Custom blocks for MV photon beams need to be shaped in what manner to effectively shield critical structures?

For custom blocks used in megavolt (MV) photon beam therapy, a diverging shape is essential for effectively shielding critical structures. This is because the divergence of the beam must be taken into account to ensure that the radiation dose is uniform and precisely targeted to the treatment area, while minimizing exposure to surrounding healthy tissues.

In a diverging design, the edges of the block are crafted to match the divergence of the photon beam as it moves away from the source. This accounts for the geometric distribution of the beam, which expands as it travels through space. By shaping the blocks to be diverging, the dosimetry calculations can ensure that the coverage of the tumor is optimized while simultaneously sparing critical organs located nearby.

This method of shaping allows the clinician to better conform the radiation dose to irregularly shaped tumors, adapting the treatment plan to the specific anatomy of the patient. It also assists in reducing the dose to surrounding healthy tissues, enhancing treatment effectiveness while minimizing potential side effects.