Session Item

To investigate the ability of the Photon Optimizer, PO, to reduce the dose rate mimicking sectors of 0 monitor units with VMAT technique.

20 left breast cancer patients, in DIBH, were planned for VMAT technique with the Varian Eclipse TPS to deliver 40.5 Gy in 15 fractions to the whole breast. Two planning strategies were adopted: PartArc, with 2 partial arcs about 200° long; and AvoidArc, with the same partial arcs and additional avoidance sectors (arc sectors with 0 dose rate) set in the PO to reduce the dose to contralateral breast and lung.

RapidPlan, RP, models were generated based on the two plan sets, named RP_Part and RP_Avoid.

A new set of plans was optimized using only the two RP models. The beam geometry was the same as the initial plans, without setting avoidance sectors inside the optimizer. The RP generated plans were compared with the original initial plans.

In a second part of the study a third RP model was generated, with plans similar to the AvoidArc, but forcing the beam geometry to the avoidance sectors (2+2Short plans): the two partial arcs were split in two short arcs each. The model was named RP_2+2. The plan differences between the AvoidArc and the 2+2Short plans were minimal and not significant since they were in practice using the same geometry, the first set inside the optimizer as dose rate setting on a sector, the second set as beam geometry with split shorter arcs. Plans generated with RP_2+2, with the geometry without splitting the partial arcs, were compared with the AvoidArc plans to determine if the initial beam geometry information in the RP model can improve the PO performances.

The target coverage and dose homogeneity were acceptable in all cases, with manual and RP based plans. The following table reports the mean doses, averaged over the whole patient’s cohort, and standard deviation to the homolateral structures, left lung and heart, and the contralateral structures, right breast and lung, for the manual plans PartArc and AvoidArc, and for the RP generated RP_Part, RP_Avoid and RP_2+2.

The PO, using the RP_Avoid, was unable to reduce the dose rate in the avoidance sectors when they were not manually defined, although the RP_Avoid estimated DVHs of the contralateral structures were in agreement with the AvoidArc plans. Similarly, in the case of the RP model generated with the split short arcs, the PO did not reduce the contralateral doses.

Only in the case with the arcs split into short arcs in the initial geometry, the dose to the contralateral structures was correctly achieved. The last row in the table reports the case of RP_2+2 and split arcs as initial geometry, where the contralateral structures are similar to the AvoidArc and 2+2Short cases.

The PO showed to be unable to modulate the dose rate enough to produce the RP DVH estimation. In particular, it does not reduce the dose rate in the beam toward structures where a very low dose (scattering) is required.