Absolute dose output and plan normalization

How each treatment plan is normalized is one of the most critical parts of a treatment planning system, since it deter- mines how the monitor units should be calculated, which in turn determines the actual doses delivered to the patient. Study of all the different methods of plan normalization which are available in the RTP system is critical to confirm that~1! they work as expected; and ~2! the treatment delivery system in the department uses them correctly. This section deals primarily with external beam planning, while parallel issues in brachytherapy such as dose specification criteria and dwell time normalization are addressed in Appendix 5.

4.8.1. General guidelines for QA for normalization and MU calculation. The first and most basic recommendation

of the task group on this subject is the following:

A complete check of the entire treatment plan normaliza- tion and monitor unit calculation process must be performed for a series of different kinds of plans. Each plan should be normalized in a number of different ways, and for each method, the user should utilize the available methods to cal- culate the monitor units required to treat the plan. The dif- ferent methods should then be compared to assure that (1) the correct monitor units and doses are always achieved; and (2) the results of the different methods are the same (within tolerance).

TABLE4-4. Suggested Format for Acceptability Criteria for External Beam Dose Calculations, with Example Criteria*

(The criteria shown are based on the collective expectations of the members of the task group and are not to be used as goals or requirements for any particular situation.) Situation Abs. Dose @normpt ~%!** Central Axis ~%! Inner Beam ~%! Penumbra ~nm! Outer Beam ~%! Buildup Region ~%! Homogeneous phantoms: Square fields 0.5 1 1.5 2 2 20 Rectangular fields 0.5 1.5 2 2 2 20 Asymmetric fields 1 2 3 2 3 20 Blocked fields 1 2 3 2 5 50 MLC-shaped fields 1 2 3 3 5 20 Wedged fields 2 2 5 3 5 50

External surface variations 0.5 1 3 2 5 20

SSD variations 1 1 1.5 2 2 40

Inhomogeneous phantoms***:

Slab inhomogeneities 3 3 5 5 5 -

3-D inhomogeneities 5 5 7 7 7 -

*Percentages are quoted as a percent of the central ray normalization dose. The criteria shown as examples in the table are based on the collective expectations of the members of the task group and are not to be used as goals or requirements for any particular situation.

**Absolute dose values for the dose at the beam normalization point are relative to a standard beam calibration point. They do not include all the uncertainties associated with determining the absolute dose under standard calibration conditions.

Study of the normalization/MU calculation process should be performed in each clinic. Clinics should of course attempt to standardize this process, to minimize the complexity and possibility for misinterpretation of input data or results. The radiation oncology physicist should attempt to ensure that the process will perform as expected for any likely combina- tion of situations, even in the face of deliberate errors or misuse of the system functions. A careful analysis of the possible hazards associated with this aspect of the system should be performed at each institution, since plan prescrip- tion, normalization, and monitor unit calculation methods vary quite a bit from institution to institution. A detailed knowledge of the design, methodologies, algorithms, and safety checks which are part of the RTP system design is required. The task group recommends that vendors provide enough information so that the user can carry out such an analysis of the normalization/MU calculation process.

The task group also recommends that vendors incorporate into the RTP system design automated checks of geometric and dosimetric information to be performed during beam and plan normalization.13 Such checks can detect not only soft- ware errors but also incorrect system use and errors in judg- ment in choice of normalization points and/or methods. Error or warning messages generated by the system can help users avoid inappropriate or incorrect normalization situations that might lead to incorrect treatment.

4.8.2. Verification of the steps in the process. In order to

determine the monitor units required to give a prescribed dose to a particular treatment plan, various steps in the plan- ning process are involved, including:

• The relative beam weights are set as part of the plan technique.

• The overall relative plan normalization method is cho- sen for the treatment plan.

• The total dose and fractionation are prescribed by the physician.

• A particular prescription point or isodose level is cho- sen by the physician.

• Monitor units are calculated so that the prescribed dose is delivered.

Each step in this process should be carefully studied and appropriate testing carried out.

Relative beam weights. In order to add the doses from

several beams together, some method of determining the relative beam weight of each beam is used in each RTP system. This relative weight may be the dose defined at the beam normalization point, the relative number of MU for the field, or may be related to the energy fluence. Typically, the RTP system calculates the relative dose to be delivered to the normalization point ~beam norm-pt! chosen for each beam

~in older systems, this point may be at dmax on the central

axis for each beam, or it may be the isocenter for an isocen- tric plan!. In more complex systems, the beam normalization point may be different for each beam, since dmaxor isocenter

may not always be appropriate. After the point is identified, some relative dose ~called the beam weight! is delivered to this point for each beam, and then individual beam dose

distributions are summed to yield the dose distribution for the plan. Table 4-5 lists some beam weight issues to be checked. Comparable questions must be asked for any of the methods used for beam weights inside the RTP system.

Overall relative plan normalization. After the relative

dose distribution is obtained, most RTP systems allow the normalization of the entire distribution to give a specified dose at some defined point ~the plan normalization point, or plan norm-pt!. The value at the plan norm-pt might be in terms of relative dose, absolute dose for one fraction, or dose for the entire treatment. Testing issues are listed in Table 4-6.

Isodose level chosen for dose prescription. A common

use of plan normalization features is to normalize the plan to 100% at the isocenter of the plan, and then to choose a mini-

TABLE4-5. Relative Beam Weight Issues

How is the beam norm-pt chosen? Are different norm-pts allowed for dif- ferent beams?

Does the identification of the beam norm-pt agree with the coordinates chosen, for all options available?

What happens if the beam norm-pt is near or under a block or MLC edge? How close to the beam edge can the norm-pt be placed?

What happens if the beam norm-pt is within or behind an inhomogeneity?

What happens if the beam norm-pt is outside the patient external surface?

What happens if objects such as the CT couch are in the patient representation? What happens if there are serious CT artifacts?

How is the norm-pt dose calculated? Dose it take into account effects of blocks/MLC, beam modifiers, inhomogeneity corrections?

Are warnings given when inappropriate norm-pts are chosen?

TABLE4-6. Overall Plan Normalization Issues

How is the plan norm-pt chosen?

Does the identification of the plan norm-pt agree with the coordinates chosen, for all options available?

What happens if the plan norm-pt is near or under a block or MLC edge?

What happens if the plan norm-pt is within or behind an inhomogeneity?

What happens if the plan norm-pt is outside the patient external surface?

How is the norm-pt dose calculated, for each normalization method available? Does it take into account effects of blocks/MLC, beam modifiers, inhomogeneity connections?

Are dose units handled correctly?

Does the plan normalization cause appropriate changes in other related parameters~e.g., dose at beam norm-pts!?

mum isodose line ~isodose surface! that encloses the plan- ning target volume ~PTV!, and to use this isodose level as the prescription dose. It is critical that this part of the pre- scription process be included in any monitor unit calculation methods. Alternately, the same result can be accomplished by increasing the dose at the plan normalization point~e.g., to 105%! so that the 100% isodose level covers the PTV.

Calculation of monitor units (MU) to deliver prescribed dose for a plan. QA for the calculation of monitor units for a

particular plan is of course very dependent on the methods used inside the RTP system and any external MU calculation program or techniques, if used. It is here that all of the ques- tions about exactly how the planning system calculates and displays dose to the beam normalization points and the plan normalization point become most important. The MU calcu- lation methodology must be completely tied to the methods of normalization used inside the RTP system, or incorrect doses delivered to the patient will result. Table 4-7 contains several additional recommendations.