We report a 12-year-old girl who presented with 3 months of left hemiparesis, ataxia, and slurred speech. On examination, she was drowsy, had right-sided sixth-nerve palsy and cerebellar signs. A CT scan revealed an isodense enhancing lesion measuring 3·5×2·2 cm in the posterior fossa, aff ecting the pontine region. MRI examination of the brain confi rmed the CT fi ndings. The lesion was isointense on T1-weighted imaging and had patchy enhancing areas in contrast study. In T2-weighted imaging, the lesion was moderately hyperintense. On the basis of the patients’ clinical features and radiological characteristics, we diagnosed her with a diff use pontine glioma and considered her for direct radiotherapy. Radiotherapy was given by use of parallelopposed bilateral portals in the supine position, with the scalp skin being healthy at this stage. 54 Gy in 30 fractions over 6 weeks was delivered by localised fi elds with a cobalt60 γ-ray unit. When the patient received 36 Gy of treatment, she developed high-grade fever with chills. Her blood count was healthy and she had a negative smear for malarial parasites (vivax, ovale, falciparum, and malariae). Nevertheless, she was given an empirical course of chloroquine consisting of an immediate dose of 500 mg chloroquine phosphate, followed by 250 mg given after 6 h, and 250 mg given once per day for the next 2 days. Before the initiation of chloroquine use, the patient had only a mild skin erythema in the irradiated area, which was consistent with the radiotherapy dose she had received. On day 3 of chloroquine treatment, she developed localised brisk bullous eruptions in the irradiated area, which peeled off rapidly in 6–7 h and developed into a patch of fulminant moist desquamation. The underlying subcutaneous tissues were healthy with no bleeding. The irradiated skin surrounding this patch developed severe erythema with characteristic, sharply demarcated borders (fi gure). Radiotherapy was withheld and the patient was managed conservatively with topical amniotic membrane and gentian violet application. 1 week later, her fever subsided and the moist desquamation had almost healed. Radiotherapy was restarted and completed uneventfully, with no recurrence of skin reactions. The patient was assessed 2 months after radiotherapy and showed substantial clinical improvement with restoration of motor power, resolution of cranial nerve palsies, and improved cerebellar signs. Repeat MRI revealed bulk and expanded pons with an ill-defi ned, heterogeneously hyperintense lesion on T2-weighted images. We saw patchy contrast enhancement that was suggestive of postradiotherapy change. A one-voxel MRI scan undertaken through the lesion showed a raised choline-to-creatine ratio that suggested residual disease. Comparison with the pretreatment MRI showed that the original lesion had reduced in size, but since the lesion in the follow-up scan was ill-defi ned, a formal comparison of sizes was not possible. The patient has since been put on regular follow-up. Although the radiosensitising activity of chloroquine has been well known on the basis of in vitro and animal studies, its eff ect in clinical practice is largely unknown. Chloroquine has been used as a radiosensitiser in small groups of patients with glioblastoma multiforme, who are given continuous treatment of low-dose chloroquine over an extended period during radiotherapy. Sotelo and colleagues did a randomised, double-blind, placebocontrolled trial of 30 patients with surgically confi rmed glioblastoma. Oral chloroquine at 150 mg/day or placebo was given for 12 months, beginning on day 5 after surgery. All patients received conventional chemotherapy and radiotherapy. Median survival after surgery was 24 months for chloroquine-treated patients and 11 months for controls. Although not signifi cantly diff erent, the rate of death in patients receiving chloroquine was about half of that in patients receiving placebo (p=0·139). Our patient had a rapid progressive moist desquamation with chloroquine, and the drug seemed to be the most probable cause. When reviewing the physics calculations and radiotherapy planning, we did not identify any error, Lancet Oncol 2006; 7: 608–09