9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 177

There will inevitably be a high-dose zone around each source. Although this zone is often small and well tolerated, the exact tolerance dose and volume for interstitial therapy are not known yet. However, for intercomparison, there must be agreement on how to describe the high-dose volumes. A dose of approximately 100 Gy is likely to be significant in determining late effects. In those patients who receive 50 - 60 Gy as Minimum Target Dose or 60 - 70 Gy as Mean Central Dose (MCD), 100 Gy corresponds approximately to 150% of the MCD. It is therefore recommended in ICRU-Report 58 (45) that the size of the region receiving more than 150% of the MCD should be reported. The high-dose region should be defined as that encompassed by the isodose corresponding to 150% of the Mean Central Dose (MCD) around the sources in any plane parallel to the central plane where a high-dose region is suspected. The maximum dimensions of all regions, in all planes calculated, should be reported. 6.4.3 Low-dose regions A low-dose region is defined as a region, within the CTV, where the dose is less than 90% of the prescribed dose. The maximum dimension of the low-dose region in any plane calculated should be reported. In implants for which the CTV is included within the Minimum Target Dose isodose, the occurrence of a low-dose region is exceptional. If the CTV is not covered by the Minimum Target Dose isodose, there will be low-dose regions due to geographical miss. Low-dose regions should be reported in order to correlate the local recurrence rate with the dose distribution. 6.4.4 Reference Volume The Reference Volume is the volume encompassed by an isodose defined in relation to the Mean Central Dose.

Fig 6.12: Dose and volume reporting in interstitial brachytherapy for breast cancer. The Minimum Target Dose was defined at ~ 10 mm from the outer needles and was 20 Gy for LDR. When the Mean Central Dose was calculated after treatment, this was 28 Gy. For comparison with published data, the “85% of the Mean Central Dose” was chosen. 85% of the Mean Central Dose was 24 Gy, the mean reference volume was 73 cm³, and the homogeneity index