Dose mapping of the rectal wall during brachytherapy with an array of scintillation dosimeters.

PURPOSE: In pelvic brachytherapy treatments, the rectum is an organ at risk. The authors have developed an array of scintillation dosimeters suitable for in vivo use that enables quality assurance of the treatment delivery and provides an alert to potential radiation accidents. Ultimately, this will provide evidence to direct treatment planning and dose escalation and correlate dose with the rectal response. METHODS: An array of 16 scintillation dosimeters in an insertable applicator has been developed. The dosimeters were calibrated simultaneously in a custom designed circular jig before use. Each dosimeter is optically interfaced to a set of pixels on a CCD camera located outside the treatment bunker. A customized software converts pixel values into dose rate and accumulates dose for presentation during treatment delivery. The performance of the array is tested by simulating brachytherapy treatments in a water phantom. The treatment plans were designed to deliver a known dose distribution on the surface of the rectal applicator, assumed to represent the dose to the rectal wall. RESULTS: The measured doses were compared to those predicted by the treatment plan and found to be in agreement to within the uncertainty in measurement, usually within 3%. The array was also used to track the progression of the source as it moved along the catheter. The measured position was found to agree with the position reported by the afterloader to within the measurement uncertainty, usually within 2 mm. CONCLUSIONS: This array is capable of measuring the actual dose received by each region of the rectal wall during brachytherapy treatments. It will provide real time monitoring of treatment delivery and raise an alert to a potential radiation accident. Real time dose mapping in the clinical environment will give the clinician additional confidence to carry out dose escalation to the tumor volume while avoiding rectal side effects.

The angular dependence and effective point of measurement of a cylindrical scintillation dosimeter with and without a radio-opaque marker for brachytherapy.

Fibre optic scintillation dosimeters, consisting of a plastic scintillator coupled to an optical fibre, are a promising dosimeter in brachytherapy applications. The combination of tissue equivalence, real-time readout and small spatial size makes them especially attractive for in vivo verification of patient treatments. Given that the orientation of the dosimeter with respect to the radioactive source changes during brachytherapy treatment, the angular dependence of the dosimeter is important. We derived the dependence of the response of a cylindrical dosimeter to a point radiation source as a function of distance along its axis and along a radius. Using the results, the effective point of measurement of a cylindrical scintillator was located for two points in the angular response curve as a function of distance between the source and dosimeter. We measured the angular response experimentally for a cylindrical scintillation dosimeter, when the source was located at a distance of 50 mm from the centre of the scintillator. A refinement of the design, in which a radio-opaque marker is incorporated into the tip for accurate localization in the patient, modifies the angular response of the dosimeter. For this new dosimeter design, we show that the dosimeter response decreases by 20% when the source is located on the axis of the scintillator, due to absorption by the marker. The dosimeter response becomes almost angle independent at 10 degrees away from the axis. Excluding this cone, a cylindrical scintillation dosimeter which incorporates a radio-opaque marker was found to be angle independent to within 2%. In most clinical brachytherapy applications, this design has an acceptable angular dependence.

Malignant papillary mesothelioma of the tunica vaginalis testes: cutaneous metastases showing pagetoid epidermal invasion.

Malignant papillary mesothelioma of the tunica vaginalis testis is an extremely rare tumor arising from the mesothelium of the tunica vaginalis. A total of 14 cases have been reported, with only two exhibiting cutaneous metastases. We present a third case of cutaneous metastases arising from this unusual tumor. In addition, this is the first reported case of the occurrence of pagetoid epidermal invasion in this condition.

Psoralen-containing sunscreen is tumorigenic in hairless mice.

Sunscreens containing 5-methoxypsoralen (5-MOP) are currently being marketed to promote tanning by inducing psoralen-mediated ultraviolet (UV) A (320-400 nm) melanogenesis. The rationale is that this may prevent UVB (290-320 nm) radiation-induced skin damage. However, mouse studies have shown that 5-MOP has the same cutaneous photocarcinogenic potential as 8-methoxypsoralen. In addition, the 5-MOP--containing sunscreen Sun System III (SS III), when combined with UVA, induces epidermal ornithine decarboxylase activity, an enzyme associated with tumor promotion. Therefore, we investigated whether SS III had sufficient psoralen concentration to be tumorigenic in hairless mice exposed to chronic, intermittent UVA radiation. SS III was applied to hairless mice 5 days per week for 20 weeks. After each application the mice were exposed to 2.5 to 10 joules/cm2 UVA radiation. All test groups developed atypical squamous papillomas in direct proportion to the dosage of UVA radiation received. A shorter latency period for tumor development was seen with larger UVA doses. Test animals followed up to 1 year developed invasive squamous cell tumors. Control groups (SS III without UVA and UVA without SS III) remained free of tumors. Animals receiving SS III plus UVA developed persistent skin thickening and increased dermal cyst formation similar to that reported with chronic exposure to UVB, a known carcinogenic wavelength. Over-the-counter sunscreens containing 5-MOP do contain sufficient psoralen concentrations to cause cutaneous phototoxicity and photocarcinogenicity in mice, and their use in humans should be discouraged in the interest of preventing further UV-induced skin damage and skin cancer.