To limit dose to an OAR during optimization, you may find yourself using numerous upper objectives. Every one of these objectives are assigned a priority, and when you have numerous objectives across numerous OARs, the optimizer can become rigid to desired changes. Then for the optimizer to respond, you may need to drastically increase the priority for a set objective which can potentially disrupt your priority scheme.
The generalized equivalent uniform dose (gEUD) can be an alternative to avoiding this problem. A ‘regular’ upper objective pinpoints to a single distinct point on the optimizer DVH curve, whereas a gEUD upper objective defines a region. Thus, you can set a single gEUD upper objective to limit dose across a stretch of volume. This stretch is determined based on the assigned alpha value which relates to radiobiology, i.e., serial versus parallel organs. The alpha value ranges from 1.0 to 40.0 with a value of 1.0 affecting the majority of the optimizer DVH curve and a value of 40.0 affecting that of the max dose.
The gEUD can be conceptualized as a collection of regular objectives put together into a single objective. However, the gEUD and regular objectives are not mutually exclusive, and can be used in combination to utilize the advantage of regular objectives that is fine tuning. Additionally, the gEUD can also be set as a lower objective functioning to provide coverage with an alpha value range of -40.0 to -1.0.
When an OAR is enveloped by the PTV such as in a hippocampal or chiasm sparing (re-treat whole brain) plan, conventionally set field sizes that encompass the entire PTV (brain) can make for a more difficult plan. For the MLC leaves to block the OAR, they must traverse a significant length of the PTV, thus, resulting in a constant push-and-pull between providing adequate coverage and reducing OAR dose during the optimization. Closing in the X1 jaw while in the Beam’s Eye View, and the X2 jaw for the subsequent arc, so that the target is located just inside the field will shorten the distance that the MLC leaves need to travel and reduce that push-and-pull. For the example shown above, the collimator has been rotated by 90° so that the MLC leaves travel in the superior to inferior direction. Following a similar line of thinking, for target volumes that require a X field size greater than 20 CM, reducing the field size to be closer to 15 CM can improve your dose distribution as the MLC leaves have a mechanical limitation of traversing a maximum of 15 CM. In both scenarios, it is important that the reduced field sizes have overlapping regions with one another.
For an OAR that abuts the PTV, positioning the MLC leaves to align to the OAR-PTV interface can improve the blocking. In the example shown below, the MLC leaves align all throughout the Brainstem-PTV interface, and the resultant dose distribution exhibited a sharp dose gradient by this region.
The normal tissue objective (NTO) is an objective for the dose fall-off outside of the target. The manual NTO is defined by the following parameters: Distance from Target Border, Start Dose, End Dose, and Fall-off.
|ID/Type||Vol [cm^2]||Vol [%]||Dose [cGy]||Actual Dose [cGy]||Priority|
|Distance from Target Border||0.20 CM|
‘Start Dose’ refers to the percentage of the upper objective target dose that the system will allow around the target up to distance defined by ‘Distance from Target Border’ without increasing cost. For the example above, the system will accept 6615 cGy (i.e., 105% of 6300 cGy) for up to 0.20 CM from the target border. If two upper objectives are used, the NTO will use the upper objective of the lower dose. The ‘Fall-off’ refers to the steepness of the dose fall-off between the ‘Start Dose’ and ‘End Dose’. The system takes into account the physical/mechanical limitations of the machines and MLC specifications, and thus, setting an unrealistic NTO may hold back the plan.
The Base Dose Plan (BDP) allows you to create a plan based off of a previous dose distribution. BDP can be helpful when the current target volume is nearby a previously treated region as the fall-off dose spread can be accounted for. For example, if a patient had just finished treatment for 30 Gy in 10 fx, and upon physician evaluation, needed to undergo treatment for a neighboring region, overlap of high dose can be prevented by using BDP. BDP can only be used for plans of the same CT scan, and so it will typically not be used for patients who recur or return for additional treatment.