The challenge of conformal radiotherapy in the curative treatment of medulloblastoma

Abstract

Despite more than two decades of investigation of the role of chemotherapy in the treatment of medulloblastoma, radiotherapy remains the most important adjuvant therapy. While the entire craniospinal axis is at risk, there has been growing interest in improving the dose distribution to the posterior fossa. The classical field borders for the posterior fossa boost have included the posterior clinoid as the anterior border, the C2 vertebral body as the inferior border, the internal occipital protuberance as the posterior border, and approximately two-thirds of the vertical dimension from the foramen magnum to the parietal vertex. The availability of MRI has eliminated the need of approximate bony landmarks to patient-specific anatomy. The opposed lateral photon approach has provided excellent coverage of the entire posterior fossa. However, the development of neurocognitive, neuroendocrine as well as ototoxicity late effects has challenged the radiation oncology community to reassess the current treatment techniques. Paulino et al. (1) report in this issue of the IJROBP their analysis of dose to surrounding normal tissue using conformal radiotherapy for the posterior fossa boost in children with medulloblastoma. This paper is timely as the radiation oncologists within the pediatric cooperative groups (now merged as Children’s Oncology Group) have been undergoing extensive discussion on the use and definition of such techniques. In fact, on the current intergroup low-stage medulloblastoma protocol, conformal techniques are allowed in place of the conventional lateral fields. In this series of 5 patients, opposed laterals (conventional technique) are compared with 2 different conformal techniques. One conformal technique utilized a pair of coplanar posterior obliques and the other used obliques and a vertex field. Both techniques actually improved the dose distribution over those for conventional techniques in terms of inclusion of the planning target volume and a lower mean dose to the pituitary gland. However, both conformal techniques gave a higher mean dose to the thyroid gland, mandible, parotid glands, and pharynx. Of interest, while the two conformal plans reduced the dose to the hypothalamic‐ pituitary axis and cochlea, the integral dose to the mandible and thyroid increased. This paper confirms some of the findings of Fukunaga-Johnsonet al. in their report of 5 cases last year in the IJROBP (2). However, in that study, while the dose to the cochlear was reduced from 100% to 68%, the dose to the pituitary actually increased from 48% to 68% in the 3D plan. These studies summarize the difficulty with conformal radiotherapy techniques including intensity modulation. The conformal techniques discussed deliver a superior distribution to the target dose with a significant reduction of dose to potential critical targets. However, the basic physical principle of dose distribution of photons (exponential loss of energy in tissue with increasing depth) leads to an appreciable integral dose distributed in nontarget tissue. This integral dose might result in serious late effects in a developing child or in an increase in the risk of late radiation-induced malignancies. In contrast to the exponential depth dose characteristics of photon beams, clinical proton beams have depth-dose distributions with a relatively uniform low-dose entrance region, followed by a region of uniform high-dose (the spread-out Bragg peak) at the tumor/target, then a steep fall-off to zero dose. Therefore, compared to protons, single beams of photons deliver higher doses to normal tissues/ organs proximal to deep-seated tumors, non-uniform dose across the target, and a significant dose beyond the target. This physical advantage of protons for single beams extends to multibeam treatments, including intensity modulation. Compared to photons in comparable beam configurations, protons have better dose localization properties. This is determined by the laws of physics and does not depend upon technique. The advantages of proton therapy may be particularly important for the treatment of pediatric malignancies such as medulloblastoma. In general, for comparable beam configurations, proton treatment plans have, on the average, a factor of 2 less integral dose than do treatment plans using photons. We find that for comparable doses to the posterior