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Comparison of manual and computer-generated customized blocks for whole brain fields used in the treatment of medulloblastoma.

BACKGROUND
By using CT simulation and three-dimensional planning, we have developed a simple but accurate method of producing customized blocks for the cranial fields in the treatment of medulloblastoma. We compare here the margins and the volume of normal tissues included in the treatment volume with those obtained using blocks generated manually.


PROCEDURE
Customized blocks for the whole brain field are generated using CT planning and autoblock function. The clinical target volume (CTV) is defined as the whole cerebrospinal fluid pathway, and the whole brain and spinal cord are contoured. A margin of 1.1 cm is generated using the autoblock function to account for set-up errors (3-5 mm) and penumbra (approximately 7 mm). A separate set of blocks was generated manually without the knowledge of the ones generated by the CT-simulator. These 2 sets of blocks were compared for a cohort of 7 consecutive patients.


RESULTS
Overall, the manual blocks and the computer-generated blocks were quite similar. Those generated manually had more variations; they were always tighter (median of 6 mm tighter; range: 3-7 mm) at the level of the cribriform plate and in 5/7 patients were more generous (median of 6 mm more generous, range: 0-6 mm) at the temporal lobes. Dosimetric analysis showed that both methods provide adequate coverage of the CTV, with 100% of the CTV receiving > 95% of the prescribed dose for both.


CONCLUSIONS
The customized block method for whole brain fields is simple to use and ensures adequate coverage of the target volume.