Session Item

TROG 13.01 SAFRON II have shown similar 1-year toxicity, OS, PFS and local control between single and multi fractions Lung SBRT. While preparing the planning protocol for single fraction Lung SBRT, dosimetric measurements for different dose calculation algorithms and a different number of VMAT arcs were performed to ensure plan deliverability and agreement between planning and delivered dose (28Gy/1 fraction).

5 VMAT plans (AXB-6FFF-3A, AXB-10FFF-3A, AAA-10FFF-3A, AAA-10FFF-4A and AAA-10FFF-6A) were output by an experienced dosimetrist on 5 retrospective patients. 3A, 4A and 6A denote the number of radiation beam arcs and AXB or AAA refers to the dose algorithm used for optimizing the plan. 1mm and 2mm calculation grid size and dose to medium were used in dose reporting for AXB algorithm. PSQA was performed using point-dose measurement (Static QUASAR^TM phantom with Cedarwood insert; both Pinpoint and Farmer Chamber from PTW were used) and PD. Global gamma passing rates at 1%/1mm, 2%/1mm, 1%/2mm and 2%/2mm were reported for the PD QA. Lastly, plan complexity comparisons were evaluated using modulation complexity score (MCS) across all the plans.

The results show that the 4A and 6A plans are less complex (P-value < 0.05) as compared to other plans and the respective MCS for each plan types for all patients are as shown in Figure 1.

Figure 1(A) shows the MCS for all plan types across all patients and 1(B) shows the gamma passing rate of 1mm/1% for all plan types across all patients. 

The point dose difference between measurement and plan calculated with two different grid sizes using two different dose calculation algorithms are shown in Figure 2. All plans are evaluated to be less than 5% dose discrepancy except AXB-6FFF-3A. This discrepancy result between 6FFF and 10FFF is similar to that reported by Öllers MC et al.[2]. The least discrepancy occurs using the AXB dose calculation algorithm with a 1mm calculation grid size. Subsequently, PSQA using PD was done and the result follow a similar trend from point dose measurement. AXB-6FFF-3A gives a lower gamma passing rate for all criteria. Despite being less complex, 4 and 6 arcs plan does not result in a statistically significant higher gamma passing rate. Lastly, the dose in OAR and PTV were compared between the two different dose calculation algorithms. The main differences occur in PTV D95 and D98 where using 6FFF beams gave less than 1 Gy difference while 10FFF beams gave less than 3 Gy difference. The difference in doses in the OARs are less than 0.5%. 

Figure 2 shows the percentage difference in point dose between treatment plans (AXB and AAA; 1mm and 2mm) and measurement (Pinpoint and Farmer).

This study shows that 10FFF beam is recommended for hypo-fractionated treatment and similar dosimetric accuracy are obtained for 3, 4 and 6 arcs plan. The point dose measurement results also support the use of the AXB dose calculation dose algorithm with a 1mm calculation grid size.