[Interobserver variability study for daily cone beam computed tomography registration of prostate volumetric modulated arc therapy]

[Partial delegation to radiation therapists of the control by cone beam CT of prostate positioning]

PURPOSE

Intensity modulated radiotherapy for prostate cancer involves daily monitoring of the positioning of the prostate, possible with cone beam CT (CBCT). It allows increased accuracy compared to readjustments but induces an increase in the time dedicated to these medical checks. The aim of the study was to evaluate the possibility of delegation of this task to the radiation therapists by comparing their readjustments to the doctors.

PATIENTS AND METHODS

Five consecutive patients treated with radiation for prostate cancer (76Gy) were analysed. All had a daily CBCT for position control. The movements of the prostate relative to the bony part, the positional variations of the prostate measured by the radiation therapists and the doctors and medical time required to analyse imagery (filling of the rectum and bladder and perform a recalibration) were measured.

RESULTS

One hundred seventy-six CBCT were analysed or 980 steps in the three axes. The movements of the prostate relative to bony part were respectively at least 5mm in 19%, 7% and 3% in the anterior-posterior, upper-lower and right-left axes. Changes readjustments between radiation therapists and doctors were in 95% of cases at the most 4mm in the anterior-posterior and upper-lower axis, and 3mm in the left-right axis. The time for medical use of the CBCT averaged 8min 40 [4 to 22min].

CONCLUSION

The daily readjustment on the prostate using CBCT may be delegated to radiation therapists with acceptable concordance of less than 4mm for 95% of measurements. An initial and ongoing training will ensure treatment safety.

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.

[Expected benefit of lymph node and seminal vesical dissection to decrease high-risk prostate cancer radiotherapy]

PURPOSE

In case of pelvic lymph node and seminal vesicle dissection followed by prostate cancer intensity-modulated radiotherapy, the objective of the study was to evaluate the dosimetric benefit of reducing the target volume.

PATIENTS AND METHODS

A total of 25 patients with high-risk prostate cancer had surgery first followed by intensity-modulated radiotherapy and androgen deprivation. Four treatment planning were simulated for each patient, based on two CT scans performed before and after surgery. The target volumes were: prostate-seminal vesicles-lymph nodes, prostate-lymph nodes, prostate-seminal vesicles and prostate only. The total dose was 46Gy in the seminal vesicles and lymph nodes, and 80Gy in the prostate.

RESULTS

Compared to prostate target volume only, the addition of seminal vesicles and lymph nodes multiplied by a factor of 1.6 and 6.5 the target volume, respectively. Decreasing the target volume from prostate-seminal vesicles-lymph nodes to prostate-seminal vesicles, to prostate only decreased the rectal wall mean dose from 49Gy to 42Gy, to 36Gy, and the risk of late rectal bleeding from 4.4% to 3.2%, to 2.4% (P<0.05), respectively. The bladder wall mean dose decreased from 51Gy to 40Gy, to 35Gy (P<0,05), respectively. Not irradiating the lymph nodes decreased the absolute risk of diarrhea by 11%.

CONCLUSION

Lymph node and seminal vesicle dissection before prostate cancer intensity-modulated radiotherapy allows decreasing moderately the risk of digestive toxicity.