20 Prostate Cancer

Prostate Cancer

12

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

Table 21.5: Physical and equi-effective dose values (EQD2) for LDR brachytherapy.

RADIONUCLIDE

INDICES

Iodine-125

Palladium-103 Caesium 131

Prescription Dose (Gy) EQD2 (Gy)

145.0

125.0

115.0

66.2

68.3

58.0-68.7

The radiobiological values for those calculations are taken by the AAPM Report (Table I in Nath et al . [33]): a=0.15 Gy−1, ß=0.05 Gy−2, a/ß =3.0 Gy, Tp=42 days, and repair half-time of 0.27 h. Initial dose rates were 7cGy/h, 21cGy/h and 30cGy/h for 125 I, 103 Pd and 131 Cs sources, respectively.

CT is the usual imaging modality for post-planning due to its wide availability and the excellent resolution for identifying and reconstructing the implanted sources in addition to reasonable soft tissue contrast in the pelvic region. CT imaging is not as re- liable as MRI for prostate and other normal tissue delineation. On the other hand multiple scans are required with MRI for op- timal viewing of tissue and sources. In general, CT overestimates the prostate volume [34]. CT based post-planning should use an axial, 2-3 mm, contiguous image series. When MR imaging is considered for prostate and relevant anatomy delineation a T2 weighted 3mm contiguous image series should be acquired. [31] [33]. Table 21.5 offers representative values for Biological Effective Dose (EQD0=BED) and Biological Equivalent Dose (EQD2) to be delivered in 2Gy fractions assuming homogeneous dose dis- tribution at the corresponding prescribed dose using the formal- ism and parameters as described in [33] for iodine, palladium and caesium sources. HDR temporary implants In contrast to LDR permanent implants where the treatment dose is accumulated over several weeks or months depending on the half-life of the radionuclide used, in HDR temporary im- plants the treatment dose is delivered in fractions each of 10-15 minutes duration (comparable to the dose rate in external beam radiotherapy). Iridium-192 high strength sources (40,3 kU or 10Ci) are the gold standard and are in use with HDR afterload- ers for temporary implants. Some use a Co60 source rather than iridium which having a much longer half-life requires source changes far less often; dosimetry from the two isotopes is com- parable. Ultrasound (US) based implantation guided by the clinical ex- perience of the team is followed by either US-based or CT-based planning and treatment delivery. Currently commercial systems are available allowing an intraoperative 3D-US pre-planning and 3D US-based planning for the final treatment delivery. With such technology patient transfers from implantation to imaging and then to treatment room can be avoided and the whole proce- dure can be realized within an implantation room with adequate shielding for iridium-192 HDR treatments [17], [26]. This ena- bles a total procedure duration comparable to that achieved with permanent seeds implants of 1-2 hours. The current GEC ESTRO recommendations define the CTV as the entire prostate gland defined by the capsule in case of T1 and

Figure 21.8 demonstrates an example of dose distribution and DVHs for a CT-based HDR prostate implant.

Figure 21.9 demonstrates an example of dose distribution and DVHs for a 3D-US-based HDR prostate implant.