22 Penis cancer

Penis cancer

9

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 15/07/2022

3D printed applicator[18] or a bivalved lucite cylinder (diameter range 3 - 4.5 cm)[19]. The penis should fit snugly in the cylinder. To avoid excessive dose to the penile skin/mucosa, longitudinal source channels should be embedded 5mm from the inner surface which is in contact with the penis, and should be close enough to provide optimum homogeneity and minimize hyperdose sleeves around each source track. CT simulation is performed with the penis placed in the posterior half of the cylinder, supported and positioned to be horizontal. The GTV (or CTV if no GTV due to prior excision) is marked with wire based on pathology and prior clinical photos. The anterior half of the cylinder is then positioned and attached. The transparent material allows easy verification of penile position. A lucite “cork” with a convex end is advanced into the open end of the cylinder until it touches the glans. Any redundant skin on the shaft of the penis should be retracted out of the way using a penile constriction ring so it does not slide down into the treated volume while treatment is ongoing (Figure 7). Although the patient can be catheterized for the CT simulation, even without a catheter the urethra can generally be identified on the CT images from themeatus through the length of the applicator, located between and ventral to the corpora cavernosa. The GTV/ CTV is contoured. For the surface mold technique, because of some potential variability in positioning from day to day, a 5 mm PTV margin is recommended along the longitudinal axis of the penis. Treatment is accomplished as an outpatient, twice daily, 6 hours apart, over 5 days. LDR interstitial The majority of the interstitial penile brachytherapy experience over the decades (Table 1) has been obtained following the Paris System recommendations for geometry and dose prescription (Figure 4). Needles are inserted parallel and equidistant in parallel planes.The dose rate minimumbetween the planes is known as the Basal Dose Rate. The prescription dose (60-65Gy) is delivered to the Reference Isodose which is 85% of the Basal Dose Rate (Figure 3). From the needle spacing, the lateral clearance margins can be calculated. This algorithm ensured that the hyperdose sleeves around each source would not be excessive and generally <20%. HDR and PDR interstitial After CT simulation (slices at 1.2mm), the GTV is contoured based on prior clinical examination and imaging. A minimum 5 mm margin is added to define the CTV. Homogeneity constraints are important to minimize the risk of necrosis, especially with HDR. For PDR, dose may be prescribed to the 85% isodose following Paris System rules as for an LDR implant [20]. However, manual isodose optimization can be performed to improve target coverage andminimize hyperdose sleeves close to the urethra and/or mucosa. As described before, critical structures are the urethra and the penile mucosa though the treatment volume. As can be appreciated from Table 2, there is no agreed prescription for HDR penile brachytherapy. We recommend limiting the dose to the urethra and penile mucosa to less than 125% of the prescription dose, though there is no firmdata to confirm this recommendation. See Table 3 for the EQD2 and BED of various fractionation schemes. 9. TREATMENT PLANNING

With this recommendation, the dose limit to the organs at risk will depend on the prescription and can be seen to range from 45-52.5 Gy with EQD2 ranging from 59-70 Gy. It is preferable to report urethral dose as EQD2 per volume of urethra in cm 3 . This is less ambiguous than reporting the dose to a percentage of total contoured volume, as there are no accepted guidelines as to what constitutes the appropriate contoured volume. In an unpublished study based on PDR data, the risk of meatal stenosis correlated with distal urethra D 0.1cm 3 and D 0.2cm 3 , with a threshold of 82 Gy (HR=0.12, 95%CI = 0.04-0.38) for D 0.1cm 3 and 78 Gy (HR=0.19, 95%CI = 0.06-0.56) for D 0.2cm 3 [21]. Similar data for HDR will be of great benefit. Table 4 suggests minimal reporting requirements for HDR interstitial penile brachytherapy. Suggested homogeneity constraints for HDR include V 125% < 40% of the treated volume, V 150% < 20% and V 200% < 5%. With the V 125% < 40% of the treated volume, it should be possible to keep the urethra and skin under 125%but the dose distribution needs to be carefully evaluated and optimized as necessary to protect these critical structures. For PDR treatments, slightly higher inhomogeneity can be accepted, provided the Dose Non-uniformity Ratio V150/ V100 is maintained < 25%. GTV should receive 100% of prescription (GTV D 98% ≥ 100%) and CTV 90% (CTV D 98% ≥ 90%). Wherever external needles have been placed, bolus material is required between the surface and the external needles, generally 5 mm superflab. HDR surface mold In our experience 40 Gy/10 fractions/5 days, is prescribed at 100% to the GTV/CTV. A PTV margin is advisable because of some uncertainty of set up, especially along the longitudinal axis, but is not required at a depth. With skin contoured as the outermost 2 mm of the penile structure, skin D 0.1cm 3 should be < EQD2 70Gy (125% of a prescription dose of 40Gy (19)). Since this is a surface prescription with a maximum 3 mm thickness, urethral dose is not a concern and will be much less than prescription. A tighter skin constraint of D 0.1cm 3 EQD2 58.8 Gy for the same prescription (skin dose limited to 105% of 40 Gy/10/5days) has been associated with a higher toll of local recurrence (18). LDR and PDR interstitial Decades of experience with LDR interstitial penile brachytherapy employed manually afterloaded Ir-192 wire. Recommended activity was 1-1.5 mCi/cm (37-55 MBq/cm) but variable length and spacing of sources per individual implant resulted in a range of dose rates generally from 40-70 cGy/hr. Prescribed dose was 60-65 Gy, delivered over 4-5 days. The use of PDR permits prescription at a classic average low dose rate of 42-55 cGy/hr through brief hourly HDR pulses [22] without the need for a correction for Radiobiologic Effect (RBE) with respect to classic LDR. The largest brachytherapy series for penile glans tumours, including 205 patients treated over 45 years at Institut Gustave Roussy, has shown that a dose ≥ 62 Gy correlated with better local control than lower doses in node negative patients (p=0.038). The risk of complications correlated with the prescribed dose at the 85% 10. DOSE, DOSE RATE, FRACTIONATION