9 Reporting in Brachytherapy: Dose and Volume Specification

204 Reporting in Brachytherapy

Fig 6.28 : continued.

(a)

(b)

(c)

(d)

(e)

Width of the GTV

32 mm

38 mm

48 mm

26 mm

26 mm

85 Gy reference volume width

58 mm 78 mm 80 cm³ 68 mm 90 mm 180 cm³

61 mm 82 mm 106 cm³ 90 mm 110 mm 420 cm³

67 mm 88 mm 148 cm³ 100 mm 120 mm 620 cm³

55 mm 80 mm 59 cm³ 60 mm 88 mm 105 cm³

58 mm 84 mm 87 cm³ 66 mm 94 mm 145 cm³

height

volume (approx.)

60 Gy reference volume width Volume (approx.) Total dose to point A height

75 Gy 25 Gy

85 Gy 45 Gy

95 Gy 45 Gy

60 Gy 0 Gy

75 Gy

External beam dose to point A 0 Gy In cases (a-c), the 85 Gy reference volume is equal to the Treated Volume. In case (b), the 85 Gy reference volume is also equal to the “point A” volume. In case (d), the 60 Gy reference volume is equal to the Treated Volume and to the “point A” volume. In case (e), the Treated Volume (75 Gy) is equal to the “point A” volume. In addition, the reference volume is important when evaluating and comparing late side effects. Published data underline that the 60 Gy reference volume is relevant – in combination with ICRU reference points - for the prediction and reduction of late side effects (7). 8.4.3.4 Practical application of the reference volume concept Brachytherapy alone For comparing treatments with brachytherapy alone, physical doses have to be weighted to take into account possible differences in time-dose patterns as indicated in section 6.5. This radiobiological weighting influences the dimensions of the reference volumes. For example within the prospective Manchester trial on dose rate, a dose weighting factor of 15% was found: 75 Gy brachytherapy alone with radium (0.5 Gy/hour) could be replaced by 67.5 Gy with cesium-137 (1.4-1.8 Gy/hour) for the same Treated Volume with a comparable clinical result with regard to tumour control and side effects (38). A prospective randomised trial was performed at IGR comparing 0.4 and 0.8 Gy/hour: the increase in dose rate led to a significant increase in acute and late side effects (34). Combination of brachytherapy and external beam therapy The situation becomes more complicated when external beam therapy is combined with brachytherapy (12,22,33,59,107). Usually, external beam is used to treat a large PTV (PTV1: the whole pelvis) to 45-50 Gy. In the second phase of the treatment, brachytherapy is used to treat a smaller PTV which requires a higher dose (PTV2: usually the volume closely related to the GTV). To compare the brachytherapy applications the dose which has been delivered by external beam therapy must first be subtracted from the total dose. The weighting for differences in time-dose pattern must then be applied, as discussed above, before treatments can be appropriately compared. For example, for a large tumour if 85 Gy seems to be appropriate to define the reference volume, and a dose of 45 Gy is delivered to the whole pelvis by external beam therapy, the dose level for definition of the reference volume for the brachytherapy application is: 85 Gy – 45 Gy = 40 Gy. However, for a given dose, the biological effects produced by external beam and brachytherapy, may be different due to the differences in time-dose pattern and in dose–volume distribution.