Session Item

The conventional and currently most used method to perform TBI consists of two static either LAT or AP/PA opposed photon beams, delivered with a LINAC at an extended SSD, to cover the entire patient’s body with one field. With this technique, treatment delivery takes at least 2 hours. Continuous developments on LINACs and MLCs allow to introduce VMAT technique for TBI. Purpose of this dosimetric study is to verify the feasibility of VMAT-TBI treatment in our centre.

The whole-body CT acquisition of Alderson Rando phantom filled with 200GR-A lithium fluoride TLDs was performed with a Canon Aquilion® Exceed LB CT. Whole body, lungs, and PTV (the whole body reduced by 3 mm from the skin and excluding lungs contracted by 3 mm) were contoured with Monaco®5.51.10 (Elekta AB) TPS. The VMAT-TBI treatment plan consisted of four equally spaced isocentres and the same number of dual-arc 6MV overlapping fields (with junction regions between adjacent fields up to 4 cm), delivered with an Elekta LINAC equipped with Agility®(Elekta AB) MLC. Treatment goals were to cover almost 95% of PTV volume with 95% of prescription dose (Dp, 12 Gy in 6 fractions), with the near-minimum and the near-maximum doses within 90% of Dp and 110% of Dp respectively, and to reduce the mean dose to lungs below 10 Gy. TLDs placed inside Alderson Rando for point measurements were calibrated at 6 MV with the LINAC used for measurements and read with a RADOS RE-2000 TLD reader (RadPro International GmbH). In order to measure dose distributions of junction regions, a new method was developed by acquiring each couple of adjacent fields, as well as each single beam, with Delta4 phantom+ (ScandiDos AB).

PTV V95% was 95.70% (D98%=11.10 Gy, D2%=12.73 Gy), while the mean dose to lungs was 8.94 Gy. MUs of each beam ranged from 490.69 to 1199.75 and the total beam-on time was 15 min. Mean and maximum percentage differences between calculated doses (TPS) and measured doses (TLDs) were 2.3% and 10.4% respectively (10.4% in lungs where there was a high gradient region). No significative differences were observed between measures performed by TLDs placed in junction regions between adjacent fields and those placed in isocentre regions, meaning that Monaco TPS is able to properly optimize beams with different isocentres. Dose distributions acquired with Delta4 had a gamma passing rate (3%/3 mm, global, 10% threshold) for single beams always higher than 99% and higher than 90% for coupled beams. Quantitative estimate of the Rando setup residual error was at most 1.8 mm in lateral direction and lower than 1 mm in the other directions.

This dosimetric study confirms the feasibility of VMAT-TBI in our centre and shows several potential advantages of this technique, including a more comfortable positioning of the patient, shorter delivery times and the possibility of a more personalized treatment.