RESULTS
Table 1 shows the patient and dosimetric characteristics for VMAT-TBI plans. The mean global Dmax was 118.8%±4.0% and the PTV V110% ranged from 0.1% to 5.3%. All lung Dmean values were below 7.2 Gy and lungs-1cm Dmean was 36.5%±4.8%. For patients that required kidney sparing, the average kidney Dmean was 67.5%±4.0% of the prescription dose. Achieved testes/ovaries Dmean and Dmax were 0.66±0.07Gy and 0.90±0.03Gy, respectively. The average brain-1 cm Dmean was 55.9%±3.7%. Thyroid sparing was performed for two patients with Dmean = 1.45Gy±0.07Gy. Lenses were spared to 90% of prescription for all patients.
Table 2 shows the comparison between VMAT and 2D plans for target and lung dosimetric indices. The target coverage, PTV D90%, was significantly superior for VMAT plans as compared to 2D plans with a mean difference of 6.1%±2.4%, p<0.001. No significant difference was observed between the VMAT and 2D plans for global Dmax and PTV V110% (mean differences of -2.0±6.7% and 0.0±2.9%, respectively). With the 2D plans, lungs received an average of 80.6% of the prescription dose (range: 72.0% - 90.0%). The average lungs-1cm Dmean for 2D TBI plans was 70.6%±8.1%.
The mean lungs and lungs-1cm dose for VMAT-TBI plans was significantly lower as compared to the 2D plans: reductions of -25.6%±11.5% and -34.1%±10.1%, respectively, were observed (p<0.001). In addition, VMAT-TBI plans spared kidneys, brain, thyroid, testes/ovaries, and lenses, while 2D plans delivered the prescribed dose to those organs.
Gonadal sparing between 2D and VMAT-TBI plans was compared for two patients (female - patient 4 and male – patient 3). For patient 4, Figure 1 shows the dose distribution comparison between the 2D and VMAT plans. The ovaries were spared to mean dose of 65.8 cGy and maximum dose of 87.8 cGy with the VMAT plan. Although gonadal sparing would not be practical with the 2D approach due to difficulties in ovary localization in the standing AP/PA treatment in the booth, the 2D plan was created for dosimetric comparison. With the 2D plan, the ovaries received 147.1 cGy Dmean and 150.0 cGy Dmax (prescription dose of 2 Gy). Figure 2 shows the DVH comparison between the VMAT and 2D Conventional TBI plans for patient 3. The maximum and mean doses to the testes were 71.9 cGy and 44.7 cGy, respectively, for the VMAT plan compared to 156.4 cGy Dmax and 136.2 cGy Dmean with the 2D plan.
All ten patients tolerated treatment well. The image guidance included CBCT at chest isocenter and MV portal images for the consequent isocenters. The average treatment time was 44.4 min, range: 25.1 min to 57.4 min. Three patients were treated under anaesthesia, and seven remaining patients were watching movies during radiotherapy.