Leukaemia

Figure evolve 33.1 ). Patients are immobilized in a head shell and treated with lateral-opposed 4–6 MV megavoltage fields which may be centred on outer canthus to minimize divergence into the contralateral lens. Shielding will cover the face, dentition, nasal structures and lenses. The clinical target volume (CTV) includes the intracranial meninges extending inferiorly to the lower border of the second or third cervical vertebra. Great care is taken to include the cribriform fossa, temporal lobe and base of skull. Although the lens is shielded, as much of the posterior orbit as possible is included as ocular relapses occasionally occur. The CTV is localized with lateral simulator radiographs or, more recently, computed tomography (CT) simulation. The prescribed dose in current UK protocols is 24 Gy in 15 fractions of 1.6 Gy daily.

Boys who suffer a testicular relapse are treated with testicular radiotherapy (Figure 33.2). The technique employed is an anterior field, generally electrons or orthovoltage (200–300 kV). The CTV includes both testes, scrotum and inguinal canal superlaterally as far as the deep inguinal ring with shielding of non-target skin and perineum. The prescribed dose is 24 Gy in 12 fractions of 2.0 Gy daily.