Daily Variations in Delivered Doses In Patients Treated With Radiotherapy for Localized Prostate Cancer

K.M. Langen, IJROBP Volume 66, Issue 3, Page S341 (1 November 2006)

M.D. Anderson Cancer Center-Orlando, Orlando, FL

To study the variations in delivered doses to the prostate gland, rectum and bladder during a full course of image-guided external beam radiotherapy.

Ten localized prostate cancer patients were treated with helical tomotherapy to 78 Gy at 2 Gy per fraction, in 39 fractions. Prior to each fraction, a megavoltage pelvic CT study was obtained, resulting in a total of 390 CT studies. The prostate, rectum and bladder were manually contoured on each CT study by a single physician. Daily dosimetric analysis was performed with dose recalculation. The specific study endpoints were D95 (dose to 95% of the prostate), rV2 (absolute rectal volume receiving 2 Gy), and bV2 (absolute bladder volume receiving 2 Gy). The average + S.D. and range of each parameter were analyzed either per patient or in the entire study cohort.

For the entire cohort and all 390 fractions, the average daily D95 (+S.D.) was 2.02 + 0.04 Gy (range: 1.79 - 2.20 Gy). This shows remarkable constancy of delivered prostate doses throughout the course of treatment in this group of patients with daily alignment performed on intraprostatic fiducials. For the entire cohort, the average rV2 (+S.D.) was 7.0 + 8.1 cc (range: 0.1 - 67.3 cc). For the entire cohort, the average bV2 (+S.D.) was 8.7 + 6.8 cc (range: 0.3 - 36.8 cc). Fig. 1 shows the variation in rectal doses by displaying the rectal volume receiving 2 Gy in the individual 10 patients; the rV2 value as per the initial plan, the mean rV2 for all 39 fractions per patient, the S.D. of the rV2 for all 39 fractions per patients, the minimum rV2 for all 39 fractions per patient, and the maximum rV2 for all 39 fractions per patient. This shows stiriking variation in rectal doses from patient to patient, and large deviations from day to day in individual patients, e.g. patient #2. In this cohort of patients, rectal doses were, on average, higher than expected in the large majority of patients.

Daily CT images can be used to validate doses delivered during a course of external beam radiotherapy for localized prostate cancer. Unlike for the prostate gland, there is significant day to day variation in rectal and bladder doses, mostly due to daily variations in volume and shape of these organs. Image guidance for the targeting of the prostate, even with intraprostatic fiducials, does not take into account the variation in actual rectal and bladder doses. Techniques that take such dosimetric parameters into account in daily patient set-ups should be investigated.