Single field treatments are not used in curative therapy because they:

Single field treatments are not typically used in curative therapy primarily due to the issue of dose non-uniformity across the target volume. When using a single treatment field, there can be significant areas within the target that receive not only insufficient doses but may also experience varying levels of radiation due to the geometry of radiation delivery. This means that while some areas of the tumor might receive adequate radiation, other parts could be underdosed, leading to inadequate treatment of the malignancy.

In curative therapies, the goal is to deliver a uniform and effective dose of radiation to the entire tumor while minimizing the dose to surrounding healthy tissue. Therefore, utilizing only a single field often compromises the overall treatment effectiveness and poses challenges in achieving a homogeneous distribution of the therapeutic dose required to effectively treat cancer.

In contrast to the other options, which might highlight some perceived benefits of single field treatments such as speed and patient comfort, the critical factor in curative treatment is achieving an effective and uniform dose across the tumor. This is why multi-field techniques, such as IMRT or VMAT, are preferred, as they allow for better dose shaping and distribution that can adequately cover the tumor while sparing surrounding normal tissues.