Radiobiological comparison of two radiotherapy treatment techniques for high-risk prostate cancer

Abstract

Background

To make a radiobiological comparison, for high risk prostate cancer (T3a, PSA[[ce:hsp sp="0.25"/]]>[[ce:hsp sp="0.25"/]]20[[ce:hsp sp="0.25"/]]ng/ml or Gleason[[ce:hsp sp="0.25"/]]>[[ce:hsp sp="0.25"/]]7) of two radiotherapy treatment techniques. One technique consists of a treatment in three phases of the pelvic nodes, vesicles and prostate using a conventional fractionation scheme of 2[[ce:hsp sp="0.25"/]]Gy/fraction (SIMRT). The other technique consists of a treatment in two phases that gives simultaneously different dose levels in each phase, 2[[ce:hsp sp="0.25"/]]Gy/fraction, 2.25[[ce:hsp sp="0.25"/]]Gy/fraction and 2.5[[ce:hsp sp="0.25"/]]Gy/fraction to the pelvic nodes, vesicles and prostate, respectively (SIBIMRT).

Materials and methods

The equivalent dose at fractionation of 2[[ce:hsp sp="0.25"/]]Gy (EQD₂), calculated using the linear quadratic model with α/β_prostate[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1.5[[ce:hsp sp="0.25"/]]Gy, was the same for both treatment strategies. For comparison the parameters employed were D95, mean dose and Tumour Control Probabilities for prostate PTV and D15, D25, D35, D50, mean dose and Normal Tissue Complication Probabilities for the rectum and bladder, with physical doses converted to EQD₂. Parameters were obtained for α/β_prostate[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1.5, 3 and 10[[ce:hsp sp="0.25"/]]Gy and for α/β_oar[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1, 2, 3, 4, 6 and 8.

Results

For prostate PTV, both treatment strategies are equivalent for α/β_prostate[[ce:hsp sp="0.25"/]]=[[ce:hsp sp="0.25"/]]1.5[[ce:hsp sp="0.25"/]]Gy but for higher α/β_prostate, EQD₂ and TCP, decrease for the SIBIMRT technique. For the rectum and bladder when α/β_oar[[ce:hsp sp="0.25"/]]≤[[ce:hsp sp="0.25"/]]2[[ce:hsp sp="0.25"/]]Gy, EQD₂ and NTCP are lower for the SIMRT technique or equal in both techniques. For α/β_oar[[ce:hsp sp="0.25"/]]≥[[ce:hsp sp="0.25"/]]2–3[[ce:hsp sp="0.25"/]]Gy, EQD₂ and NTCP increase for the SIMRT treatment.

Conclusions

A comparison between two radiotherapy techniques is presented. The SIBIMRT technique reduces EQD₂ and NTCP for α/β_oar from 2 to 8[[ce:hsp sp="0.25"/]]Gy.