Consideration of the likely benefit from implementation of prostate image-guided radiotherapy using current margin sizes: a radiobiological analysis

Clinical and radiobiological evaluation of a method for planning target volume generation dependent on organ-at-risk exclusions in magnetic resonance imaging-based prostate radiotherapy

Background and purpose

Due to a smaller target volume when delineating prostate on magnetic resonance imaging (MRI), margins may be too tight as compared to computed tomography (CT) delineation, potentially reducing tumor control probability (TCP) in prostate radiotherapy. This study evaluated a clinically implemented MRI-based target expansion method to provide adequate margins yet limit organ-at-risk (OAR) dose as compared to CT-based delineation.

Methods and materials

Patients in this study were treated to 79.2 Gy in 44 fractions via intensity modulated radiotherapy using an MRI-based expansion method, which excluded OARs when performing a 5 mm isotropic (except 4 mm posterior) expansion from gross tumor volume to clinical target volume (CTV), followed by an isotropic 5 mm expansion to generate the planning target volume (PTV). Ten cases were re-planned using CT-delineated prostate with CTV-to-PTV expansion of isotropic 8 mm, except for a 5 mm posterior expansion, with comparison of PTV volumes, TCP and normal tissue complication probability (NTCP) to the MRI-based method. Under IRB approved protocol, we retrospectively evaluated 51 patients treated with the MRI-based method for acute bladder and rectal toxicity with CTC-AE version 4.0 used for scoring.

Results

MRI-based PTV volume differed by 4% compared to CT-based PTV volume. Radiobiological calculated TCP of the MRI-based method was found comparable to CT-based methods with an average equivalent uniform dose of 80.5 Gy and 80.1 Gy respectively. Statistically significant decrease in bladder NTCP (toxicity Grade 2 and above for 5% complications within 5 years post radiotherapy) was observed in the MRI-based method. Outcomes data collected showed 65% and 100% of patients studied experienced Grade 0/1 bladder and rectal acute toxicity respectively. Grade 2 bladder toxicity was indicated in the remaining 35% of patients studied with no Grade 3 toxicity reported.

Conclusions

Results showed comparable PTV volume with MRI-based method, and NTCP was reduced while maintaining TCP. Clinically, bladder and rectal toxicities were observed to be minimal.

Target margins in radiotherapy of prostate cancer

We reviewed the literature on the use of margins in radiotherapy of patients with prostate cancer, focusing on different options for image guidance (IG) and technical issues. The search in PubMed database was limited to include studies that involved external beam radiotherapy of the intact prostate. Post-prostatectomy studies, brachytherapy and particle therapy were excluded. Each article was characterized according to the IG strategy used: positioning on external marks using room lasers, bone anatomy and soft tissue match, usage of fiducial markers, electromagnetic tracking and adapted delivery. A lack of uniformity in margin selection among institutions was evident from the review. In general, introduction of pre- and in-treatment IG was associated with smaller planning target volume (PTV) margins, but there was a lack of definitive experimental/clinical studies providing robust information on selection of exact PTV values. In addition, there is a lack of comparative research regarding the cost-benefit ratio of the different strategies: insertion of fiducial markers or electromagnetic transponders facilitates prostate gland localization but at a price of invasive procedure; frequent pre-treatment imaging increases patient in-room time, dose and labour; online plan adaptation should improve radiation delivery accuracy but requires fast and precise computation. Finally, optimal protocols for quality assurance procedures need to be established.

Image-guided radiotherapy of the prostate using daily CBCT: the feasibility and likely benefit of implementing a margin reduction

OBJECTIVE

To investigate whether planning target volume (PTV) margins may be safely reduced in radiotherapy of localized prostate cancer incorporating daily online tube potential-cone beam CT (CBCT) image guidance and the anticipated benefit in predicted rectal toxicity.

METHODS

The prostate-only clinical target volume (CTV2) and rectum were delineated on 1 pre-treatment CBCT each week in 18 randomly selected patients. By transposing these contours onto the original plan, dose-volume histograms (DVHs) for CTV2 and the rectum were each calculated and combined, for each patient, to produce a single mean DVH representative of the dose delivered over the treatment course. Plans were reoptimized using reduced CTV2 to PTV2 margins and the consequent radiobiological impact modelled by the tumour control probability (TCP) and normal tissue complication probability (NTCP) of the rectum.

RESULTS

All CBCT images were deemed of sufficient quality to identify the CTV and rectum. No loss of TCP was observed when plans using the standard 5-mm CTV2 to PTV2 margin of the centre were reoptimized with a 4- or 3-mm margin. Margin reduction was associated with a significant decrease in rectal NTCP (5-4 mm; p < 0.05 and 5-3 mm; p < 0.01).

CONCLUSION

Using daily online image guidance with CBCT, a reduction in CTV2 to PTV2 margins to 3 mm is achievable without compromising tumour control. The consequent sparing of surrounding normal tissues is associated with reduced anticipated rectal toxicity.

ADVANCES IN KNOWLEDGE

Margin reduction is feasible and potentially beneficial. Centres with image-guided radiotherapy capability should consider assessing whether margin reduction is possible within their institutes.

Quantitative Evaluation of the Benefit of Fiducial Image-Guidance for Prostate Cancer Intensity Modulated Radiation Therapy Using Daily Dose Volume Histogram Analysis

To quantitatively evaluate the extent to which fiducial-based image-guidance improves dose coverage of the target volume and sparing of critical organs for prostate cancer patients treated with intensity modulated radiotherapy (IMRT) and determination of planning margins by original approach of detailed daily dose volume histogram (DVH) and patient's position correction analysis. Sixty-two patients divided in two groups (clinical target volume (CTV) → planning target volume (PTV) margin 10 and 7 mm) were treated with IMRT using implanted fiducial markers. Each patient's treatment fraction was recalculated as it would have been treated without fiducial-guided positioning. For both plans (IGRT and non-IGRT), equivalent uniform doses (EUD), maximal and minimal doses for target volumes, normal tissue complication probability (NTCP), maximum and mean doses for organs at risk and the whole DVH differences were assessed. In the group with 10 mm margins, the only significant difference was worse rectal NTCP by 4.5%, but the CTV dose coverage remained at the same level. Recalculated plans with 7 mm margin could not achieve the prescribed target volume coverage, and the EUD decreased by 3.7 and 0.6 Gy for PTV and CTV, respectively. Desired CTV → PTV margin for non-IGRT plans should be no lower than 12 mm to guarantee 95% instances when delivered dose to CTV maintain as planned, for IGRT plans decrease this requirement to 2 mm. Prostate IMRT strategies involving margin reduction below 7 mm require image-guidance to maintain the planned dose coverage. Using fiducial-based image-guidance and large margins seems to be superfluous.