Reduction of Late Urinary Toxicity from Prostate Cancer Radiotherapy via Intrafractional MV-kV Image Guidance

PURPOSE/OBJECTIVE(S)

Prostate cancer localization during radiotherapy is difficult and introduces positional variability during treatment. Here we evaluate the impact on treatment toxicities of an in-house system that uses MV and kV imaging guidance (MKIG) to track implanted fiducials during stereotactic body radiotherapy (SBRT) for localized prostate cancer.

MATERIALS/METHODS

A 3D MKIG platform that tracks prostate implanted fiducials in real-time was built and clinically translated to replace a prior commercial approach called intrafraction motion review (IMR), which tracks thefiducials in 2D kV view only. MKIG has been shown to correct both superior-inferior and anterior-posterior (AP) motions that are harmful to critical organs and is superior to IMR for intrafractional motion management, which is less sensitive to AP motions. From 2017 to 2019, 150 patients with localized prostate cancer were treated with SBRT to 40 Gy in 5 fractions. During the delivery of volumetric modulated arc therapy, orthogonal MV-kV pairs were simultaneously acquired at every 20° gantry rotation and rigidly registered to the reference image templates created from the planning CT. Calculated 3D translations of implanted fiducials were used to localize the prostate and alert the therapist to interrupt and reposition the prostate when exceeding a 1.5-2 mm threshold. A comparison cohort of 121 prostate patients was treated from 2015 to 2016 with the same prescription dose and treatment technique but instead managed by IMR, where the therapist interrupted treatment based on a 2mm expansion of the fiducial contours superimposed on the kV images. Statistics of intrafractional interruptions, patient shifts, and overall delivery time were collected to evaluate the efficacy of the clinical workflow. The incidence of late grade ≥2 toxicities was analyzed to assess clinical complications. The median follow-up time was 5.5 years (range of 3.6 to 8.0 years).

RESULTS

The MKIG cohort had more interruptions per fraction (1.09 vs. 0.28) and longer average delivery time per fraction (579±205s vs. 357±117s) than IMR. 75% of shifts resulting from MKIG were less than 3mm, compared to 51% in IMR, indicating that MKIG tended to detect and correct smaller deviations (p<0.001). The baseline International Prostate Symptom Score was 7.9 in the MKIG cohort vs. 8.4 in IMR (p = 0.41). The incidence of late grade ≥2 urinary toxicity was lower in the MKIG than IMR cohort: 10.7% vs. 19.8% (p = 0.05). One grade ≥2 rectal toxicity was observed in the IMR cohort but none in MKIG.

CONCLUSION

Wehave demonstrated that MKIG is a clinically practical and effective method for monitoring and correcting prostate positional deviations during SBRT of prostate cancer. MKIG is better suited than 2D IMR to localize the prostate and trigger patient repositioning during treatment. A statistically and clinically significant reduction in urinary toxicity was observed. The potential expansion of MKIG to other clinical sites and translation to other centers should be considered.

Clinical experience with intensity modulated radiation therapy (IMRT) in prostate cancer

PURPOSE

To compare acute and late toxicities of high-dose radiation for prostate cancer delivered by either conventional three-dimensional conformal radiation therapy (3D-CRT) or intensity modulated radiation therapy (IMRT).

MATERIALS AND METHODS

Between September 1992 and February 1998, 61 patients with clinical stage T1c- T3 prostate cancer were treated with 3D-CRT and 171 with IMRT to a prescribed dose of 81 Gy. To quantitatively evaluate the differences between conventional 3D-CRT and IMRT, 20 randomly selected patients were planned concomitantly by both techniques and the resulting treatment plans were compared. Acute and late radiation-induced morbidity was evaluated in all patients and graded according to the Radiation Therapy Oncology Group toxicity scale.

RESULTS

Compared with conventional 3D-CRT, IMRT improved the coverage of the clinical target volume (CTV) by the prescription dose and reduced the volumes of the rectal and bladder walls carried to high dose levels (P<0.01), indicating improved conformality with IMRT. Acute and late urinary toxicities were not significantly different for the two methods. However, the combined rates of acute grade 1 and 2 rectal toxicities and the risk of late grade 2 rectal bleeding were significantly lower in the IMRT patients. The 2-year actuarial risk of grade 2 bleeding was 2% for IMRT and 10% for conventional 3D-CRT (P<0.001).

CONCLUSIONS

The data demonstrate the feasibility and safety of high-dose IMRT for patients with localized prostate cancer and provide a proof-of-principle that this method improves dose conformality relative to tumor coverage and exposure to normal tissues.

Preliminary results of conformal radiation therapy for medulloblastoma

Radiation therapy for medulloblastoma consists of postoperative irradiation of the intracranial and spinal subarachnoid volume with an additional boost to the primary site of disease in the posterior fossa. The entire posterior fossa is usually included in the boost volume. Conformal radiation therapy techniques may be used to boost the primary site alone and substantially reduce the dose received by normal tissues, including the supratentorial brain, the middle and inner ear, and the hypothalamus. Using these techniques to irradiate only the tumor bed or residual tumor and not the entire posterior fossa represents a new paradigm in the treatment of medulloblastoma. In this study, we examine the use of conformal radiation therapy in the treatment of 14 patients with medulloblastoma. These patients were treated with multiple static, individually shaped, noncoplanar beams directed at the primary site after craniospinal irradiation. Excluding two patients who had previously received irradiation to the posterior fossa, the mean dose delivered to the primary site was 5715 cGy. Among the medulloblastoma patients (n = 10) who received immediate postoperative radiation therapy, no failures have occurred with a median follow-up of 42 months (range from 30 to 54 months). To demonstrate the differences in the distribution of dose to normal tissues when comparing conventional and conformal techniques, dose-volume histograms of the total brain, middle and inner ear, hypothalamus, and temporal lobe were created and presented for an example case. The neurologic, neuroendocrine, and neurocognitive outcome for patients with medulloblastoma may be influenced with the use of conformal radiation therapy. The use of these techniques should be formally tested in prospective studies of rigorously staged patients with failure rate monitoring.

Quantification and predictors of prostate position variability in 50 patients evaluated with multiple CT scans during conformal radiotherapy

PURPOSE

To determine the extent and predictors for prostatic motion in a large number of patients evaluated with multiple CT scans during radiotherapy, and evaluate the implications of these data on the design of appropriate treatment margins for patients receiving high-dose three-dimensional conformal radiotherapy.

MATERIALS AND METHODS

Fifty patients underwent four serial computerized tomography (CT) scans, consisting of an initial planning scan and subsequent scans at the beginning, middle, and end of the treatment course. Each scan was performed with the patient in the prone treatment position within an immobilization device used during therapy. Contours of the prostate and seminal vesicles were drawn on the axial CT slices of each scan, and the scans were matched by alignment of the pelvic bones with a chamfer matching algorithm. Using the contour information, distributions of the displacement of the organ center of mass and organ border from the planning position were determined separately for the prostate and seminal vesicles in each of the three principle directions: anterior-posterior (AP), superior-inferior (SI) and left-right (LR). Each distribution was fitted to a normal (Gaussian) distribution to determine confidence limits in the center of mass and border displacements and thereby evaluate for the optimal margins needed to contain target motion.

RESULTS

The most common directions of displacement of the prostate center of mass (COM) were in the AP and SI directions and were significantly larger than any LR movement. The mean prostate COM displacement (+/- 1 standard deviation, SD) for the entire population was -1.2 +/- 2.9 mm, -0.5 +/- 3.3 mm and -0.6 +/- 0.8 mm in the, AP and SI and LR directions respectively (negative values indicate posterior, inferior or left displacement). The mean (+/- 1 SD) seminal vesicle COM displacement for the entire population was - 1.4 +/- 4.9 mm, 1.3 +/- 5.5 mm and -0.8 +/- 3.1 mm in the AP and SI and LR directions, respectively. The data indicate a tendency for the population towards posterior displacements of the prostate from the planning position and both posterior and superior displacements of the seminal vesicles. AP movement of both the prostate and seminal vesicles were correlated with changes in rectal volume (P = 0.0014 and < 0.0001, respectively) more than with changes in bladder volume (P = 0.030 for seminal vesicles and 0.19 for prostate). A logistic regression analysis identified the combination of rectal volume > 60 cm3 and bladder volumes > 40 cm3 as the only predictor of large ( > 3 mm) systematic deviations for the prostate and seminal vesicles (P = 0.05) defined for each patient as the difference between organ position in the planning scan and mean position as calculated from the three subsequent scans.

CONCLUSIONS

Prostatic displacement during a course of radiotherapy is more pronounced among patients with initial planning scans with large rectal and bladder volumes. Such patients may require more generous margins around the CTV to assure its enclosure within the prescription dose region. Identification and correction of patients with large systematic errors will minimize the extent of the margin required and decrease the volume of normal tissue exposed to higher radiation doses.

The effect of treatment positioning on normal tissue dose in patients with prostate cancer treated with three-dimensional conformal radiotherapy

PURPOSE

To prospectively assess the effect of supine vs. prone treatment position on the dose to normal tissues in prostate cancer patients treated with the three-dimensional conformal technique.

METHODS AND MATERIALS

Twenty-six patients underwent three-dimensional treatment planning in both the supine and prone treatment positions. The planning target volume and normal tissue structures were outlined on each CAT scan slice, and treatment plans were compared to assess the effect of treatment position on the volume of rectum, bladder, and bowel exposed to the high dose of irradiation.

RESULTS

The average dose to the rectal wall and the V95 (volume of rectal wall receiving at least 95% of the prescription dose) for the prone position were 64 and 24% of the prescription dose, respectively, compared to 72 and 29%, respectively, for the supine position (p < 0.05). When the average rectal wall dose was used as an endpoint, 14 of the 26 patients (54%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantage for the supine position (p < 0.0002). Similarly, when V95 of the rectal wall was used as a measure of comparison, 15 patients (58%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantage for the supine position (p < 0.0002). In 13 patients (50%), a change from supine to the prone position was associated with reduction of the V95 to levels < 30% of the prescription dose compared to 3 patients (11%) in whom such an advantage resulted from change of the prone to the supine position (p < 0.005). The effect of treatment position on the rectal wall dose was most pronounced in the region of the seminal vesicles. An increased volume of bowel was also noted in the supine position. The treatment position, however, had no significant impact on the dose to the bladder wall.

CONCLUSIONS

Three-dimensional conformal radiotherapy for prostate cancer in the prone position is associated with significant reduction of the dose to the rectum and bowel resulting in an improvement in the therapeutic ratio.

Neoadjuvant hormonal therapy improves the therapeutic ratio in patients with bulky prostatic cancer treated with three-dimensional conformal radiation therapy

PURPOSE

To determine the extent of reduction of volume of normal tissue structures exposed to high doses of radiation therapy (RT) after administration of neoadjuvant hormonal therapy (NHT) in patients with bulky, geometrically unfavorable prostatic cancers.

METHODS AND MATERIALS

Twenty-two patients with bulky prostatic cancers were treated with a 3 month course of neoadjuvant leuprolide acetate and eulexin prior to three-dimensional (3-D) conformal radiotherapy. Patients were included if 3-D treatment planning revealed that either > 30% of the rectal wall would receive 95% of the prescription dose (D95) (n = 13); > or = 50% of the bladder wall would receive D95 (n = 10); or that any volume of small bowel would receive > or = 65% of the prescription dose (n = 16). All patients underwent simulation and conformal treatment planning before and after NHT. Pre and posthormone cumulative dose volume histogram (DVH) calculations for all normal tissue structures were analyzed and compared for each patient.

RESULTS

The median percentage of target volume reduction after NHT was 25% (range: 3-52%). Ten of 13 patients (78%) whose prehormone rectal DVH demonstrated > 30% of the rectal wall receiving D95 responded to NHT with a median 25% (range: 16-48%) reduction of rectal volume receiving the D95. A median reduction of 50% (range: 6-64%) of the bladder volume receiving D95 was observed in nine of ten patients (90%), while 13 of 16 (81%) showed a reduction of small bowel volume to a median percentage of 88% (range: 67-100%) of the prehormonal values.

CONCLUSION

Neoadjuvant hormonal therapy is an effective method for decreasing the size of bulky prostatic tumors as well as for optimizing the geometry of the target volume in relation to the adjacent normal tissue structures prior to radiation therapy. Such an approach allows for reduction of the volume of normal tissues exposed to high doses in the majority of treated patients. Currently, studies are underway to determine whether NHT will lead to a decreased likelihood of long-term complications associated with radiotherapy of bulky, geometrically unfavorable prostatic tumors, and permit the safe delivery of escalated dose levels using conformal treatment techniques.