D shift value of all the fractions of radiotherapy combined (lateral, longitudinal and vertical shift) was 0.285. There was significant increase in vertical shift when the mean setup errors of the 1^st three fractions were compared with rest of the treatment using daily imaging (p value: 0.004). Central tumours had less setup uncertainty as compared to peripheral tumours. When the effect of 6D couch was nullified (pitch, roll and rotation were negated) all directions (vertical, longitudinal and lateral) showed increased error with longitudinal shift being significant (0.19cm vs. 0.27cm; p value: 0.001). The average number of fractions per patient with a setup error of >0.3cm were more when the effect of 6D couch was nullified (12 vs. 5; p value: 0.001). There was a significant decrease in volume of brain parenchyma irradiated when margin of PTV was reduced from 0.5cm to 0.3cm (difference of 45.77cc and 61.47cc in 60Gy and 46Gy volume respectively) (p value: 0.001) 

Daily CBCT at the time of radiotherapy treatment along with corrections using 6D couch can reduce setup error margins which will improve therapeutic index.