  Purpose: Retinoblastoma (RB) is the most common eye tumor in childhood and
can be treated external radiotherapy. The purpose of this work is to evaluate
the adequacy of Monte Carlo simulations and the accuracy of a commercial
treatment planning system by means of experimental measurements. Dose
measurements in water were performed using a dedicated collimator.
  Methods: A 6MV Varian Clinac 2100 C/D and a dedicated collimator are used for
RB treatment. The collimator conforms a D-shaped off-axis field whose
irradiated area can be either 5.2 or 3.1cm$^2$. Depth dose distributions and
lateral profiles were measured and compared with Monte Carlo simulations run
with PENELOPE and with calculations performed with the analytical anisotropic
algorithm (AAA) using the gamma test.
  Results: PENELOPE simulations agree well with the experimental data with
discrepancies in the dose profiles less than 3mm of distance-to-agreement and
3% of dose. Discrepancies between the results of AAA and the experimental data
reach 3mm and 6%. The agreement in the penumbra region between AAA and the
experiment is noticeably worse than that between the latter and PENELOPE. The
percentage of voxels passing the gamma test when comparing PENELOPE (AAA) and
the experiment is on average 99% (93%) assuming a 3mm distance-to-agreement and
a discrepancy of 3% of dose.
  Conclusions: Although the discrepancies between AAA and experimental results
are noticeable, it is possible to consider this algorithm for routine treatment
planning of RB patients, provided the limitations of the algorithm are known
and taken into account by the medical physicist. Monte Carlo simulation is
essential for knowing these limitations. Monte Carlo simulation is required for
optimizing the treatment technique and the dedicated collimator.
