Dosimetric comparison of conventionally and ultra-hypofractionated RT boost in prostate cancer
Jarosław Krawczyk,
Poland
PO-1892
Abstract
Dosimetric comparison of conventionally and ultra-hypofractionated RT boost in prostate cancer
Authors: Jarosław Krawczyk1, Andrzej Ginter1
1Greater Poland Cancer Centre, Radiotherapy Department I, Poznań, Poland
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Purpose or Objective
Prostate cancer being the leading cancer disease for men shows
high sensitivity to increasing fractional dose. Therefore, ultra-hypofractionated
radiotherapy based on high fractional doses is an alternative to the classical
radiotherapy scheme.
Material and Methods
In this study we performed a retrospective analysis of dose
metrics in selected group of patients classified for the HYPOPROST trial comparing ultra-hypofractionated
radiotherapy (UF-RT) used as a boost to conventionally fractionated
radiotherapy (CF-RT) in high-risk prostate cancer. This dosimetrical analysis
includes 179 patients treated by VMAT technique - 86 plans for UF-RT and 93
plans for CF-RT.
The physical doses obtained from the second phase of UF-RT were
recalculated to biologically equivalent doses using EQD2 formula based on the
linear quadratic model. The a/b
ratios used in the recalculations were 1,5 for the tumor and 3,0 for the OARs.
Before the plan acceptance for delivery, the biologically equivalent doses from
the first and second phases were summed and checked for adherence to required
dose constraints for OARs as well as to adequate dose distribution in the PTVs.
The ICRU-83 plan normalization criteria for the PTVs were followed, with
prescription to the median dose.
The treatment
plans for the CF-RT and UF-RT arms were analyzed for the potential differences
between dose metrics using the following parameters:
- ·
the mean EQD2 [Gy], and the EQD2 related to 2% (D2 [Gy]), and 95%
(D95 [Gy]) of the PTV (the prostate and
the basal volume of seminal vesicles),
- ·
the EQD2 related to 5% (D5 [Gy]), 25% (D25 [Gy]), 30% (D30 [Gy]),
and 40% (D40 [Gy]) of the rectum volume,
- ·
the EQD2 related to 10% (D10 [Gy]), 25% (D25 [Gy]), 30% (D30
[Gy]), and 40% (D40 [Gy]) of the bladder volume,
- ·
the EQD2 related to 10% (D10 [Gy]) of the volumes of femoral heads
and the bowels.
Results
Despite the increase in dose delivered to PTV (e.g., mean EQD2 for
UF-RT equal to 84,6 Gy vs 76,3 Gy for CF-RT) the doses delivered to OARs were
clinically comparable. Due to the clinical constraints, the statistical
superiority of UF-RT for the doses D25 (CF-RT 54,6 Gy vs UF-RT 51,2 Gy), D30 (CF-RT 51,9 Gy vs UF-RT 48,0 Gy), D40 (CF-RT 47,4
Gy vs UF-RT 43,5 Gy) of the rectum, and D10 (CF-RT 38,5 Gy vs UF-RT 34,5 Gy) of
the femoral heads were clinically not
relevant.
Conclusion
The statistical differences between mean values of EQD2 parameters
obtained for PTV are caused by the scheme of treatment. Dosimetric analysis of the boost plans prepared for UF-RT and CF-RT
schemes showed that biologically equivalent doses EQD2 for organs at risk were
statistically close for both schemes in agreement with clinically observed
similar toxicity outcomes for these patient groups.