20 Prostate Cancer

Prostate Cancer

11

THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II: Clinical Practice Version 1 - 01/12/2014

145 Gy isodose In yellow

PTV in lilac

urethra

Seed position

Rectal wall

Fig 21.6: Map of needle positions for LDR based on the standard template design with rows designated by letters along the x axis and numbers up the y axis: There are several clinical implantation rules. Place the needles inside the prostate contour and start from the upper rows due to scatter of the needles at ultrasound. Usually 1cm spacing is used on rows and columns and treatment is performed using a checkerboard pattern. The needles should avoid the urethra (red area). There should be no needles in the column including the urethra (red area).

Fig 21.7: LDR dose distribution for 145Gy I125

Table 21.4: Dose volume parameters for LDR brachytherapy

DOSE-VOLUME PARAMETER

DOSE RECOMMENDA- TION 145Gy (100%) for Io- dine-125

PRE-PLAN

POST-PLAN

VOI

Prescription

Yes

yes

V

Yes Yes Yes Yes Yes No No Yes Yes No

yes yes yes yes yes yes yes yes yes yes

≥ 95% ≥ 100% ≤ 50% ≤100% < 200Gy

PTV

100

D

90

V

150

D

2cm³

D

0.1cm³

OAR: Rectum

V V

--- ---

100

150

D D

< 150% < 130%

10

OAR: prostatic urethra

30

D

---

5

and 1.4-2.2 U (1.1-1.7 mCi) for Palladium-103 and 1.6-2.2 U (2.5-3.4 mCi). U is the unit for air kerma strength, 1U = 1 cGy cm² h -1 . Due to the much higher initial dose rate (typically 7 cGy.h-1 for Iodine-125 versus 21 cGy.h -1 for Palladium-103 and more than 30 cGy.h-1 for Caesium-131) and the much shorter half-life, the prescribed dose for caesium is 115 Gy, for palladium is 125 Gy, while for iodine the usual dose at the periphery of the planning target volume is 145 Gy (Fig 21.7) using the AAPM TG43 for- malism. Ninety percent of the treatment dose is delivered within 197 days for iodine, within 56 days for palladium and 32 days for caesium. Technically, the implantation procedure is identical for all three isotopes. According to ESTRO recommendations [31] there is a set of dose-volume parameters for both PTV and OARs which have to be reported and considered for both pre-planning and post-plan-

ning procedures. Table 21.4 summarizes those parameters and the corresponding recommended dose limits [31]. In practice in order to avoid very high dose volumes (above 150% of the prescription dose) the number of seeds in the cen- tre of the PTV is reduced achieving thus a modified peripheral loading pattern (Fig 21.6, Fig 21.7). However, adequate cover- age of base and apex prostate region is mandatory and this can be achieved by placing an inner ring of applicators and loading them only at the base and apex. It is recommended that post implant dosimetry for iodine-125 implants is performed based on cross sectional imaging at 4-6 weeks after implantation [31], although some centres have rec- ommended that post plan dosimetry should be undertaken at 24 hours post implant (32). In the case of palladium-103 implants an optimal imaging time is considered to be 16 days after im- plantation [33].