Session Item

To establish the dose-response relation for skin dose and temporary alopecia in patients treated with stereotactic or conventionally fractionated photon radiotherapy to intracranial lesions using MRI imaging, for implementation as soft constraint in the radiotherapy planning system.

46 patients receiving a follow-up MRI examination after cranial irradiation by photons (VMAT technique) (~ 90 days after last fraction) were screened for alopecia, of which 10 showed local temporary alopecia. For the reported analysis 5 patients with alopecia (3 stereotactic regime, 2 conventional fractionation) were found eligible for analysis, based on the following criteria: no prior cranial irradiation, stereotactic or conventional fractionation. Alopecic areas were marked with a gadolinium-filled line-marker after which the follow-up MRI scan was obtained according to local protocol. A T1-weighted 3D scan (1mm³ resolution) was used to automatically delineate the scalp skin with a thickness of 3 mm, which is known to bear the scalp hair follicles. Skin delineations, which were divided in an alopecic volume and an unaffected hair-bearing volume using the line-marker, were rigidly registered to the planning CT scan and corresponding radiotherapy dose distribution (1mm³ dose grid). Subsequently, the dose-per-voxel was obtained for the two volumes. Afterwards, a logistic probability curve (dose vs. effect) was fitted to the dose-per-voxel values to estimate the normal-tissue complication probability curve.

3D visualization showed that the affected skin delineation corresponded consequently to areas with a higher skin dose (see example in Figure 1). After combining voxel data of all patients, it was shown that the median dose in the alopecic volume was higher than for the unaffected regions (0.2 vs. 1.5 Gy for a single fraction delivered in a stereotactic regime and 5.7 vs. 16.9 Gy for conventionally fractionated radiotherapy in 28-30 fractions, for in total 44,262 and 16,423 voxels respectively). Using the logistic fit, a RD₅₀ (meaning 50% alopecia probability) was found at a physical dose of 3.2 Gy (single fraction), and 20.3 Gy (28-30 fractions); RD₁₀ (10% alopecia probability) was found at 1.6 and 3.3 Gy, respectively (Figure 2). All fit parameters had a p-value << 0.01.

We present a workflow and results to derive radiation induced alopecia probability after cranial irradiation. To our knowledge this is the first study presenting data on temporary alopecia probability. After expanding the number of patient in the analysis, we envision that the observed relationship between alopecia and dose can be used to set soft constraints during treatment planning to improve quality of life for patients with primary or metastatic brain tumours.