29 Skin Cancer

Skin Cancer

12

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/04/2017

Fig. 31.16: Lower eyelid implant with 2 plastic tubes and dosimetry for interstitial HDR brachytherapy.

For 3D image based treatment planning with interstitial brachytherapy CT scan slices or MRI, with dummies inside the tubes or needles are used, with markers on the edges of the GTV or CTV. GTV and CTV are delineated. In the case of parallel tubes or needles the modified Paris System (SSDS van der Laarse) can be used and the prescription is usually to 90% isodose of the Mean Central Dose (Fig. 31.16). In 3D planning D90 and D98 values should be reported which are related to the GTV and the CTV as defined on 3D imaging. Optimized dosimetry will also adjust isodose curves to the CTV drawn on the scan, to ensure target coverage and avoid hot spots by keeping the DNR (Dose-non-uniformity-ratio) under 0.36 [29]. The DNR (which is the ratio of the volume that receives 150% of the prescribed dose over the volume that receives the prescribed dose = V150/ V100) is a simple and easy to interpret parameter to quantify the inhomogeneous dose distribution around interstitial sources. To achieve the highest degree of homogeneity it is advised to prescribe not only to an isodose related to the MCD in the central plane, but also to the mean longitudinal minimum doses between the sources throughout the whole treated volume. The lowest DNR is achieved by the combination of dose point optimization with geometrical optimization.

between fractions, dose: typically 8 times 4 Gy twice a day in 4-5 days. Depending on the volume to be treated and the organs at risk (eye, cartilage, bone) a lower or higher per fraction (with corresponding total dose) is used. Doses between 3 and 5 Gy can be chosen, in order to finish the treatment in a similar time to the LDR techniques: five days to avoid the gap of the weekend, or eight-ten days with lower doses per fraction. When using contact brachytherapy, the schedule is more similar to electrons, and 3-4 Gy per fraction three days per week in 4-5 weeks is effective. In large areas such as scalp, a 2-3 Gy per day fractionation is preferable. In small epithelial tumours, 5-7 Gy per fraction 2-3 days per week can be used. With skin surface applicators the chosen dose is 7 fractions of 6 Gy or 5- 6 fractions of 7 Gy, twice a week. In cases of very thin skin or with underlying cartilage, such as the nose, lower doses per fraction probably allow better cosmetic long-term results. No clear recommendations can be done due to the great variety of published schedules and the prescribed dose is based more in experience that in evidence. The depth of prescription must be always indicated, because the same prescribed dose can result in different doses to the skin surface. The total dose depends on the chosen dose per fraction, common regimens may include 51-54 Gy (17-8 fractions of 3Gy), 44-48 Gy (11-12 fractions of 4 Gy ), 40 Gy (8 fractions of 5Gy), 42 Gy (6 fractions of 7 Gy or 7 fractions of 6 Gy), 35 Gy (5 fractions of 7 Gy).

10. DOSE, DOSE RATE, FRACTIONATION

11. MONITORING

With LDR Ir-192 wires, the prescribed dose was 60 Gy at the 85% of the Mean Central Dose isodose. This assumed to be the minimum target dose (peripheral dose) covering the CTV, at dose rates between 45-70 cGy/h. Depending on the linear activity and the source spacing and length, the required dose used to be delivered in 4 to 6 days. Although doses up to 70 Gy were given in some large tumours, without unacceptable sequelae, the increase in cosmetic damage from a dose increase above 60 Gy is greater than the gain in local control. With PDR similar doses are recommended. With interstitial HDR brachytherapy, high dose per fraction is used, twice a day in interstitial implants, separated at least 6 hours

Moist desquamation develops 1 week after implantation in the mucosa and after ± 12 days in the cutaneous areas. The reaction is maximal at about three weeks, and heals progressively (depending on the area) in 5 to 8 weeks. No special care is required, except for daily cleaning and application of a topical antiseptic. Local application of silicone-coated wound dressing, seems to be a good alternative to treat larger zones of moist desquamation. Sun exposure must be avoided, and a sunscreen should be routinely used.